Tachypleus tridentatus and T. gigas are sometimes the subject of misidentification. In China, three horseshoe crab nature reserves were established in Guangdong Province in 2004 for the protection of T. gigas (Kwan et al. 2016), even though T. gigas has not been recorded in these areas (Liao and Li 2001, Liao et al. 2001a).

In Hong Kong, the occurrence of T. gigas, T. tridentatus and C. rotundicauda was reported (Mikkelsen 1988), but recent population studies did not reveal any presence of T. gigas in Hong Kong waters (Li 2008, Shin et al. 2009, Morton and Lee 2010) and a recent literature review suggests previous reports of the presence of T. gigas in Hong Kong was a case of mistaken identity, based on the wrong labelling of photographs in a popular natural history book at the time (Laurie 2011, unpublished).

On 23 May 2012, five horseshoe crabs were seized at Sam Ratulangi International Airport, Manado, North Sulawesi, Indonesia under PP No. 7/1999 (Indonesia Government Regulation No. 7/1999). Although described as T. gigas, photographs indicate the seized animals were T. tridentatus (BKIPM 2012).

This assessment integrates information on major threats, habitat and population responses, population genetics, past and predicted future levels of exploitation and management and conservation actions. Population responses, in terms of abundance, geographic range and viability inform risk at the country level, which in turn, informs the assessment of the species' risk of extinction.

Population Reduction

Tachypleus tridentatus is an inshore, coastal species. Its distribution area is vast and complex, extending in a north to south range, from Japan with a temperate climate in the north, through mainland of the People’s Republic of China (hereafter abbreviated to China), Taiwan (province of China, hereafter abbreviated to Taiwan), Hong Kong (a Special Administrative Region of China, hereafter abbreviated to Hong Kong), Viet Nam, Philippines, Brunei Darussalam and East Malaysia to Indonesia, with a tropical climate in the south. There is genetic evidence for subpopulations within its range and it appears to exhibit differing population profiles, with historically dense populations in its northern range (Japan, China, Taiwan, Hong Kong, Vietnam) and low population densities in its southern range (Philippines, Brunei Darussalam, Malaysia, Indonesia).

Its life-history habits with five distinct stages make it vulnerable to a variety of threats at each stage of its development. High tide spawning sites and intertidal juvenile nursery habitat are critical to its survival needs, but the occurrence of such habitats is not universal; they occur intermittently throughout its range based on hydrology, geomorphology, sedimentology and the species' own habitat preferences. Whilst the coastlines where T. tridentatus ranges are long, these critical areas of occupancy, comprising the actual used and useable areas utilized as spawning habitat and intertidal juvenile nursery grounds, are relatively small in area. As a consequence, within its range, spawning and intertidal habitats do not equate to all available coastline because only select areas of it constitute suitable habitat. Furthermore, where horseshoe crab species occur sympatrically (such as localities in China and Hong Kong where T. tridentatus and Carcinoscorpius rotundicauda coexist together) juvenile nursery grounds on intertidal flats are spatially delineated into sandy facies (favoured by juvenile T. tridentatus) and muddy facies (favoured by juvenile C. rotundicauda), further restricting the extent of useable area available for juvenile T. tridentatus to forage.

Where data are available, significant population declines (See Table 1 in the Supplementary Material) and habitat declines (See Table 2 in the Supplementary Material) have been documented in Japan, China, Taiwan, Hong Kong and Viet Nam, with declining population trends being reported by fishermen in the Philippines, Malaysia and Indonesia.

Because of their coastal location, T. tridentatus spawning habitat and juvenile nursery grounds are highly susceptible to reclamation, mariculture projects or are suffering from the adverse impacts of coastal infrastructure construction and sea sand extraction. The species has experienced different levels of habitat loss or degradation throughout much of its range, with studies indicating that infrastructure projects have contributed to extirpations, instances of restricted gene flow, low haplotype diversity, and the creation of population bottlenecks indicative of impending extirpation. Whilst many habitats have been lost completely, recruitment has ceased at many previously healthy spawning sites, for reasons that are still not clear. The result is the presence of remnant populations in Japan, China, Taiwan and Hong Kong. In the Philippines, Malaysia and Indonesia population densities are low, common threats occur and population declines have been reported. Population data are not available for Vietnam or Brunei Darussalam. The risk profiles vary between countries.

Japan: T. tridentatus was once abundant on all of the coasts of the Seto Inland Sea. There is no T. tridentatus fishery in Japan. Populations have declined significantly because of habitat loss or degradation. Between 1930 to 1994 over 80% of horseshoe crab habitats disappeared in the Seto Inland Sea and the area of tidal flats in Japan has decreased by over 40% since 1945, whilst between the 1970’s to 2000’s seagrass beds, some of which support juvenile horseshoe crab populations were reduced by 40% in area. Tachypleus tridentatus was assessed to be Critically Endangered in 2006 mainly because of the loss or deterioration of its tidal flat habitats. Reports of extirpations in Japan are not uncommon. Tachypleus tridentatus has disappeared from many of its spawning grounds, whilst other sites have recorded significant decreases in visits by spawning pairs.  Small, remnant populations remain, some are conservation-reliant for their survival and spawning habitat and juvenile nursery grounds are still threatened with development.

China: T. tridentatus was once abundant in high densities throughout its range in China. Mass spawning events were a common sight and it was regarded as an important economic species, both to supply the biomedical industry and for consumption. In both cases, all of the animal is used, resulting in 100% mortality. Population declines of ≥90% have been recorded throughout much of its range in China and a number of important fisheries collapsed between the 1970s and 1990s. In parallel to overfishing, land reclamation between the 1950s and the 1990s drastically shrunk intertidal habitats. Tachypleus tridentatus was assessed as Endangered in 2004 because populations had seriously declined due to its over-exploitation as a source of raw material for chitin, the utilization of its blood in the medical and biomedical industries and the use of its meat for food. Subsequent to this, between 2008 and 2012 the mangrove area in China decreased by about 66% because of land reclamation and sea sand extraction has caused significant habitat degradation along almost the entire range of T. tridentatus on the mainland coast of China. Reports of extirpations are not uncommon. Between 2006 and 2010, no spawning was observed during surveys of 27 nationally recognized spawning sites and a few juveniles were found at only six of the sites, representing an 88% decline in habitat utilization. Dongshan Bay in Fujian went from having abundant adult populations in the early 1980s to no spawning being observed and only juvenile populations being found during surveys between 2006 and 2010. Commercially, limuloid resources are exhausted in China, yet as late as 2011, the Guangxi Government was still promoting the horseshoe crab fishery in Beibu Gulf, demand for T. tridentatus as a culinary treasure and to supply the biomedical industry is high and remaining populations are being targeted to meet these demands.

Taiwan: T. tridentatus was once abundant in high densities throughout its range in Taiwan, but populations have been extirpated or declined significantly because of habitat loss or degradation. On Taiwan Island, 55% of the natural coastline, particularly on the west coast, which was home to the majority of T. tridentatus spawning grounds in Taiwan has been lost to reclamation or degraded because of coastal infrastructure construction, to the extent it is extirpated at many localities and is on the verge of extirpation on Taiwan Island. Similar habitat loss and degradation has occurred on Kinmen Island, due to the reclamation, dredging of subtidal areas and sea sand extraction. No adults or mating pairs have been recorded on the intertidal flats of Taiwan Island since the 1960’s, nor more recently from Kinmen. When population surveys commenced in Taiwan in 2003, they were conducted on juvenile populations because of the lack of adults. Where juveniles still exist, populations are declining and most of the remaining sites support non-viable juvenile populations.

Hong Kong: T. tridentatus was once widespread and abundant in Hong Kong with thriving populations until the 1980s, but by the early 1990s it had disappeared from much of its range. Significant population declines occurred because of overfishing to supply the biomedical industry, combined with significant levels of habitat loss or degradation. Population declines of ≥90% can be inferred in Hong Kong. Spawning was last scientifically observed in 1986 and when population surveys commenced in 2002, they were conducted on juvenile populations because of the lack of adults.

Viet Nam: Up to the early 1980s major concentrations of T. tridentatus could be found along the coast of Viet Nam, at least as far south as Nha-Trang, but populations have declined significantly through a combination of overfishing to supply the biomedical industry and to meet consumer demand in China, in parallel with significant levels of habitat loss or degradation. Tachypeus tridentatus was assessed as Vulnerable in Viet Nam in 2007, based on a population decline of 50%, a fisheries catch decline of 20% and a decline in area of occupancy of 50% between 1990 and 2007. Subsequent to this, Viet Nam lost 40% of its mangroves between 2006 and 2012 and 56% of the remainder are planted mangroves and considered of low ecological value. Seagrass bed coverage in Viet Nam decreased from 40% to 70% between 2009 and 2014 and the coast of Viet Nam’s central coastal provinces, where T. tridentatus was once most abundant has been transformed since 1999 through a combination of shrimp farming, tourism infrastructure construction and sand mining, which have destroyed thousands of hectares of the ecosystem, so that its ecosystem service functions are now severely diminished. Tachypleus tridentatus is a legally exploitable resource in Viet Nam and demand from China to supply the biomedical market and to meet consumer demand remains high.

Philippines: Historic information suggests T. tridentatus was once widely distributed throughout the Sulu Sea, but it experienced significant declines in numbers in the decades preceding 2000. Spawning sites and nursery grounds are threatened by coastal construction and sand mining and because of their close proximity to human population centres, they suffer from high rates of incompatible habitat use. Surveys in 2001 and 2002 indicated the presence of small juvenile populations at two renowned nursery beaches in Palawan.

Malaysia: T. tridentatus is only found in Sabah and Sarawak. Where surveys have been conducted in Sabah, they indicate the presence of small, low density populations. Horseshoe crabs, including T. tridentatus form an important income component for poor fishermen and are a dietary component of Bajau Laut fishermen. Population declines have been reported by fishermen. The adverse impact of selective fishing practices on adult populations is likely evident in the male-biased Operational Sex Ratios (OSR) observed in Sabah, which is believed to reflect the selective targeting of gravid females by coastal and artisanal fisheries. Because of their close proximity to human population centres, spawning sites and nursery grounds suffer from high rates of incompatible habitat use. No population, habitat or threat data is currently available for Sarawak,

Brunei Darussalam: Formal studies have just commenced, so population, habitat and threat data are not available. Because of its small geographic area, it is unlikely significant populations are resident here.

Indonesia: Formal studies have just commenced, so population, habitat and threat data are not available. Population declines have been reported by fishermen. Because of their close proximity to human population centres, spawning sites and nursery grounds suffer from high rates of incompatible habitat use. Land reclamation of spawning habitats and juvenile nursery grounds has occurred in Indonesia.

A significant contributor to past population declines and the future near-term threat to T. tridentatus is the unmanaged, unsustainable harvest of adults to supply the biomedical industry and the selective targeting of gravid females to meet consumer demands (See Table 3 in the Supplementary Material). The long-term threat to T. tridentatus is the loss or degradation of spawning habitat and juvenile nursery grounds by reclamation, mariculture projects or the adverse impacts of coastal infrastructure construction and sea sand extraction, because these factors are leading to local populations being extirpated and their impacts are not reversible.

The generation length of T. tridentatus in its northern range has been estimated to be 20.25 years (giving a 3-generation length of 60.75 years [rounded to 60 years]). There is evidence to suggest different age-to-maturity rates at different latitudes, so that the generation length for southern populations could be considerably shorter.

In terms of future exploitation, the increasing demand driven by the projected growth of the Amebocyte Lysate industry and the categorization of T. tridentatus as a culinary treasure in China constitute high risk threats that will have considerable impact on remaining populations, and with the exception of Japan which has no horseshoe crab fishery, it will be aggressively targeted to meet these demands. In its southern range, artisanal fisheries, particularly the selective harvesting of gravid females in small or depleted populations can and in future will have significant negative impact, as evidenced by the male-biased Operational Sex Ratio (OSR) recorded in eastern Malaysia. 

Furthermore, once recruitment at a site has stopped, the chances of recolonization and natural population recovery are low. Specific spawning and juvenile nursery habitat preferences, limited dispersal capabilities of larvae and juveniles, limited adult migratory behavior, topographic barriers to migration, genetic isolation and restricted or limited gene flow between geographically close populations and evidence of endemism means natural recruitment to abandoned spawning locations could take generation length time spans for populations to re-establish themselves and at many sites, because of lasting changes to local hydrology and sedimentology, recolonization may not occur because the changes may not be reversible.

Because of the combination of over-harvest and habitat loss, an ongoing population reduction, based on past estimates starting circa 1980 and future projections extending at least for the next two decades, suggest a reduction of ≥50% over a period of about 60 years or 3 generations in a significant portion of the species range.


Risk Assessment
Tachypleus tridentatus is facing relentless and unremitting anthropogenic threats from multiple stressors on a range of fronts. Major population declines have been reported throughout much of its range, all life-history stages face a combination of threats, including targeting of adults for its high value blood, chitin and eggs, high levels of habitat degradation, loss or abandonment, low resilience to exploitation including long age-to-maturity time, low juvenile recruitment rates, specific spawning and juvenile nursery habitat preferences and largely unregulated, continuing fishing pressure and these threats have not ceased and in some cases they are not reversible.

Up to the early 1980s, when survey data are first available, T. tridentatus was abundant with high population densities in much of its northern range, but since then, it has experienced significant population declines throughout much of its range, caused by a combination of unmanaged exploitation and significant levels of habitat loss or degradation It is extirpate at many localities and is still threatened with habitat loss throughout much of its range. The increasing demand for T. tridentatus, driven by the projected growth of the Amebocyte Lysate market and to meet consumer demands following its categorization as a culinary treasure in China constitute high risk threats that will result in future exploitation of remaining populations, and with the exception of Japan, it will be aggressively targeted to meet these demands. Its exploitation as an all-parts-use animal means populations have little chance of recovery. Tachypleus tridentatus has historically been subject to high levels of exploitation and it is predicted to be the subject of future high levels of exploitation, compounded by significant levels of habitat loss and degradation in the past and likely losses of spawning habitat and juvenile nursery grounds in the future. In summary, these threats have not ceased, in many instances they are not reversible and the future risk and impact of these threats are assessed to be high to extremely high throughout its range. Based on these factors and because of an ongoing population reduction of ≥50% over a period of about 60 years (three generations), T. tridentatus is assessed as Endangered.

In addition to the Endangered global assessment, a number of country level assessments have been designated for T. tridentatus as follows: Critically Endangered in Japan (2006), Endangered in China (2009) and Vulnerable in Viet Nam (2007).

Horseshoe crab fossils are found in deposits ranging from the Palaeozoic to the Cenozoic. The earliest fossil forms date back 475 million years ago (mya) (Van Roy et al. 2010) and by 310 mya the unmistakable horseshoe crab body form was clearly established (Shuster 2001, p. 36). Modern horseshoe crabs have existed for more than 200 million years (Botton and Ropes 1987, Shuster 2001, Tanacredi 2001, Anderson and Shuster 2003, Blazejowski 2015).

There are four extant species of horseshoe crab found in two regions of the world. Limulus polyphemus (Linnaeus 1758), the American Horseshoe Crab, is found along the Atlantic and Gulf of Mexico coastlines of North America ranging from the Gulf of Maine, USA (42°N) to the Yucatán Peninsula, México (19°N) and west to the northcentral Gulf of Mexico, USA (30°N, 88°W). Three species—Tachypleus tridentatus (Leach 1819), T. gigas (Müller 1785) and Carcinoscorpius rotundicauda (Latreille 1802)—inhabit the coastal waters of Asia (Sekiguchi 1988, p. 1).

The first formal surveys of the range, abundance and populations of T. tridentatus were conducted between 1975 to 1981 in its southern range and 1980 to 1982 in its northern range (Sekiguchi 1988, pp. 24–45). Tachypleus tridentatus broadly ranges in distribution from the north to the south (Sekiguchi 1988, p. 34), extending from Japan in a southerly direction through China (Zhejiang, Fujian, Guangdong, Hong Kong, Guangxi, Hainan), Taiwan, Vietnam, Philippines (Palawan and Sulu Sea), Malaysia (Sarawak and Sabah), Brunei Darussalam and Indonesia, while T. gigas and C. rotundicauda range from the east to the west (Sekiguchi 1988, p. 35).

All recent horseshoe crabs, including T. tridentatus are limited to relatively narrow areas and their distribution ranges are discontinuous (Sekiguchi 1988, p. 410). During surveys conducted between 1975 and 1982, population densities were highest on the China coast in its northern range, especially in Fujian and on the west coast of Hainan, whilst population densities in its southern range were extremely low (Sekiguchi 1988, pp. 24–25, 34). In the south eastern part of its range T. tridentatus overlaps with both T. gigas and C. rotundicauda (Sekiguchi 1988, p. 28), where the sympatric occurrence of all three species sometimes occurs (Sekiguchi 1988, p. 28, 35; Robert et al. 2014; Mashar et al. 2017a).
A single female T. tridentatus was recorded from Udo Island, Jeju, South Korea in 1979 (Yang and Ko 2015), but because this was not associated with a known spawning or juvenile nursery habitat, it is not considered within the species range occurrence in this assessment.

Northern extent: The northern most range of T. tridentatus is Kasaoka in Okayama Prefecture on Honshu Island in Japan and the Japanese populations represent the northernmost range for T. tridentatus. Spawning habitats are distributed intermittently along the coastal regions of Southern Honshu, Shikoku and Northern Kyushu, with a historic concentration around the Seto Inland Sea (Sekiguchi 1988, pp. 34, 39–45; Nishida et al. 2015).

Southern extent: Horseshoe crabs, including T. tridentatus inhabit the north coast of Java, Indonesia but no horseshoe crabs have been recorded from South Java (Nishida 2012a), the possible reason being the sea south of Java is deep close to shore (Laurie/Meilana 2015, pers. comm.). 

Eastern extent: Manado in North Sulawesi, Indonesia (Sekiguchi 1988, pp. 32, 36; Yamasaki et al. 1988; BKIPM 2012) represents the eastern most occurrence of T. tridentatus populations. The Wallace Line, a boundary separating Asian and Australasian ecozones, may represent a barrier to the significant eastward expansion of T. tridentatus populations (Laurie/Meilana 2015, pers. comm).

Western extent: Sibolga in North Sumatra Province and Padang in West Sumatra Province, Indonesia (Sekiguchi 1988, pp. 30, 36; Yamasaki et al. 1988, Fig. V-21, p. 96) represent the western most occurrences of T. tridentatus populations.

For a more detailed description of the distribution of T. tridentatus within its range states, and the spread of the species through transportation and introductions, see the 'Geographic Range' section in the Supplementary Information.

The population is split into a series of genetically distinct subpopulations across the range of T. tridentatus (for a full discussion of genetic diversity and dispersal patterns, see the 'Population' section in the Supplementary Information).
A number of factors may contribute to the presence of subpopulations in T. tridentatus. In terms of its ecology, T. tridentatus exhibits limited larval dispersal (Botton and Loveland 2003), confined juvenile mobility (Kawahara 1982, Kaiser 2002, Almendral and Schoppe 2005, Kwan et al. 2015b), limited adult movement (Wada et al. 2016) and spawning beach fidelity (Wada et al. 2010, Chan et al. 2016, unpublished), which may all be factors contributing to observed patterns of local genetic variation.
Peninsulas
Where noticeable genetic differentiation has been detected, it may be due to geographic separation due to peninsulas including the differences in the eastern and western populations in Japan, separated by the Itoshima Peninsula (Nishida et al. 2015) or differences in the Beihai and Zhanjiang populations, separated by the Leizhou Peninsula in southwest China (Weng et al. 2013).

Enclosed Bays
Another geographic factor influencing genetic differentiation appears to be the geographic isolation of populations in enclosed bays leading to restricted gene flow. Examples of this type of isolation include the population in Hirao Bay in the eastern Japan group, which exhibit unique genetic features when compared with neighbouring sites (Nishida et al. 2015), the limited genetic diversity in the Tiexianwei population in the semi-enclosed Magong Bay in Taiwan (Yang et al. 2007), the genetic differentiation seen in the Zhangpu population from the large but semi-enclosed Dangshan Bay in China (Weng et al. 2013), and the low haplotype diversity observed in populations on the enclosed north western coast of Lantau Island in Hong Kong (Chan et al. 2016, unpublished). In terms of bathymetrics, South-east Asia comprises shallow marginal seas, island arcs and marine basins bounded by deep sea trenches (Hutchinson 1986, 1989) and it is important to note, this underwater ‘topography’ is equally, if not more important in limiting aquatic dispersal, but in respect of T. tridentatus this potential limitation has not yet been tested.

At the extreme end of the range, the presence of the population at Budai, Taiwan with a single haplotype and no genetic diversity is indicative of a population on the verge of extinction (Yang et al. 2009a).

The limited dispersal potential and the subdivision of at least some T. tridentatus populations to local embayments parallels the documented population structure in the American Horseshoe Crab (Limulus polyphemus), especially in the northern and southern portions of its geographic range (Smith et al. 2016).

Population Trends
Data availability determined whether quantitative analysis was valid for trend estimation. In the absence of a quantitative analysis, descriptive summaries of observations were included to infer qualitative trends in relative abundance and distribution (Smith et al. 2016). Population trends are here assessed comparing previous records, which usually involve mortalities together with recent survey data.

Japan
There is a consensus among Japanese scientists that T. tridentatus populations have decreased precipitously throughout Japan since the 1950s and populations within the Seto Inland Sea, where they were once common are almost extinct (Botton 2001, p. 44; Nagasaki Prefecture 2001; Iwaoka and Okayama 2009, p. 581). The word ‘decimation’ has been used to describe these declines (Botton 2001, p. 48). In particular, T. tridentatus was once abundant on all of the coasts of the Seto Inland Sea, but coastal change due to human activities has resulted in habitat loss, leading to reduced ranges and reduced populations (Sekiguchi 1988, p. 39).

During the reclamation of Tomioka Bay in 1958, the remains of approximately 10,000 adult T. tridentatus were found stranded (Nishii 1973, Seino et al. 2003, p. 8) whilst when Kasaoka Bay was reclaimed commencing in 1969, approximately 100,000 adult T. tridentatus and millions of juvenile horseshoe crabs were believed to have perished (Seino et al. 2003, p. 8).

In the 1970s, it is estimated that the spawning T. tridentatus population at Tatara Beach (a protected horseshoe crab breeding area in Imari Bay on northern Kyushu) may have been 500 pairs (Sekiguchi 1988, p. 44). Surveys conducted at Tatara Beach during the 1979 and 1980 peak spawning season recorded a daily peak count of 23 pairs on one day at diurnal high tide, with normally fewer than 20 pairs spawning at any one time, with the largest number of spawning pairs being recorded during a season being less than 30 pairs. A maximum number of 48 spawning pairs were recorded on 22 July 1982 during a night high tide (Sekiguchi 1988, p. 63-64), whilst a similar survey at the same location in 1994 recorded a total of 29 mated pairs, with a daily peak count of five pairs on two consecutive days (Botton et al. 1996).

Sone Tidal Flat is 517 hectares (5.17 km²) when exposed and hosts the largest juvenile T. tridentatus intertidal nursery ground in Japan. Annual surveys from 1995 to 2013 recorded 120 pairs in 1995 to a maximum of 1581 pairs in 2005, with fluctuating counts, ranging from 513 pairs in 2006; 265 pairs in 2007; 738 pairs in 2012; and 1,079 pairs in 2013 (Hayashi 2015). In 2016, the adult T. tridentatus population was estimated to be around 2,400 individuals at the Sone-Higata tidal flat according to the Fukuoka branch of the Horseshoe Crab Preservation Society of Japan (Nihon Kabutogani o Mamoru Kai) (Takahashi 2016).

In Japan, cooperation has been sought from fishermen to assist in horseshoe crab research by reporting or contributing their catches. Between 2002 and 2005, fishermen at three locations of the Kujukushima Islands in Nagasaki Prefecture reported their catches year-round. During this period, 81 adults were caught, with most being collected between May and October, which covered the pre-spawning, peak-spawning and post-spawning periods (Iwaoka and Okayama 2009, pp. 577–578. Table 1, p. 578).

Between 1999 and 2016, annual T. tridentatus surveys including records of offshore capture by fishermen and intertidal surveys at known spawning sites were conducted in Etajima and Takehara cities in Hiroshima Prefecture. At Takehara City, during intertidal surveys, a total of seven adults were recorded, including one amplexed spawning pair in 2015, two amplexed spawning pairs and a single adult female in 2016. The maximum number of juveniles observed at one time during these surveys was 46 individuals in April 2016. At Etajima City, 37 adults were recorded in the study period, with a maximum count of 15 individuals in 2008 whilst 40 juveniles were recorded, with a maximum count of 16 individuals in 2008 and spawning was observed once in 2000 (Ohtsuka et al. 2017, Laurie/Ohtsuka 2018, pers. comm, Laurie/Nishihara 2018, pers. comm.). Both populations are nearly extinct (Laurie/Ohtsuka 2018, pers. comm).

In parallel, a continuous monitoring survey of known T. tridentatus spawning sites conducted on the Tsuyazaki coast in Fukuoka Prefecture reported a decrease in spawning pairs from 139 pairs recorded in 2005, to 70 pairs in 2006, 45 pairs in 2007 and 40 pairs in 2008 (Wada et al. 2010), a 70% reduction in three years. The decreases are believed to be due to habitat degradation caused by coastal development in the area.

China
Nationally:  In the 1970s T. tridentatus was once widely distributed and abundant along the southeast coast of China, below the Yangtze River with high population densities (Sekiguchi 1988, Hong 2011). It could be found ‘throughout the seas’ and was regarded as an important economic species, but subsequently it suffered significant population declines (Liao and Ye 2000; China Species Red List 2009a,b; Li and Hu 2011; Hong 2011, p. 153; Weng et al. 2012b).

Fujian Province:  Xiamen was a major locality for T. tridentatus, where they could be seen all along the beaches at high tide in the spawning season, from May to September and in the 1970s, it was always easy to find them in the coastal waters around Xiamen (Hong 2011, p. 153). They were easy to catch, from the high tide line to water 5 m deep, where they could be found by following their bubbles. Artisanal fishermen could easily catch 50 to 60 pairs at a time, amounting to overall productivity of 5,000 pairs or more a year (Hong 2011, p. 160). However, interviews with fishermen, field surveys and other research undertaken between 2002 and 2004 revealed that T. tridentatus populations had decreased sharply after the 1970s and had become scarce (Hong 2011, pp. 159-161).

Pingtan in Fujian was once reputed to have the largest population of T. tridentatus on the coast of China, but the population has since experienced dramatic declines (Huang 2011). Huang et al. (2002) also reported that in the 1970s, the productivity of the horseshoe crab fishery in Pingtan was reduced by approximately 80 to 90% compared to the 1950s and by the 1990s there were not enough individuals to support the local fishery. Based on official records kept by the Pingtan Ocean and Aquatic Bureau, the estimated productivity of T. tridentatus in the Pingtan area was 15,000 pairs in 1984; 9,500 pairs in 1995; 3,700 pairs in 1998; and 1,000 pairs in 2002 (Huang et al. 2002), representing a reported fisheries capture decline of 93% between 1984 to 2002. This is on top of the 80 to 90% declines experienced between the 1950’s and 1990’s, representing a shifting baseline in estimates.

Another site in Fujian where T. tridentatus abounded up to the early 1980s was the seas around Dongshan Island (Mikkelsen 1988 p. 70), but surveys between 2006 and 2010 observed no spawning and only recorded the presence of juvenile populations (Weng et al. 2012b).

Field surveys and interviews with local fishermen in previously healthy horseshoe crab habitats in 2005 and 2006 in Fujian by Weng et al. (2007) revealed T. tridentatus was extirpated at Qianqi in Fuding County and Shandu Island in Ningde City, whilst spawning horseshoe crabs were no longer seen on beaches at Huangcuo in Xiamen City, Meizhou Island in Putian City or on Pingtan Island. In July 2015, surveys of the benthic macrofauna in the sandy intertidal zone of Shanqi and Tannan Bay in the Chinese Horseshoe Crab Reserve in Pingtan Island, Fujian failed to find any evidence for the presence of adult or juvenile T. tridentatus (Li et al. 2017).

Guangdong Province:  In the 1980s, it was estimated up to 1,500,000 T. tridentatus pairs were distributed in Guangdong waters, particularly around the Leizhou Peninsula (Hong 2011, p. 208) but by 1990, this was reduced to between 600,000 to 700,000 pairs (Hong 2011, p. 154). Since then, T. tridentatus are difficult to find around the Pearl River delta (Hong 2011, p. 153) and one of the justifications for establishing a conservation zone in the Shantou Tidal Zone Wetland included the identification of T. tridentatus as a ‘rare species’ (UNEP 2005).

Guangxi Zhuang Autonomous Region:  In Guangxi, prior to the 1980s, mass spawning of T. tridentatus was frequently encountered on the shores of Beibu Gulf (Weng et al. 2012b) and according to fishermen, before the 1990s, T. tridentatus were common in Guangxi, being widely distributed and abundant and seen everywhere, but after the 1990s they were rare to see (Hong 2011, p. 202).

Oceanic research reports estimated populations in excess of 600,000 to 700,000 T. tridentatus pairs in Beibu Gulf before the 1990s (Hong 2011, p. 154), but by the late 1990s these had dropped to 300,000 pairs or less (Liao and Li 2001; Hong 2011, p. 202). Historically, up to 1,000,000 pairs of adult T. tridentatus may have been exploited in Beibu Gulf to satisfy the demand for consumption and biomedical applications (Liao and Ye 2000, Li and Hu 2011) and according to Professor Guangyao Liang of the Guangxi Oceanic Research Institute, the output of T. tridentatus in Guangxi dropped by 90% between 1990 and 2000 largely because of human consumption (People’s Daily 2000).

Because adult T. tridentatus are so rarely encountered, surveys are now conducted on juveniles in Guangxi. In 2009, a survey of three intertidal mudflats in Guangxi (Hu et al. 2011) recorded 1,742 juveniles at Xi Bei Ling, with a distribution of 1.79 individuals/100 m², 2,753 juveniles at Jin Hai Wai, with a distribution of 3.19 individuals/100 m² and 441 juveniles at Xi Cheng, with a distribution of 0.88 individuals/100 m².

By 2014, it was estimated populations of T. tridentatus in Beibu Gulf had decreased by over 90% and about 10% of the remaining adults were being harvested annually. The local biomedical industry estimated that over 80% of the harvested T. tridentatus were being consumed as food and 20% were being bled to produce Tachypleus Amebocyte Lysate (TAL) (Fauna and Flora International 2014). Such a decline was supported by responses to a survey in September 2015 of 407 respondents from 29 communities along Beibu Gulf (Liao et al. 2017). Elder respondents recalled commonly seeing T. tridentatus spawning on the shores when they were younger, but almost all respondents had noted an overall reduction in T. tridentatus populations, with the average daily harvest of adult T. tridentatus declining sharply from an approximate range of around 50 to 1,000 individuals in the 1990s to 0 to 30 individuals by 2015. Nearly all respondents opined there had been an overall reduction in the abundance of T. tridentatus in the previous five years.

Hainan Province: Up to the early 1980s, T. tridentatus could be found in major concentrations on the west coast of Hainan (Mikkelsen 1988. p. 71; Sekiguchi 1988. p. 34) and before the 1990s, it was estimated up to 300,000 pairs of T. tridentatus were distributed in the seas to the north of Hainan (Hong 2011, p. 154).

From 2006 to 2010, interviews with fishermen and surveys were conducted at 27 nationally recognized T. tridentatus spawning sites on the coasts of Zhejiang, Fujian, Guangdong, Guangxi and Hainan (Weng et al. 2012b). Fishermen reported populations had decreased greatly in most of the coasts surveyed, no spawning was observed at the 27 sites, a few juveniles were found at only 6 of the 27 sites and no ‘new recruit’ Instar I to Instar III juveniles were found at Longkou in Dongshan Bay in Fujian. Such data indicate that T. tridentatus had almost disappeared from much of its range in China, with the exception of Dongshan Bay in Fujian, where over 1,000 juveniles were found and Beibu Bay in Guangxi. By 2004, it was nationally recognized that T. tridentatus populations had seriously declined due to over-exploitation (China Species Red List 2009b), such that it was considered to be on the brink of extinction in Chinese waters (Xie and Weng 2011) and in need of urgent protection (Weng et al. 2012b).

Taiwan
Tachypleus tridentatus was once widespread and abundant along the west coast of Taiwan Island, in the Penghu Islands and Kinmen with thriving populations, as attested to by evidence from historical records, literature, colloquial language, daily usage and place names (Chen et al. 2004, Chen and Chen 2011), but populations in Taiwan are declining (Hsieh and Chen 2015). No individual adults or mating adult pairs have been recorded from the intertidal flats of the Taiwan Island coast since the 1960s, nor more recently from Kinmen Island (Hsieh and Chen 2015). However, over the past decade, each year there have been occasional reports of adult horseshoe crabs being landed at fishing wharfs in Taiwan (maximum four or five individuals ever documented from each report), suggesting the adult population in the waters surrounding Taiwan is limited.
 
Because adult T. tridentatus are so uncommon, surveys have been conducted on juveniles in Taiwan. In 2003, surveys over several weeks on the Penghu Islands found only 20 juveniles (Hsieh and Chen 2015) and on Taiwan Island a small population of juveniles was documented at the Haomeiliao Nature Reserve on the Longgong River Estuary in Budai, Chiayi County between 2004 and 2005. However, none have been found there since 2007 (Yang et al. 2009b), whilst one juvenile was reported from Xiangshan intertidal flat at Hsinchu City in Taiwan Island in 2013 (Hsieh and Chen 2015).

Further reported by Hsieh and Chen (2015), a study of juvenile densities at Beishan, Nanshan, Hsiashu on Kinmen between 2003 and 2009 showed rapid and significant declines in the juvenile populations at Nanshan and Hsiashu. Juvenile abundance at Nanshan diminished from 0.18 individuals/m² in 2003 to 0.01 individuals/m² in 2009, whilst at Hsiashu, juvenile abundance declined from 1.02 individuals/m² on average in 2003 to 0.18 individuals/m² on average in 2009. In contrast, the juvenile population at Beishan changed little and fluctuated between 0.37 individuals/m² in 2003 and 0.03 individuals/m² in 2007. Beishan and Nanshan are located within the Kinmen Kuningtou Northwest Intertidal Terrain Horseshoe Crab Conservation Area (Hsieh and Chen 2009) and when data from Beishan and Nanshan are combined, the juvenile density decreased from 0.30 individuals/m² in 2003 to a range from 0.02 to 0.17 individuals/m² from 2004 to 2009. In 2006, the Kinmen Fisheries Research Institute purchased 57 adults from fishermen and 335 individuals in 2007, but no adults were offered by fishermen in 2008, whilst the Penghu Fisheries Research Institute purchased 209 adults from fishermen in 2008. A survey for adult T. tridentatus using drift gillnets over three nights around Penghu Islands in November 2008 only caught two sub-adult males, whilst a separate survey conducted for one day by Kinmen Fisheries Research Institute using bottom trawl nets in 2008  harvested 12 adults (Hsieh and Chen 2015).

Tachypleus tridentatus is now extirpated at many locations along the west coast of Taiwan Island and between 1998 and 2012 surveys found only scattered juvenile populations left (Yang et al. 2007, Chen 2011, Chang 2012). It is found at two locations on the Penghu Islands, with viable juvenile populations restricted to a number of sites on Kinmen Island, but these populations are declining, even in the Kinmen Kuningtou Northwest Intertidal Terrain Horseshoe Crab Conservation Area, a designated reserve (Hsieh and Chen 2015).

Hong Kong
Tachypleus tridentatus was once widespread and abundant throughout Hong Kong waters with thriving populations, as attested to by evidence from anecdotal accounts and place names (Shin et al. 2014). Major concentrations of T. tridentatus could be found in Hong Kong up to the early 1980s (Mikkelsen 1988, p. 71), but populations are declining (Huang et al. 1998, Chiu and Morton 1999, Chiu 2003, Shin et al. 2014). Fishermen and older residents recalled T. tridentatus being abundant in Hong Kong. In 1962 a retired police officer remembered seeing ‘hundreds’ of pairs of T. tridentatus on a beach at Lung Kwu Sheung Tan and from the 1960s to 1970s, fishermen in Ha Pak Nai would collect up to 200 T. tridentatus as bycatch in their fishing nets in a single night. There were so many on the mudflats on some days, people had to be careful where they walked, whilst in 1979 another police officer  recalled seeing ‘thousands’ of T. tridentatus covering the beaches on Chep Lap Kok, spread out in long chains as they were mating (Laurie/Various pers. comm).

The historic widespread occurrence of horseshoe crabs in Hong Kong may be indicated indirectly from place names (Shin et al. 2014) and a study of historical distributions of horseshoe crabs through interviews with local fishermen and elderly villagers revealed mating pairs of T. tridentatus were often seen in great numbers on many beaches in Hong Kong during the summer months until the 1980s and adults were evenly distributed throughout the waters of Hong Kong (Chiu and Morton 1999). However, by the early 1990s mating pairs were no longer seen on beaches where they had previously occurred and according to local fishermen in 1998 (Huang et al. 1998), the Pak Nai mudflat used to attract large numbers of spawning T. tridentatus in summer, but few in the last decade.

Tachypleus tridentatus has since disappeared from much of its range, whilst reclamation has claimed many historic spawning and juvenile nursery grounds and only lesser numbers of individuals can be found in Deep Bay as compared with anecdotal records (Chiu and Morton 1999, Chiu 2003). According to monthly reports related to a marine ecological survey by the Provisional Airport Authority in 1995 (Huang 1997), no horseshoe crabs were collected from the Sha Chau area, adjacent to Hong Kong's new airport at Chek Lap Kok, although the area was once said to be a thriving habitat for T. tridentatus.

Because adult horseshoe crab densities are very low in the seas around Hong Kong (Li 2008), the visual spawning survey of mating pairs on the shores or sea-bottom trawling of adults used for estimating populations of American Horseshoe Crabs (Ehlinger et al. 2003, Hata and Berkson 2003) are not practical. The last time a mating pair of T. tridentatus was scientifically observed in Hong Kong was at Lung Kwu Sheung Tan in 1986 (Huang et al. 1998, Chiu and Morton 1999). Surveys at wholesale fish markets, seafood restaurants, fish sellers and with local fishermen indicated 332 adult horseshoe crabs, mainly T. tridentatus were caught in Hong Kong waters from 2004 to 2005 (Shin et al. 2009), whilst in a series of studies to investigate the effectiveness of a trawling ban in Hong Kong on demersal fish and crustacean communities, only 13 sub-adults of T. tridentatus were recovered from systematic trawl surveys conducted at 12 sampling sites in three different areas of Hong Kong waters between 2012 and 2015 (Laurie/Tao 2018,  pers. comm). Where fishermen at Ha Pak Nai caught up to 200 T. tridentatus in a single night, they now catch two to three pairs annually (Laurie/Various pers. comm). As adult T. tridentatus are so rare in Hong Kong, population estimates and trends are by necessity based on juvenile surveys (Kwan et al. 2016).

The first set of quantitative data on juvenile T. tridentatus in Hong Kong was reported from a study along the northwestern coastline of the New Territories in 2002 (Morton and Lee 2003), taken as the baseline, followed by surveys in 2004 and 2005 (Shin et al. 2009), 2007 (Morton and Lee 2010) and 2012 to 2014 (Kwan et al. 2016). In 2002, surveys at four shores in Deep Bay recorded displayed mean juvenile densities from 0.10-1.97 individuals/100 m² (Morton and Lee 2010); in 2004, the same localities displayed mean juvenile densities from 0.08-0.23 individuals/100 m² (Shin et al. 2009); in 2007 from 0.00-0.86 individuals/100 m²  (Morton and Lee 2010); in 2012 from 0.00-1.17 individuals/100 m² (Kwan et al. 2016) and in 2014 from 0.1-1.97 individuals/100 m² (Kwan et al. 2016). Surveys at other localities found similar low mean juvenile densities and from surveys conducted between 2012 and 2014, no ‘new recruit’ Instar I to Instar III juveniles were found throughout Hong Kong (Kwan et al. 2016).

The presence of small and discrete T. tridentatus juvenile populations on only a few shores in Hong Kong, coupled with relatively few new recruits (Kwan et al. 2016), slow growth rate and length of time of between 13 to 14 years to reach maturity (Hu et al. 2015), suggest these populations are fragile and vulnerable to local extirpation (Kwan et al. 2016). In 2016, the population of juvenile T. tridentatus was estimated to be between 2,100 to 4,300 individuals on all horseshoe crab juvenile nursery grounds in Hong Kong (Kwan et al. 2016), about 60% of which reside on the intertidal mudflats at Pak Nai and Ha Pak Nai (Shin et al. 2014).

Viet Nam
Up to the early 1980s major concentrations of T. tridentatus could be found along the coast of Viet Nam, at least as far south as Nha-Trang (Mikkelsen 1988), but populations are decreasing. Before 1990 T. tridentatus was commonly found on sand banks in tidal areas and coastal estuaries along the coast of Viet Nam, most abundantly in the Central Coastal Province, but between 1990 and 2007 it is estimated the number of individuals declined by 50%, the area of occupancy in Viet Nam  declined by 50% and harvest yield declined by 20% (Nguyen 2007). Since then, habitat loss has continued unabated (Vietnam NBSAP 2015) and populations have been targeted to supply the TAL industry and consumer markets in China (Laurie/Novitsky pers. comm. 2014, Laurie/Do pers. comm. 2014). Tachypleus tridentatus, which was once historically abundant in the Gulf of Tonkin, is now considered endangered because of over-exploitation (Liu 2013).

Philippines
Historic information suggests T. tridentatus was once widely distributed throughout the Sulu Sea, including on both the west and east shores of Palawan, Danjugan Island on the east coast of Negros Occidental, Tangalan on the northwest coast of Panay, Tigbauan on the south coast of Panay, Cagayancillo in the centre of the Sulu Sea and Turtle Island and Tawitawi in the south of the Sulu Sea (Schoppe 2002). However, T. tridentatus experienced significant declines in population numbers throughout its range in the decades preceding 2000 (Schoppe 2002). Tachypleus tridentatus was also recorded from Zambales on the east coast of Luzon (Sekiguchi 1988 p 35), but surveys in 2011 failed to find any evidence of its presence there (Nishida 2012b).

In Palawan, surveys conducted between 1999 and 2001 indicated T. tridentatus was present in coastal municipalities extending the length of Palawan including Quezon, Aborlan and El Nido on the west coast and Brookes Point, Espanola, Narra, Puerto Princesa City, Roxas, Dumaran, Araceli, Taytay and the islands of Agutaya, Magsaysay and Cagayancillo (Schoppe 2002). Juvenile nursery beaches have only been studied at San Pedro, Puerto Princesa (Kaiser 2002) and Aventura Beach, Puerto Princesa on the east coast of Palawan (Almendral and Schoppe 2005). It has also been recorded from Busuanga Island in the northern end of the Palawan Island Chain (WWF 2010; Bautista et al. 2016, p. 84).

There is currently no information available on T. tridentatus population trends in the Philippines, although the juvenile populations that have been studied at nursery beaches are small. In an eight-month survey of San Pedro nursery beach between May and December 2001, 374 juveniles were recorded (Dorkas 2002), whilst in a five-month study from June to October 2002 at Aventura Beach, 125 juveniles were recorded (Almendral and Schoppe 2005).

Malaysia
Most horseshoe crab research in Malaysia has focused on Peninsula Malaysia on the two horseshoe crab species that occur there, T. gigas and Carcinoscorpius rotundicauda (John et al. 2012, Mohamad et al. 2015). Tachypleus tridentatus population studies have only recently commenced in Sabah (Robert et al. 2014, Manca et al. 2017, Mohamad et al. 2017) and are yet to be systematically undertaken in Sarawak (Laurie/John 2017, pers. comm). As such, the status of T. tridentatus in East Malaysia is still unknown, but anecdotal reports (Manca et al. 2017) and interviews with locals (Laurie/Robert 2014, pers. comm.) suggest the populations are decreasing.

Over the course of an adult survey conducted over four days during daytime of the highest spring tide between April and October 2014, 195 adult individuals as amplexed pairs (98 males; 97 females) were visually located along an 8,000 m stretch of a spawning beach  at Tanjung Limau, Sabah. In the same period, in waters 1 to 3 km off the coast of Inderasabah, Sabah, because no spawning beach could be located, 113 non-spawning adult individuals (80 males; 33 females) were caught using gill nets deployed at up to 10 m depth (Mohamad et al. 2016). In another 5-month study of T. tridentatus in Tawau conducted from October 2014 to September 2015 using gill nets deployed in waters up to 5 m deep approximately 1 to 3 km from the shore,  271 adults, representing 267 individuals (188 males; 88 females) with four recaptures were recovered (Manca et al. 2017). From the above two studies, male-biased Operational Sex Ratios (OSR) were recorded, with 2.42:1 at Inderasabah, Sabah (Mohamad et al. 2016) and 2.08:1 at Tawau, Sabah (Manca et al. 2017).

Brunei Darussalam
Horseshoe crab studies commenced in 2017 in Brunei Darussalam, so apart from confirming occurrence, there is currently no information available on T. tridentatus populations (Laurie/Marshall 2018, pers. comm.), although because of its small geographic size, it is unlikely significant populations are resident here.

Indonesia
According to the Biodiversity Action Plan for Indonesia in 1993, all three species of Asian horseshoe crab, T. tridentatus, T. gigas and Carcinoscorpius rotundicauda were considered either Rare (R), Vulnerable (V) or Endangered (E), but were listed as insufficiently known (K) because of lack of information (Indonesia NBSAP 1993, p. 86).

Basic research on horseshoe crabs in Indonesia was conducted from 1990 to 2000, then stopped (Mashar et al. 2017b), when the focus was on T. gigas and C. rotundicauda (Meilana et al. 2015). Recent research has focused on establishing baseline distribution data (Nishida 2012, Mashar et al. 2017a), including identifying population and stock status, locations of spawning grounds, nursery grounds, adult habitat range and rates of exploitation for all three horseshoe crab species, including T. tridentatus (Mashar et al. 2017b). In this respect, T. tridentatus has until recently been understudied in Indonesia and is therefore still listed as insufficiently known (K), but interviews with fishermen on the north coast of Java revealed horseshoe crab catches are declining (Meilana and Fang 2017).

All four extant species of horseshoe crab have different habitat requirements at different stages of their life cycle and their habitat requirements and life-history characteristics appear to be similar (Shuster 1982). Each of the species has specific and specialized habitat requirements related to spawning, juvenile development and adulthood and the close proximity of different habitats to meet the differing demands of their various stages of development is critical to their survival needs (Shuster and Sekiguchi 2009).

In T. tridentatus, beaches are needed for laying eggs along the high tide mark. There must be intertidal mudflats in close proximity, preferably with seagrass beds, so juveniles can develop and mature. It is also essential to have shallow water offshore in which large juveniles and sub-adults can feed and deeper water offshore for adults to forage after the spawning season is over (Itow 1993, Seino et al. 2003, Wada et al. 2016).

Horseshoe crab spawning and juvenile intertidal habitats are subject to spatial and temporal variations because of the complex interrelationship between hydrology, geomorphology, light, temperature, nutrients and other environmental factors, as well as chemical variations, caused by factors such as variable river discharge, tributaries with different input composition, precipitation and influx of materials at the sediment-water interface. At a given time, environmental conditions and hydrographic features together may influence certain biological processes including distribution, abundance, physiology and reproduction, as well as the phytoplankton which supports the benthic macro- and meio-fauna at the base of the food chain (Rathod 1992). Such factors guide site selection and site avoidance for all life-style stages.

Populations of T. tridentatus in the northern part of its range are found in coastal muddy bottom regions, whilst the southern populations live along sandy coasts and often in coral reefs, indicating adaption to different living environments through its range (Sekiguchi 1988, pp. 32, 53, 414).

For more details on spawning, larval development, growth, mortality, and biology and ecology, see the 'Habitat, Biology & Ecology' section in the Supplementary Information.


Generation Length

Generation length in T. tridentatus has been estimated based on the following:

Although there is variation in generation length among horseshoe crab species, all are relatively slow maturing, exhibiting particularly slow growth rates, and age-to-maturity for T. tridentatus is the longest of the four extant species.

Studies of Limulus polyphemus indicated it takes 9-10 years to reach maturity (Shuster and Sekiguchi 2003); the longest live retrieval in an adult tagging study in a published report was 10 years (Swan 2005), whilst in the United States Fish and Wildlife Services adult Horseshoe Crab Tagging Program 13 crabs were retrieved alive 15-17 years after they were tagged (Laurie/Newhard 2018, pers. comm.), suggesting at least similar times can be used for age-to-maturity and age-at-maturity estimates.

In T. tridentatus there is evidence to suggest different age-to-maturity rates at different latitudes, but in Japan and China, where data are available, estimates for age-to-maturity are between 13 to 14 years, the female taking longer than the male to reach maturity (Sekiguchi 1988, p. 194; Hu et al. 2015).

Based on the retrieval times recorded in tagging studies for L. polyphemus, this assessment adopts a conservative approach and uses similar times for age-to-maturity and age-at-maturity, giving a generation length of 20.25 years (maturity plus one half reproductive longevity) for T. tridentatus, resulting in a 3-generation length of 60.75 years (rounded to 60 years).

The following sub-sections summarize the country level assessment and list the potential threats of T. tridentatus throughout its range. As concluded by Hu et al. (2015) from a study of three juvenile nursery grounds in Beibu Gulf, China, once a local horseshoe crab population is extirpated, even under ideal conditions, it would take a long period of time for them to re-establish themselves.

Assessments of Country Threats

Country level assessment on T. tridentatus has been conducted only in Japan, China and Vietnam thus far and a synopsis is given below.

In Japan, T. tridentatus was assessed to be Critically Endangered (CR + EN) in the Red List and the Red Data Book of Japan in 2006 (Ministry of the Environment Japan 2006), mainly because of the loss or deterioration of its tidal flat habitats (Japan NBSAP 2014, p. 15) and marine and coastal ecosystem habitat and biodiversity loss in Japan has not been contained or halted. It is a continuing trend (Japan NBSAP 2014, p. 22).

In China, T. tridentatus was assessed in 2004 (year of assessment) to be Endangered (EN A2ac) in the China Species Red List because populations had seriously declined due to its over-exploitation as a source of raw material for chitin, the utilization of its blood in the medical and biomedical industries and the use of its meat for food. Annual yields in the 1970’s amounted to 200,000 pairs but by 2004, it had become difficult to obtain (China Species Red List 2009a, b).

In Viet Nam, T. tridentatus was assessed to be Vulnerable (VU A1c, Ba, b, c) in the Viet Nam Red Data Book in 2007 because it was estimated its population declined by 50% and its area of occupancy declined by 50% between 1990 and 2007 (Nguyen 2007).

Biomedical Harvest

Horseshoe crabs are harvested by the biomedical industry for the manufacture of Amebocyte Lysate and because of its size, T. tridentatus is the primary source of Amebocyte Lysate in Asia, where the practice is to drain all of the animal’s blood resulting in its death (Hong 2011, p. 154; Laurie/Novitsky 2014, pers. comm).

According to market analysts, demand for Amebocyte Lysate is high and globally, the Amebocyte Lysate market maintained an average annual growth rate of 7.25% from US$231 million in 2013 to US$285 million in 2016. It is predicted the market will further expand, so that by 2021, the Amebocyte Lysate market size will reach US$386 million, a 35% increase in market size in five years (BisReport Information Consulting 2017).

Having largely depleted Chinese waters of commercially viable catches, the TAL industry is targeting Viet Nam as a supply source (Laurie/Novitsky 2014, pers. comm., Laurie/Do 2014, pers. comm.) and is also looking farther afield for new sources of supply, such as Indonesia (Laurie/John 2017, pers. comm). Scientists in Malaysia are also exploring the opportunities of capitalizing on horseshoe crab stocks, including T. tridentatus in Sabah to produce TAL and other pharmaceutical products (Noraznawati 2015).

Horseshoe crab carapaces are also in demand in China and overseas as a source of chitin, which is satisfied as a by-product of the TAL industry (China Species Red List 2009a, b, Hong 2011, p. 154) as well as collection by children and adults to supply this demand (Hong 2011, p. 154). This is one of the factors resulting in horseshoe crabs being an ‘all parts use animal’ (Hong 2011, p. 154, Laurie/Novitsky 2014, pers. comm.). The demand for carapaces for chitin is considered to represent a significant threat to T. tridentatus in China (China Species Red List 2009b).

The absence any sort of harvest regulations is considered to be a significant cause of the decline in T. tridentatus population numbers in Asia (Gauvry 2015).


Consumption

The demand for T. tridentatus in China for consumption is high (Li et al. 2011; Hong 2011, p. 154; Weng et al. 2012; Laurie/Do 2014, pers. comm.) and moderate in Taiwan (Hsieh and Chen 2015), Hong Kong (Chiu and Morton 1999, Shin et al. 2009, 2014) and Viet Nam (Laurie/Do 2014, pers. comm).

Gravid females are the target of capture throughout its range by artisanal fisheries as a food source in the Philippines (Schoppe 2002), Indonesia (Meilana et al. 2015) and Sabah, Malaysia (Wood and Habibah 2014; Manca et al. 2015; Laurie/Mohamad 2017, pers. comm.), where female-biased harvesting may be contributing to male-biased OSR (Christianus and Saad 2007; Manca et al. 2015; Laurie/Mohamad 2017, pers. comm).


Trawling

Two complementary advances in the late 20^th century had considerable detrimental impact on T. tridentatus populations in China. Before the 1980s, fishing boats in China were largely reliant on man power, which limited the methods, capability and efficiency of fishing fleets. In the 1980’s, old fleets were gradually replaced with mechanized boats, accompanied by the introduction of trawls which could fish in water as deep as 20 m, opening up the seabed and therefore adult horseshoe crab habitat for exploitation (Hong 2011, p. 155). Capture on a commercial scale is still being promoted. According to the Department of Agriculture in Guangxi in 2011, there was an abundance of resources in the shallow seas around Guangxi, including dozens of thousands of tons of horseshoe crabs, with an annual output of 200,000 pairs (Department of Agriculture, Guangxi 2011).

Of all of the fishing methods, bottom trawling poses the highest risk to adult T. tridentatus populations because of its indiscriminate nature and the fact that T. tridentatus is an economically viable bycatch.


Artisanal Fisheries

In Palawan, Philippines artisanal fishing activities including push netting, gill netting and the construction of closed lines forming fish corrals in subtidal areas facing spawning beaches pose major threats to T. tridentatus populations (Schoppe 2002). Increasing fishing activities around Puerto Princesa pose a serious threat to remaining populations (Almendral and Schoppe 2005).

In Malaysia, horseshoe crabs, including T. tridentatus, are collected for consumption, both locally and for export, forming an important income component for ‘poor’ fishermen in some areas (Mohamad et al. 2015) and commencing in 2017, instances of Thai tourists visiting Inderesabah in Sabah to buy gravid female T. tridentatus, extract their eggs, then take them back to Thailand have been reported (Laurie/Mohamad 2018, pers. comm).

The Bajau Laut, nomadic sea-farers along the south east coast of Sabah, consume horseshoe crabs as a component of their diets (Wood and Habibah 2014). They are specialist marine hunter-gatherers who make a living from freediving (Cullen et al. 2007) and represent one of the most widely dispersed Indigenous groups in Southeast Asia. They comprise a total population of approximately 1.1 million, with around 200,000 living in the islands of eastern Indonesia, 347,000 in Sabah, Malaysia and 564,000 in the Philippines (Stacey et al. 2018).  Whilst such artisanal fisheries are low technology in nature, the impact of targeting gravid females can be high and have considerable negative impact, particularly when small or already depleted populations are involved. Their fishing activities may help explain the male biased OSR of 2.42:1 observed at Inderasabah (Mohamad et al. 2016) and OSR of 2.08:1 observed at Tawau (Manca et al. 2017).


Bycatch

Throughout Asia, horseshoe crabs are often considered as a nuisance by local fishermen and disposed of, because of their nuisance value in entangling nets, including in Japan (Iwaoka and Okayama 2009, p. 574; Nishida et al. 2015), China (Hong 2011, p. 155), Hong Kong (Laurie pers. obs.) and Malaysia, where they are killed as bycatch, or get trapped and die in abandoned ghost nets (Mohamad et al. 2015).


Smuggling

Tachypleus tridentatus eggs are being targeted for smuggling in Sabah, Malaysia (Laurie/Mohamad 2018, pers. comm.) and in Indonesia, where even though they are protected, there is a substantial illegal trade in horseshoe crabs, which are usually smuggled via ports on the northeast coast of Sumatra to Malaysia, for onward transmission to Thailand (Laurie/Meilana 2015, pers. comm.; Laurie/John 2017, pers. comm.). Whilst the principal focus of this trade is gravid T. gigas for consumption in Thailand, seizures of T. tridentatus are also made (BKIPM 2012).


Habitat Loss through Reclamation

Reclamation and related activities pose a considerable threat to T. tridentatus populations, through destruction or degradation of all lifestyle stage habitats.

In Japan, following World War II, extensive land reclamation was carried out as a national policy to increase agricultural output (Seino et al. 2003, p. 6). Tidal flats, which often exist in deeply indented bays tended to be developed. Hence, the area of tidal flats nationally was reduced by more than 40% between 1945 and 1995 due to landfilling and draining for land reclamation (Japan NBSAP 2014, p. 15) and following large scale reclamations, many small scale land reclamations were also carried out (Seino et al. 2003, p. 6). These activities were more prevalent in some areas than others. For example, over 80% of horseshoe crab habitats disappeared in the Seto Inland Sea from 1930 to 1994 (Shuster and Sekiguchi 2009), due to reclamation or revetment projects, which greatly diminished or totally destroyed spawning beaches there (Itow 1993, Tsuchiya 2009).

The effects of such land reclamations had a devastating effect on horseshoe crab populations. During the reclamation of the 106 ha Tomioka Bay in 1958, the remains of approximately 10,000 horseshoe crabs were found stranded (Nishii 1973; Seino et al. 2003, p. 8). When 1,800 ha of land were reclaimed in Kasaoka Bay, commencing in 1968, approximately 100,000 adult horseshoe crabs were estimated to be affected, and whilst efforts were made to transport the stranded adults to the sea, it was assessed that millions of juvenile horseshoe crabs perished (Seino et al. 2003, p. 8). In terms of long-term impact, a comparison of horseshoe crab spawning grounds on the Tenjin and Azuma coasts of Kasaoka showed a decrease from a maximum of around 400 breeding sites in the 1970s to no breeding sites after 1981 (Seino et al. 2003, pp. 5−6, Fig. 3).

In parallel, it is estimated seagrass beds in Japan were reduced by 40% in area from the 1970s to 2000s, but in addition to loss of habitat, rising sea temperature has been suggested as one of the causes for this decrease (Japan NBSAP 2014, p. 15). Threats to T. tridentatus habitat in Japan have not ceased. In 2017, the local government at Sone Tidal flat, home to possibly the largest remaining horseshoe crab population in Japan was considering proposals to reclaim the intertidal flat to build a wind turbine farm (Laurie/Iwasaki 2017, pers. comm).

China recognizes that land reclamation from wetlands undertaken from the 1950s to the 1990s has drastically shrunk wetland habitat and the area of land reclamation from tidal flats is continuing (China NBSAP 5 2014). From the same report, the area of land reclamation from the seas from 2008 to 2012 reached 650.6 km² and as a result of land reclamation from tidal flats, mangrove area in China decreased by about two-thirds (66%), causing direct damage to habitats and reproduction sites for some important protected species. Before the 1990s, many coastal areas in China were remote and isolated because they lacked paved roads, but with economic development, new road networks were put in place to open up the coastal areas, which in turn allowed fishermen to transport their catches, including T. tridentatus to new markets (Hong 2011, p. 155). Despite being illegal, there is now considerable inter-province trade in which T. tridentatus is being internally smuggled from Guangdong and Guangxi to meet consumer demand in Zhejiang and Fujian (Weng et al. 2012). In the past, T. tridentatus spawning beaches were also protected in many areas in Fujian because they were located in restricted military zones, but a relaxation of these restrictions has led to the opening up of these coastal areas to development (Hong 2011, p. 160).

In Taiwan, which comprises Taiwan Island, the Penghu Islands, Kinmen and the Matsu Islands, two-thirds of Taiwan Island is hilly or mountainous, bisecting the island along a centre line into east and west corridors. The east of the Taiwan Island is scarcely populated, whilst 95% of the population has settled along the west coast which comprises flat, plain terrain (Chang 2008), which was home to almost all T. tridentatus spawning grounds in Taiwan (Hsieh and Chen 2015). On Taiwan Island, habitat destruction resulting from landfill projects and the construction of breakwaters and fishing ports has led to the loss of 55% of the natural coastline, particularly of intertidal areas on the west coast (CPAMI 2007), so almost all T. tridentatus spawning grounds have been degraded or lost (Hsieh and Chen 2015). On Kinmen, horseshoe crabs were historically afforded some protection because they were located in restricted military zones (Hsieh and Chen 2009). However, as restrictions were relaxed, the major threat to T. tridentatus on Kinmen was habitat loss due to the construction of a commercial port on spawning and nursery grounds in 1997 through reclamation of intertidal flats and dredging of subtidal areas, destroying the ecological integrity of the whole bay (Hsieh and Chen 2015).

In Hong Kong, early maps showed uninterrupted coastlines, natural streams, pristine beaches and extensive and undisturbed mudflats, and the historic widespread occurrence of horseshoe crabs may be indicated indirectly from the names of some locations such as Hau Dei (Ground for Horseshoe Crabs) and Hau Hok Wan (Horseshoe Crab Carapace Bay), but Hau Dei was reclaimed in the 1960’s and is now part of a market town (Shin et al. 2014). No juvenile or adult horseshoe crabs have been observed at Hau Hok Wan in recent years (Kwan et al. 2016). Major reclamation projects since the 1970’s resulted in entire suites of T. tridentatus habitats disappearing (Shin et al. 2009), including the habitat at Nim Wan, where juvenile T. tridentatus were previously found, but which was developed as a landfill site in 1993.

Horseshoe crabs in Indonesia face a series of common threats, including land reclamation of spawning habitats and nursery grounds, houses built along spawning beaches, mangrove loss and degradation, and a high utilization rate of spawning areas and nursery grounds (Meilana et al. 2015), whilst in Palawan, Philippines all sandy shore lines in the vicinity of Puerto Princesa City have been subject to coastal construction (Schoppe 2002).


Impingement of Coastal Infrastructure

The slow growth of the juveniles and their long life span on the shores make them highly susceptible to direct loss of habitat through construction of coastal defences, such as seawalls and reclamation works and disturbances caused by urban developments in the vicinity of these spawning/nursery shores (Shin et al. 2014). Modifications of the hydrography of nearby waterways from coastal developments can also adversely affect the return of adult horseshoe crabs to the shores for spawning, thus resulting in less recruitment and decrease in juvenile populations (Sekiguchi and Shuster 2009).

In Japan, artificial coastlines rapidly increased in the 1960s and 1970s. The length of coastlines, where banks, revetments and other protection structures were developed, reached about 10,000 km, accounting for about 30% of the total length of the country’s coastline. Meanwhile, the length of natural coastlines which do not have artificial structures decreased to about 50% of the total length of coastlines by 1998 (Japan NBSAP 2014, p. 15). As reported in Itow (1993), around the Seto Inland Sea the construction of landfills and revetments has caused habitat deterioration through the loss of silt and mud from some tidal flats, turning them into pebble beaches, making it impossible for juvenile horseshoe crabs to survive in these areas.

In Taiwan, horseshoe crabs are also facing habitat deterioration due to construction of dikes close to their spawning grounds (Hsieh and Chen 2015). For example, the extirpation of T. tridentatus at Budai, one of the last spawning habitats and nursery grounds on the west coast of Taiwan Island was believed to have been caused by the blocking of the Longgong Estuary by an expansion of Budai Harbour in 2004 (Yang et al. 2009), construction of sea walls adjacent to the Haomeiliao Nature Reserve and excessive opening of fish ponds (BirdLife International 2001). Such alterations in land use contributed to changing the local hydrology so that the sandy substrate that comprised the juvenile nursery ground was replaced by the deposition of a muddy substrate instead (Laurie/Yang 2017, pers. comm).

Even small-scale infrastructure projects can have significant impacts, such as the construction of coastal footpaths and piers at Nim Shue Wan and Lai Chi Wo in Hong Kong (Laurie 2014, 2016 pers. obs). Such footpaths and piers cover high tide spawning locations or bisect intertidal flats and can cause profound effects to the local hydrology and sedimentology, resulting in the extirpation of once thriving juvenile populations.


Coastal Modification and Mariculture

In Viet Nam, marine and coastal waters support the livelihoods of approximately 20 million people and demand for fisheries-related products has placed increasing pressure on already over-exploited natural stocks. This has resulted in further aquaculture expansion which has caused a continuous decline in the quality of intertidal areas, mangroves and sea grass ecosystems and points towards future “coastal desertification” (Viet Nam NBSAP 2015). Mangrove degradation has been significant, leading to areas of natural mangrove almost completely disappearing through destruction or conversion to aquaculture to support shrimp farming, whilst mangrove afforestation has increased on tidal flats, to support shellfish aquaculture, with vast areas of intertidal estuaries dwindling in size due to the construction of clam ponds. In 1943, Vietnam had more than 408,500 ha of mangroves, by 1990, the area was about 255,000 ha, declining to 209,741 ha in 2006, 140,000 ha in 2010, so that by the end of 2012 only 131,520 ha of mangrove forests remained. Of the remainder, 56% are considered as “planted mangroves” and therefore of low ecological value (Vietnam NBSAP 2015). In parallel, sea-grass bed coverage across Vietnam has decreased from between 40 to 70% according to area, with a suite of three nationally recognized sites losing over 50% of their coverage between 2009 and 2014, some of which was due to water degradation as a result of mariculture development. The coastal sandy ecosystem, a typical ecosystem of Vietnam’s central coastal provinces, has also been remarkably changed, so that its ecosystem service functions, including preventing sand movement, protecting against coastal erosion and maintaining fresh water quality, are now severely diminished. From Vietnam NBSAP (2015), older statistics recorded the area of the coastal sandy ecosystem between Ha Tinh and Ninh Thuan as 85,100 ha, but since 1999 activities such as shrimp farming, sea sand mining and tourism infrastructure construction have destroyed thousands of hectares of the coastal sandy ecosystem in the central provinces.

In certain areas of its range, intertidal mudflats have been converted to oyster cultivation. In Taiwan on Kinmen Island (Chen 2009) and Hong Kong (Morton and Lee 2010), a traditional method of oyster farming involves bottom planting using rock plates, concrete stakes or large stones deployed in rows as protruding arrays on intertidal flats, which have created obstacles for adult T. tridentatus when they move toward high tide zones for spawning (Chen 2009). Furthermore, studies in Hong Kong have demonstrated where oyster cultivation occurs, juvenile T. tridentatus avoid the areas with cultivated oyster beds, including the significant areas of debris which have been created through the harvesting of oysters (Morton and Lee 2010, Kwan et al. 2017).


Sand and Gravel Extraction

The use of sea sand and gravel to support reclamation is a common practice in Asia. In the Seto Inland Sea, Japan the disappearance of sandbanks caused by sea gravel extraction may have led to a decrease in the populations of cornerstone species in the food chain (Japan NBSAP 2014, p. 15), and sea sand mining for supplying material for concrete is considered to be one of the factors contributing to the destruction of adult horseshoe crabs habitats there (Nishida et al. 2015) and of adult horseshoe crab habitat in Kasaoka in Okayama Prefecture (Seino 2011).

Sand or gravel extraction can not only have a biological impact, leading to a change in benthic communities, it can also change a stable area of seabed into an area of erosion and if undertaken where currents are not strong, this can lead to the preferential erosion and deposition of fine sediments from surrounding areas through a winnowing effect, to replace the sometimes coarser sediment that has been extracted (Desprez 2000). This is what appears to have happened in China, where sea sand extraction has been identified as a significant cause of the degradation of T. tridentatus spawning beaches along China’s tropical and sub-tropical southeast coast between its border with Vietnam and latitude 28°N, corresponding with the border of Fujian and Zhejiang provinces (UNDP 2000) which encompasses most of T. tridentatus range along the coast of China. It has also been identified as a primary cause of degradation in the Dongshan-Nan’ao migratory species corridor between Fujian and Guangdong (Ferguson and Wang 2009), it is implicated in the degradation of T. tridentatus habitats at Budai, Taiwan (BirdLife International 2001), in the degradation of adult, sub-adult and juvenile T. tridentatus habitats at Kinmen Island, Taiwan (Laurie/Yang 2017, pers. comm.) and of T. tridentatus spawning beaches and intertidal juvenile nursery grounds at Puerto Princesa in Palawan, Philippines (Schoppe 2002).


Habitat Disturbance

Throughout Asia, many T. tridentatus spawning and nursery grounds are either situated next to population centres or are easily accessible and popular locations for a variety of activities, including in China (Weng et al. 2012), Hong Kong (Shin et al. 2014, Kwan et al. 2016), Philippines (Schoppe 2002), Malaysia (Robert et al. 2014, Mohamad et al. 2015) and Indonesia (Meilana et al. 2015). From a home range study in analysis of area utilization, juvenile T. tridentatus were noted to exhibit limited residential and nomadic movement patterns, which might be associated with the distribution of food patches on the mudflat (Kwan et al. 2015b). These results indicate the restricted movements of juveniles and suggest their potential for high susceptibility to localized habitat disturbance.

In Hong Kong, as well as oyster farming and traditional fishing with erected nets across the shoreline, unregulated and uncontrolled recreational activities including clam digging, shellfish collection and sunset watching have caused considerable damage to nursery grounds in intertidal areas, including H. beccarii seagrass beds where juvenile horseshoe crabs forage (HKBSAP Report 2014, Kwan et al. 2016). Similar threats have been reported in Palawan, Philippines (Schoppe 2002), Sabah, Malaysia (Robert et al. 2014, Mohamad et al. 2015) and Indonesia (Meilana et al. 2015), whilst intense fishing activities in the Menggatal River, Malaysia for subsistence and commercial harvest of a myriad of species including bivalves and gastropods (Robert et al. 2014) may lead to a substantial decrease in these major dietary components of horseshoe crabs.


Water Quality and Pollution Events

In Japan, several studies have suggested that the susceptibility of eggs and embryos to pollutants is a contributing factor in the decline or extinction of local T. tridentatus populations (Botton 2001, p. 46). This is especially so along the coasts of the heavily industrialized Seto Inland Sea, where up to 42% of embryos from some locations were found to be malformed. Pollution in Japan appears to be having a greater impact on T. tridentatus than on L. polyphemus in the United States. The reasons for this are not clear, but T. tridentatus could be more sensitive to pollutants than L. polyphemus, or it could be the types and levels of contaminants in Japan are especially harmful (Botton and Itow 2009). The long incubation period of T. tridentatus of about 43 days from insemination to hatching (Sekiguchi 1988, p. 148) and the choice of spawning location along the high tide line (Sekiguchi 1988, p. 54) may lead to such high malformation incidences.

Apart from having a direct impact on eggs, pollution may also cause abnormalities during the process of egg formation in the mothers’ body, after polluting materials enter the mothers’ system from ingesting polluted silt or feeding on polluted prey, transferring environmental contaminants to the eggs before they are laid (Botton and Itow 2009). Another associated issue may be a decrease in prey animals due to the effects of pollution, so it is possible that healthy egg formation may be inhibited by lack of sufficient nourishment in the environment. Itow (1993) suggested the major source of pollution responsible for causing these abnormalities is believed to be from landfills, although the run-off of agricultural chemicals may also have had an impact.

Adverse impacts of poor water quality and changes in local hydrology on horseshoe crab habitats are also apparent. In 1928, Oe-Hama Beach, Kasaoka, Japan was designated as a ‘Horseshoe Crab Spawning Ground National Monument’, but despite this protection, the area was drained during the ‘Kasaoka Bay Land Reclamation Project’, a large-scale land reclamation which commenced in 1969 (Sekiguchi 1988, p. 40; Tsuchiya 2009, p. 559). After this, a sandy area along the Konoshima Channel was designated a protected breeding place for T. tridentatus, but in parallel the channel became the only passage for wastewater from Kasaoka City, resulting in rapid pollution and a corresponding decrease in horseshoe crab populations (Sekiguchi 1988, p. 41; Botton 2001, p. 45).

Also, before land reclamation, seawater exchange occurred in Kasaoka Bay due to tidal currents passing through the opening of the bay and the Konoshima Channel. However, this seawater exchange was cut off by land reclamation, resulting in the deterioration of water quality and Kasaoka Bay became uninhabitable for many marine organisms. The large-scale alteration of the bay shape has modified local hydrology and exerted a distinct effect on the wave field and tidal currents in the area, leading to changes in overall habitats of horseshoe crabs. As a result, sand was transported and deposited along the shores of Kasaoka Bay by littoral drift, causing the formation of some new spawning sites and the loss of others, especially where beaches became unsuitable because of a lack of wave action to stir and sort the surface sediments. The subsequent accumulation of fine materials remaining near the sand surface thus prevented sufficient exchange of groundwater around the eggs (Seino et al. 2003, p.12).

Another consequence of the narrowing of the Konoshima Channel was that the wake from passing ships was not only affecting the distribution of sand along the shorelines but at certain times also swamping the spawning locations along the high tide line (Seino et al. 2003, p. 6). The size and speed of ocean going vessels have increased in recent years, resulting in abnormally large waves repeatedly surging onto beaches, which has changed their nature, making them unsuitable for spawning. If ships are passing during a spawning event, the waves generated may also interfere with the egg laying act itself (Itow 1993).

In China, environmental pollution caused by anthropogenic activities (China NBSAP 5 2014) has been cited as one of the causes of T. tridentatus population declines (Xie and Weng 2011) and in Vietnam, pollution-related horseshoe crab die-offs have been observed (Laurie/Do 2014, pers. comm.). In Hong Kong, illegal pollution discharges into T. tridentatus spawning streams have been observed to be timed to coincide with mainly night time maximum high tides (Laurie 2009, 2011, 2013, pers. obs.), which may not only affect deposited eggs, but also coincides with the peak time when planktonic larvae choose to move from the spawning location to the intertidal area (Sakemi 1997; Botton 2001, p. 47).


Invasive Species

In recent years, T. tridentatus nursery grounds on Kinmen, Taiwan have been lost to the invasive smooth cordgrass Spartina alterniflora (Hsieh and Chen 2015) and the H. beccari seagrass beds and tidal flats at Ha Pak Nai/Pak Nai in Hong Kong are being encroached upon by the common cordgrass Spartina anglica (HKBSAP Report 2014).


Climate Change

Horseshoe crabs have proven resilience. They have survived five mass extinction events, numerous minor extinction events (Tanacredi 2001 p. 2, Rudkin and Young 2009 p. 41) and lived in geological periods which have experienced significant climate extremes (Zachos et al 2001, Moritz and Agudo 2013). However, T. tridentatus is now facing the threats of climate change with severely depleted populations throughout much of its range, which pose two obvious threats to its populations: loss of spawning habitat due to rising sea levels and rising temperatures (Arkema et al. 2013).

Rising sea levels
Sea-level rise will increase the risk of flooding of spawning habitats and, in turn, the rate at which these habitats disappear, including increasing the likelihood that spawning habit becomes compressed between the rising sea and existing housing and other infrastructure (Loveland and Botton 2015). This is a recognized risk in Asia, because many T. tridentatus spawning sites and nursery beaches are shared with resident human populations (Weng et al. 2012b, Kwan et al. 2016, Schoppe 2002). At the extreme, in Japan, spawning habitats such as Tatara Beach, a 16 m by 28 m (448 m²) patch located where a breakwater meets a sea-wall (Sekiguchi 1988, Fig. IV-10, p. 62) already requires periodic replenishment to maintain the sand (Botton 2001, p. 45) and like many spawning habitats in Japan is now conservation-reliant, depending on human support for its continued functioning.

Rising temperatures
Many aspects of T. tridendatus biology and ecology are temperature influenced. Spawning is seasonal and temperature triggered (Nishii 1975), so increasing water temperatures, particularly in its sub-tropical range, could extend the spawning period.

Eggs and trilobite larvae are the two lifestyle stages most at risk from elevated temperatures and whilst the Earth’s average annual temperature has increased at a steady pace in recent decades, global warming has increased the severity of the hottest month and day of the year (Diffenbaugh et al. 2017), with an alarming jump in the severity of the hottest days of the year during the same period. This results in short-term, extreme-heat events whose lethal effects are being caused by exposure to high ambient environmental temperatures (Papalexiou et al. 2018), and it is these events, more than gradual temperature rises, which are most likely to impact eggs and trilobite larvae.

Eggs are deposited along the high tide line, where the covering sand is periodically exposed to sub-aerial temperatures. Optimal water temperatures for incubating eggs range from 22– 31°C in Japan (Sekiguchi et al. 1988 p. 67) to 28-31.8°C in Taiwan (Chen et al. 2004). Hence, if eggs are subject to temperatures outside of these ranges, the risk of non-incubation or mortality will increase.

Trilobite larvae initially develop in their nest area, before moving to the inner shores of their natal beaches to develop (Kawahara 1982, Yeh 1999, Hu et al. 2011b). Both of these areas are periodically subjected to tide induced sub-aerial exposure, sometimes for extended periods of time, increasing the risk that these larvae will be subject to elevated levels of thermal stress. This may result in higher mortality as temperatures suddenly rise.

Juvenile foraging activity varies with latitude and climate, the emergence of juveniles and ecdysis being temperature related (Lee and Morton 2005, Morton and Lee 2010), so that rising temperatures could result in extended foraging seasons, resulting in juveniles taking a shorter time to reach sexual maturity, because they can moult more frequently at higher sediment or water temperatures (Lee and Morton 2005).

Adult horseshoe crabs have wide ranges of thermal tolerance (Mayer 1914), so the impact of rising sea temperatures on T. tridentatus is unlikely to pose a direct major threat to their survival and their exposure to elevated sub-aerial temperatures is likely to be limited, due to their spawning in the intertidal zone (Mohamad et al. 2016), or their spawning during nocturnal flood tides (Sekiguchi 1988, p. 58). However, indirect threats could result.

It is assessed the 490 dead adult T. tridentatus that were found on the shores of the Kitakyushu/Sonehigata tidal flat in Japan between January and August 2016 (Takahashi 2016) could be victims of rising sea temperatures. According to the Fukuoka Fisheries and Marine Technology Research Center's Buzen Sea Laboratory, the temperature of the Buzen Sea off of the tidal flats from May through August 2016 was between 0.9-1.6° C higher than in a typical year (Takahashi 2016). It is believed the horseshoe crabs had succumbed to predation by the Naru Eagle Ray, Aetobatus narutobiei (Kabutogani Jimankan Museum 2016), whose range could have been extended due to warming seas.

Apart from threatening T. tridentatus, rising temperatures also pose a threat to its supporting food chain, including seagrass beds as well as the intertidal meiofauna which comprises its trilobite larvae and juvenile prey. In Japan, some of the 40% reduction in area of seagrass beds from the 1970s to 2000s is believed to have been caused by rising sea temperature (Japan NBSAP 2014, p. 15).

Country impacts
Viet Nam recognizes it is likely to be one of the countries most impacted by climate change. Under current climate change scenarios, Viet Nm is predicted to house fragmented ecosystems that will undergo a high rate of loss of biological diversity resources compounding its vulnerability to climate change (Vietnam NBSAP 2015). It is because climate change combined with the degradation and depletion of forests of key watersheds can change the use of water resources, which may lead to more frequent floods, flash floods and landslides and cause heavy damage to the environment, all of which can adversely impact on coastal ecosystems. In addition, Vietnam’s main delta areas are likely to be severely impacted through sea level rise, leading to the loss of significant areas of coastal habitat (Vietnam NBSAP 2015).

Land subsidence
An issue linked to rising sea levels, which leads to flooding but not factored in to many assessments, is land subsidence. This is being recognized an emerging issue in Asia. On Taiwan Island, serious submergence of the ground layer of the southwest coastline and rising sea levels due to global weather changes have contributed to an increase in coastal infrastructure, inundation of sea water and shoreline erosion (CPAMI 2010). A total of 1,150 km² of the coastal region, representing 3.2% of the total land area, has been prone to seawater flooding due to land subsidence (Chang 2008), which has undoubtedly contributed to T. tridentatus habitat loss. This threat to T. tridentatus habitats exists elsewhere, but has yet to be quantified.

Biomedical and Pharmaceutical Industries

Tachypleus Amebocyte Lysate (TAL)
Horseshoe crabs are harvested by the biomedical industry for the manufacture of Amebocyte Lysate, which is used to test for gram-negative bacterial contamination in injectable drugs and implantable medical devices. The LAL test was commercialized in the 1970s and is currently the global standard for screening medical equipment for bacterial contamination (Levin et al. 2003). The studies of the blood-coagulating proteins of horseshoe crabs and their reaction with bacterial endotoxins aroused the interest of Japanese medical and pharmaceutical researchers and in October 1976 a symposium was held at Kasaoka, Japan entitled ‘Contribution of the Horseshoe Crabs to Medical Science’, followed two years later by a similar symposium held in the United States (Cohen 1979).

The global Amebocyte Lysate market is largely supplied from two sources, Limulus Amebocyte Lysate (LAL), derived from L. polyphemus, and Tachypleus Amebocyte Lysate (TAL), derived from T. tridentatus. Whilst blood from L. polyphemus for LAL production in the USA is obtained by collecting adult crabs and extracting a portion of their blood, then releasing them (Smith et al. 2016), the practice in Asia is to drain all of the animals blood resulting in its death (Hong 2011, p. 154; Laurie/Novitsky 2014, pers. comm).

There are two TAL producers in Japan: Seikagaku Corporation, which is the parent company of Associates of Cape Cod, USA and Wako Pure Chemical Industries Limited, which is the parent company of Wako Chemical, USA (Laurie/Novitsky pers. comm. 2014). Seikaguku Corporation commercialized a TAL reagent in 1978, but did not pursue a US licence until 1995 (Easter 2003, pp. 129-130). In China, there are eight manufacturers of TAL reagent (Cai et al. 2017). Four of them are Zhejiang Pukang Biotechnology in Zhejiang, Xiamen Bioendo Technology (Xiamen Chinese Horseshoe Crab Reagent Manufactory), Fuzhou Xinbei Biochemical (Fuzhou Xinbei Biological Industrial) in Fujian and Zhanjiang Bokang Marine Biological (Zhanjiang A&C Biological), a subsidiary of Charles River Laboratories of the USA in Guangdong.

The over-exploitation and unrestricted capture of T. tridentatus to supply blood to the lysate and biomedical industry is recognized as being a major contributor to significant population declines in China (China Species Red List 2009b, Gauvry 2015, Mizumura et al. 2017). When T. tridentatus is used in the TAL industry, all of the animal’s parts are used, because there is demand for its blood, meat and the chitin in its shell, making T. tridentatus an ‘all parts use animal’ (Hong 2011, p. 154; Laurie/Novitsky 2014, pers. comm).

In terms of recent annual usage, in the 2010’s, in Fujian, China a facility on Dongbi Island, Fuqing Bay, Fuzhou held and bled about 40,000 T. tridentatus each year which were sourced from Guangxi, Guandong, Hainan (Laurie/Novitsky 2014, pers. comm.) and Viet Nam (Laurie/Novitsky 2014, pers. comm.; Laurie/Do 2014, pers. comm.), where they were collected and penned in Beihei. Then when there was enough stock, they were transported to Dongbi by truck. In Guangxi, a lysate factory near Beihei bleeds about 60,000 crabs each year, whereas a facility in Dongxing, Fangchenggang supplies 20,000 to 30,000 pairs to Zhanjiang A&C Biological Ltd, 8,000 pairs to Xinbei and because of their higher price, 20,000 to 30,000 pairs are supplied directly to restaurants. In Hainan, several thousand pairs of crabs are supplied from a holding facility in Danzhou to Zhanjiang Bokang Marine Biological Co. Ltd. in Zhanjiang. All crabs held at these facilities are bled completely, then what remains are processed for sale as food and supply of carapaces for chitin (Laurie/Novitsky 2014, pers. comm). According to the National Institute for Food and Drug Control in Beijing, limuloid resources are exhausted in China (Pei et al. 2014).

In Hong Kong, between 1980 and 2001, a Japanese pharmaceutical company set up a clandestine, but legal horseshoe crab bleeding facility, to extract TAL from local populations of T. tridentatus. This facility was set up in a shark fin warehouse and horseshoe crabs were delivered on a daily basis from the local trawling fleet. The crabs were bled dry, then the carcasses were returned to the fishermen for disposal. In parallel with these activities, adult populations of T. tridentatus in Hong Kong collapsed, with conservative scientific estimates putting population declines at around 90% (Shin et al. 2014). At this time, the Hong Kong trawling fleet comprised approximately 1,200 trawlers, of which about 650 large trawlers mainly operated in Chinese waters, while the remaining 550 small-to-medium size trawlers operated partly or wholly in Hong Kong waters (LegCo 2010). It is therefore likely many of the horseshoe crabs captured to supply this bleeding operation were caught in Chinese waters, contributing to population collapses there.

In the Klong Yai district of Trat Province in Eastern Thailand a horseshoe crab processing factory operates, which in addition to other functions appears to be a T. tridentatus bleeding facility. The horseshoe crabs are sourced from Vietnam and once processed, products are sent abroad (Human Trafficking Organization 2007).

New research into TAL alternatives is being fueled by the realization the current industry is being supported through the unsustainable requirement for raw materials (Mizumura et al. 2017) Although a recombinant Factor C (rFC) based endotoxin test is now available, the industry has been slow to adopt it as an alternative to LAL or TAL (Li et al. 2015).

Chitin
Chitin and its derivative chitosan are of considerable interest to the biomedical, food, biotechnology and pharmaceutical fields because of their beneficial properties including biocompatibility, safety, non-toxicity, non-allergenicity and biodegradability, as well as antifungal, antibacterial, antitumor, immune-adjuvant, anti-thrombogenic, anti-cholesteremic and bio-adhesivity capabilities (Younes and Rinaudo 2015). Applications include use as cationic agents for polluted wastewater treatment, agricultural materials, food and feed additives, hypo-cholesterolemic agents, biomedical and pharmaceutical materials, wound-healing materials, blood anticoagulant, anti-thrombogenic and hemostatic materials, cosmetic ingredients, textile, paper, film and sponge sheet materials, chromatographic and immobilizing media, and analytical reagents (Hirano 1996). Horseshoe crab chitin has thus received much attention in particular because of its particular non-toxicity and biodegradability (Balasubramanian and Khan 2007).

Consumption and Use

In Japan, T. tridentatus is neither used as food nor bait (Botton 2001, p. 44) and historically, there was no target fishery for horseshoe crabs. However, those caught as bycatch were dried and used as compost (Tsuchiya 2009, pp. 560-561).
In the coastal provinces of China, T. tridentatus was historically a popular dish, being regarded as good and rare, and treated as a delicacy. Consumption was limited to the coastal provinces where it was found, due to the lack of transportation networks along the coast, so it was not traditionally consumed elsewhere. The opening up of coastal areas with the construction of roads has made it easier to transport T. tridentatus to new markets, so that considerable quantities are now transported between coastal provinces to meet demand. Furthermore, as it becomes rarer, it is being accorded the status of a culinary ‘treasure’, making it more expensive, and the rarer it becomes, the more sought after it will become, incentivising fishermen to target it as catch (Hong 2011, p. 154).

In Fujian and Guangxi, whilst some T. tridentatus are exploited for medicinal uses, the majority are harvested as food, where T. tridentatus is considered as a delicacy and highly nutritious food. In Guangxi, horseshoe crabs are sold in seafood restaurants, and some cook books and magazines teach people how to cook them and advertise the high nutritional value and good taste of their meat, particularly in soup prepared from them (Li et al. 2011). Surveys of markets and restaurants along the coastal provinces of China in 2006 to 2007 showed that despite being protected at the provincial level, T. tridentatus was widely available and to meet consumer demand in Zhejiang and Fujian, where horseshoe crabs were scarce, the crabs were smuggled across provinces from Guangdong and Guangxi (Weng et al. 2012).

In Taiwan, historically horseshoe crabs were consumed on Taiwan Island and Kinmen but as so few adults are now caught, the unpredictable catch can no longer meet the demand of restaurants (Hsieh and Chen 2015).

In Hong Kong, surveys of fish markets between 1995 and 1998 revealed adult T. tridentatus were sourced from local waters, the South China Sea and Indonesia (Chiu and Morton 1999). A further comprehensive market survey was conducted from 2004 to 2005 at wholesale fish markets, seafood restaurants, fish sellers and with local fishermen for information on the source, sale and utilization of adult horseshoe crabs. Of the 1,023 adult horseshoe crabs recorded, mainly comprising T. tridentatus, 691 individuals (68%) were caught in Chinese waters and 332 individuals (32%) were caught in Hong Kong, by shrimp trawlers and occasionally by netting and cage fishing methods. In terms of seasonal variations, more individuals were caught from September to December 2004 than in the remaining months in 2005. About one-third of the horseshoe crabs collected from trawl nets were released after capture, whilst the remaining two-thirds, about 690 individuals, were kept and sold to wholesale fish markets or seafood restaurants in Hong Kong. Of these, 425 (about 62%) were sold for use in Chinese traditional ‘set-free’ rituals, whilst 265 were retained by fish stalls and seafood restaurants, either for use as photographic props as a way to attract customers and for photograph taking by tourists, or they were served as food. ‘Set-free’ rituals are practices by Chinese Buddhist followers who believe if they release live animals back to the wild, they will have relinquished their sins of killing animals in their daily life (Shin et al. 2009).

There is currently no target fishery of horseshoe crabs in Hong Kong, although adults are sometimes caught entangled in fishing nets which are erected across the coastline of some shores, including across river mouths at high tides for harvesting fish, crabs or shrimp (Shin et al. 2014).

In Viet Nam, horseshoe crabs are not protected, but are recognized as a legally exploitable resource (Nguyen 2007), where they are collected for local consumption and for supplying demand in China, as well as for international trade (Laurie/Do 2014, pers. comm).

Targeting of gravid females

In artisanal fisheries, gravid females are targeted for their eggs in the Philippines (Schoppe 2002), Indonesia (Meilana et al. 2015) and Sabah in Malaysia, where female-biased harvesting to fulfil high demands from locals and consumers in neighbouring countries may contribute to male-biased OSR (Christianus and Saad 2007, Manca et al. 2015, Laurie/Mohamad 2017, pers. comm).
Talisman and other uses

In addition to being a source of food, T. tridentatus is used for decoration, for sale to tourists or for other uses such as talisman, spiritual or medicinal properties and dead horseshoe crabs are frequently found decorating local homes in the Philippines (Schoppe 2001), including as wall decorations to protect children from vampires (Kaiser 2002, p. 113). In certain areas of Sabah, Malaysia, locals hang T. tridentatus in their homes for protection from bad spirits (Manca et al. 2017), and when a new plantation is opened a horseshoe crab, which represents fertility, may be sacrificed to ensure a good harvest (Laurie/Mohamad 2017, pers. comm). Tachypleus tridentatus is also sold for use in traditional healing remedies in Indonesia (Meilana et al. 2015).
In Philippines, adult T. tridentatus are sold in the Puerto Princesa city market both as live animals and as decorative items (Schoppe 2001). There is also an international trade in dried T. tridentatus carapaces from Palawan, which are sold on eBay and other online trading platforms via seashell and marine curio suppliers, although the extent of this trade has not been quantified (Laurie 2009-2014, pers. obs).

Illegal Trade

Sabah, Malaysia has strict regulations relating to the collection, study and export of biodiversity effectively meaning no one can access or export biodiversity without licences issued by the Sabah Biodiversity Council (SaBC 2017). However, commencing in 2017, Thai tourists visiting Inderesabah started buying gravid female T. tridentatus, extracting their eggs, then taking them back to Thailand in almost certain violation of these regulations (Laurie/Mohamad 2018, pers. comm).
In Indonesia, horseshoe crabs were protected in Indonesia by Law No. 5 of 1990 of the Republic of Indonesia on the Conservation of Living Resources and their Ecosystems, under which it is illegal to take, keep, transport or trade in a protected species, including exporting or transporting internally within Indonesia (Indonesia Law No. 5/1990). This law only applied to T. gigas, but because of look-alike and interpretation issues, it was applied to all three horseshoe crab species in Indonesia (Laurie/Meilana 2015, pers. comm., Laurie/Wardiatno 2017a, pers. comm.). Despite this, and despite the best efforts of Indonesia law enforcement agencies, there is a substantial illegal trade in horseshoe crabs, which are usually smuggled via ports on the northeast coast of Sumatra to Malaysia, for onward transmission to Thailand. Whilst the principal focus of this trade is gravid T. gigas for consumption in Thailand, seizures of T. tridentatus are also made (Laurie/Meilana 2015, pers. comm.; Laurie/John 2017, pers. comm). Traders are also visiting remote fishing villages in Indonesia to recruit fishermen to capture horseshoe crabs for the international trade (Laurie/Meilana 2015, pers. comm). As an example, on 23 May 2012, five T. tridentatus were seized at Sam Ratulangi International Airport, Manado, North Sulawesi under PP No. 7/1999 (Indonesia Government Regulation No. 7/1999) and although described as T, gigas, photographs indicated the seized animals were T. tridentatus (BKIPM 2012).

In June 2018, Indonesia’s Ministry of Environment and Forestry upgraded PP No. 7/1999 (Indonesia Government Regulation No. 7/1999) and issued new regulations which made all three horseshoe crab species, including T. tridentatus protected species in Indonesia (Indonesia Government Regulation 20/2018).

Direct Species Protection

Tachypleus tridentatus is designated “Grade II Protected Animal of China” in the “List of State Key Protected Wildlife” and “Key Protected Aquatic Wildlife” in Zhejiang, Fujian, Guangdong and Guangxi (Huang et al. 2002). However, there is no effective management to protect the species due to a lack of scientific knowledge on its population status (Xie and Weng 2011). Despite the fact T. tridentatus is listed as a Key Protected Aquatic Wildlife in Guangxi, in a survey in September 2015 of coastal communities along Beibu Gulf in Guangxi, interviews revealed most people in the region, regardless of their age, consumed horseshoe crabs (Liao et al. 2017).

In Indonesia, horseshoe crabs were protected in Indonesia by Law No. 5 of 1990 of the Republic of Indonesia on the Conservation of Living Resources and their Ecosystems, under which it is illegal to take, keep, transport or trade in a protected species (Article 21.1. a), including exporting or transporting internally within Indonesia (Article 21.1. b) (Indonesia Law No. 5/1990), supplemented by Government Regulation No 7/1999 on Preserving Flora and Fauna Species (Indonesia Government Regulation No. 7/1999). Horseshoe crabs were also protected under Forestry Ministerial Decree No. 12/Kpts-II/1987 (Meilana and Fang 2017), which prohibits their exploitation or trade. Law No. 5 of 1990 Appendix II only identifies T. gigas as protected aquatic biota, although it appears to be applied on a look-alike basis to T. tridentatus using the Indonesian generic term for horseshoe crab ‘Ketam Tapak Kuda’ (Laurie/Wardianto 2017a, pers. comm.). In June 2018, Indonesia’s Ministry of Environment and Forestry upgraded Government Regulation No. 7/1999 and issued new regulations to protect all three horseshoe crab species, including T. tridentatus in Indonesia (Indonesia Government Regulation 20/2018).


Indirect Species Protection

Malaysia is keen to prevent bio-piracy, where biological resources are accessed and extracted without permission and developed for commercialization, so that bio-prospecting initiatives must be carried out with Prior Informed Consent (Malaysia NBSAP 5 2014). In particular, both Sarawak and Sabah have very strict regulations relating to the collection, study and export of biodiversity and each state has established a Biodiversity Council to oversee such approvals. In Sarawak no person shall collect or export biological resources for research and development or for propagation to support such research and development without permits issued by the Sarawak Biodiversity Council (SBA 2016), whilst in Sabah, no researcher or institution can access or export biodiversity without licences issued by the Sabah Biodiversity Council (SaBC 2017). In both states, these regulations are strictly enforced and apply to horseshoe crabs, and because of this, T. tridentatus is not seriously exploited for foreign export (Laurie/John 2017, pers. comm).

Similarly, in Brunei Darussalam, a permit from the Fisheries Department is required for the export of all water organisms, including horseshoe crabs (BDNSW. 2014).


Habitat Protection

In Japan in 1928, Oe-Hama Beach, Kasaoka was designated as a ‘Horseshoe Crab Spawning Ground National Monument’, but the inadequacies of this designation was demonstrated when this area was drained during the Kasaoka Bay Land Reclamation Project, a large scale land reclamation which commenced in 1969 (Sekiguchi 1988, p. 40, Tsuchiya 2009, p. 559). Areas of spawning site at Toyo region, Ehime Prefecture and Imari City are also protected as a natural monument by each local government, respectively (Nishida et al. 2015).

In China, there are two broad categories of Marine Protected Areas in China, No-take Marine Nature Reserves (No-take MNRs) and Multiple-use Marine Special Protected Areas (MSPA), but as a result of great user pressure and lack of enforcement capacity, the zoning schemes are often poorly recognized and implemented, so that even fully protected MNRs are implemented as multiple-use areas, so that certain levels of fishing and industrial activities are usually tolerated within them (Qiu et al. 2009). In Fujian, a 700 km² Ocean Special Protection Area, the Xiamen National Nature Reserve for Rare Marine Species, was designated in 2000 to protect a variety of coastal biodiversity including horseshoe crabs, with the objectives of preventing, minimizing and controlling negative human influences on horseshoe crab resources (Qiu et al. 2009; Hong 2011, p. 196). However, in the early 2000’s, an extensive sandy-mud beach planned for a local horseshoe crab reserve in Pingtan, Fujian was taken over by a businessman for a clam hatchery. Although this business failed, the reserve is gone, demonstrating the often difficult conflict that arises between conservation and economic development in China (Huang 2011), whilst in Guangdong, seven city, county or province level horseshoe crab nature reserves have been established since 2001 (Huang et al. 2002).

In Taiwan, T. tridentatus is protected on Matsu Island, part of an offshore archipelago in the northern end of the Taiwan Strait (Hsieh and Chen 2015) and the Kinmen Kuningtou Northwest Intertidal Terrain Horseshoe Crab Conservation Area was designated as a no-take zone in 1999 under the Fisheries Law of Taiwan (Taiwan Fisheries Agency 2016). However, there are no penalties for violations within the designated area for activities such as habitat destruction or pollution (Chen et al. 2004).

In Hong Kong, there are no designated areas for T. tridentatus conservation, but three spawning and nursery grounds, Tsim Bei Tsui, Pak Nai and San Tau and Lai Chi Wo, are listed as Sites of Special Scientific Interest (SSSIs) to protect other species, in a land-use planning system in which new development is not permitted in most circumstances because of their ecological significance (Lai 1998, Chiu and Morton 2003).

In Malaysia, Kota Kinabalu Wetland in Sabah was accorded Bird Sanctuary status in 1996 and declared a State Cultural Heritage Site in 1998 (Robert et al. 2014), whilst in Indonesia, there is no conservation area dedicated to the protection of horseshoe crabs (Meilana and Fang 2017).


Temporal Conservation Measures

Apart from special measures, China has also tried to introduce temporal measures to protect T. tridentatus. In 2000, the Pingtan government in Fujian issued orders to protect horseshoe crabs, including restricting any unit from catching, selling or dealing in horseshoe crabs and horseshoe crab related products, and anyone was restricted from catching horseshoe crabs between 1 May and 31 August (Hong 2011, p. 197). In 2000, fishermen in Lianzhou Bay, Beihai, Guangxi were asked to remove their nets and fish cages, which were blocking the passage of spawning horseshoe crabs, to open up passageways between the shore and the sea, to allow horseshoe crabs to reach their spawning grounds, then return to the sea (People’s Daily 2000).


Indirect Habitat Protection from Trawling

In Hong Kong, adult T. tridentatus were vulnerable to bottom trawling, but since this was banned in December 2012, this threat to adult horseshoe crabs no longer exists, which could result in a stabilization of adult populations (Shin et al. 2014). Inshore bottom trawling is also banned in Indonesia (CITES 2000) and in Taiwan, bottom trawling was once one of the most important coastal fisheries, but annual production started to decline due to overfishing and because of bycatch problems in the 1980s, inshore bottom trawling was banned from within three nautical miles of the shoreline in 1989 (CITES 2000, Shao et al. 2012).


Habitat-Based Conservation Actions

The Japanese Society for the Preservation of the Horseshoe crab (Kabutogani wo Mamoru kai) was established at 1978 for conservation of T. tridentatus, and has branch offices at some habitats/prefectures. Some local non-governmental organizations (NGOs) and groups, including the above society, are conducting conservation activities such as field surveys, beach clean-ups, social education and other activities. Release programs/events of juveniles and/or sub-adults which are reared from eggs are carried out at some habitats for recovery of population by local government/NGO/group, although there is an ongoing discussion about its effectiveness.

Imari Bay on northern Kyushu was considered by Sekiguchi (1988) to have the largest population of T. tridentatus in Japan, but the abundance of horseshoe crabs is limited by the scarcity of suitable spawning habitat. One of the few breeding areas is Tatara Beach, which is a mere 16 m by 28 m patch located where a breakwater meets a seawall (Sekiguchi 1988, Fig. IV-10, p. 62). This site requires periodic replenishment to maintain the sand (Botton 2001, p. 45), meaning that the horseshoe crab populations are conservation-reliant (dependent on human support for survival) at this location. Similar habitat-based conservation measures have been taken to combat the effects caused by the wake of passing ships on horseshoe crab spawning grounds along the Konoshima Channel, Kasaoka City, where a series of groins were built and beach nourishment programs were initiated to minimize the impact of land alteration in the area (Seino et al. 2003, p. 6, Table 2).


Restocking

In the past, thousands of Instars I and II have been released into the wild on Kinmen, Taiwan and in Xiamen, Quanzhou and Guangdong, China, but the efficacy of such release programs has not been evaluated and juvenile survival rates after release are not known (Hsieh and Chen 2015). Between 2005 and 2009, more than 40,000 juvenile T. tridentatus were released in the vicinity of Xiamen as part of a biological resources and rare species protection and recovery program (Oceanweek.Org 2009) and in 2010 the Guangdong Provincial Government jointly held a mass release at Yangjiang City of juvenile marine animals of economic, rare and endangered species, including juvenile horseshoe crabs (Farmers Daily 2010). Although scientifically supervised juvenile release programs have also been undertaken in Taiwan (Hsieh and Chen 2015) and Hong Kong (Shin et al. 2011), their efficacy is still being assessed.


Law Enforcement

Horseshoe crabs, including T. tridentatus, are protected in China and Indonesia. Despite the significant size of both countries, periodic seizures involving T. tridentatus are reported, in both China (Weng et al. 2013, SCMP 2017) and Indonesia (BKIPM 2012), although the effectiveness of enforcement effort has not been evaluated.


Harvest Management

The absence of harvest regulations is considered the main cause of the decline in T. tridentatus population numbers throughout much of its central range (Gauvry 2015), specifically in China (China Species Red List 2009b).