Hippopus hippopus has several common names such as the Horse’s Hoof Clam and Strawberry Clam. This species is a type species of the genus Hippopus.

Hippopus hippopus is distributed in the Central Indo-Pacific region. Despite extensive surveys, it is found in moderately low numbers or is locally extinct in numerous areas, indicating a decline in its area of occupancy (AOO), which is currently estimated at around 2,000 km² in spite of its large extent of occurrence). There is observed evidence of an estimated 40% population decline (based on disappearance from 10 of the 25 previously known regions where the species occurred) over three generation lengths (= ~45 years, since ca. 1979). Given that it is associated with coral reefs, there is evidence of declining quality of habitats in this period. This species continues to face fishing pressures where individuals are collected for domestic consumption. Notably, the ornamental trade of this species is mostly of cultured origins, which suggests success in mariculture. While the mariculture knowledge of this species is well-established and successful in raising individuals in culture, there is no compelling evidence that translocation has led to an increase in stock numbers. However, the outcomes of translocating cultured clams for conservation are not well understood or are underreported. Therefore, the population decline is projected to continue in the future as the causes of reduction may not have ceased. Presently, the global population genetics is not fully understood, and the potential presence of cryptic species suggests that additional precautions are necessary. Further work on the potential for mariculture to protect or enhance wild populations is important for future conservation. CITES provides important ongoing protection for this species.

On the basis of the observed population decline (up to 40% over three generation lengths), H. hippopus is assessed as VU A2acd.

This species is found in areas throughout the Central Indo-Pacific, except for the Red Sea and Western Indian Ocean (Neo et al. 2017). Previously, it was recorded in at least 25 distinct regions but more recently it has been reported as locally extinct or uncertain presence in 10 of these areas: Taiwan, Singapore, Fiji, Guam, Northern Mariana Islands, American Samoa, Samoa, Tonga, Myanmar, and Tuvalu (Neo et al. 2017).

Broadly, the paucity of contemporary information on H. hippopus could reflect the species' rarity on reefs today (as a result of local extinctions, for instance). Some of the more recent survey efforts for this species generally confirmed that it is rare throughout its geographic range (e.g., Dolorosa and Schoppe 2005, Dumas et al. 2011, Naguit et al. 2012, Purcell et al. 2020, Rehm et al. 2021). Studies generally conclude that it is found in low numbers or only in a few localities. Notably, the presence and distribution of this species among Indonesian populations is more well-studied (e.g., Triandiza et al. 2019, Tapilatu et al. 2021, Rabiyanti et al. 2023) compared to other regions (e.g., Rehm et al. 2021). As reviewed by Neo et al. (2017), the lowest density reported was 0.2 individuals per hectare at Tarawa Atoll, Republic of Kiribati (Munro 1988), while the highest density reported was 40.7 individuals per hectare at Helen Reef, Palau (Hirschberger 1980). Furthermore, 10 (i.e., 40%) of the 25 previously recorded regions known from the late 1980s now report uncertain presence or local extinction of this species (Neo et al. 2017), including four regions where the species was present at the time of the last Red List assessment for this species (Wells 1996): Kiribati, Myanmar, Singapore, and Tuvalu.

Generation length is calculated as: Age of first reproduction + [ z * (length of the reproductive period) ], with a minimum value of z=0.5 reflecting that mortality is highly skewed toward larvae in broadcast spawning species. A sclerochronology study reported individuals with age estimates up to 25 years (Shelley 1989). Male-phase maturity of H. hippopus is at 3 years (S Yusuf, pers. comms. 2024). This provides: 3 + (0.5 * 22) = 14 years (generation length for the male phase). Female-phase maturity is at 6 years (Alcazar et al. 1993), therefore: 6 + (0.5 * 19) = 15.5 year (generation length for the female phase). Mean value = 14.75 years, so the generation length is estimated as 15 years.

Given that regional distribution is an effective proxy measure for the population of a sessile species, the reported continuing decline represents potentially a 40% decrease in population over a period of three generations (45 years, since ca 1979).

Although the mariculture knowledge on this species is well-established and successful in raising cultured individuals, the outcomes of translocating these cultured clams for conservation are not well understood or underreported, so it is unclear whether these efforts have led to an increase in stock numbers.

This species often inhabits the shallow, nearshore patches of reef, sandy areas and seagrass beds that can be exposed during low tides (Poutiers 1998, Griffiths and Klumpp 1996). Occasionally it is found as deep as 10 m (S. Andréfouët pers. obs. 2018). Individuals have been reported to grow up to 40 cm (Poutiers 1998). Yet, an individual within a marine protected area of the northeastern lagoon in New Caledonia measured 47 cm (C. Fauvelot pers. obs. 2018) and another one at the Bolinao Marine Laboratory, Philippines, reached 53.5 cm (Knop 1996). Byssal attachment is present in young individuals, but older ones mostly lie free-living (or unattached) on the substratum (Rosewater 1965).

All species of giant clam are known to be simultaneous hermaphrodites. For H. hippopus, the male and female acini lie side by side throughout the entire extent of the sexual gonads. Previous studies confirmed that this species has an annual reproductive periodicity (Stephenson 1934, Shelley and Southgate 1988). Stephenson (1934) studied over 300 individuals and found that this species was immature during the Australian winter (June and July), gametes ripened during the months of August to October, and gametes matured from November to January. In another study by Shelley and Southgate (1988), the spawning was predicted to occur between December and March. While the mariculture of this species is known, there is very little published information on its reproductive potential and developmental periods (Jameson 1976). According to Mingoa-Licuanan and Gomez (2007), H. hippopus reaches reproductive maturity at 24 cm (i.e., both male and female gametes), but no age was provided. Under mariculture conditions, this species was reported to reach male-phase maturity at 3 years (Syafyudin Yusuf pers. comm. 2024). Alcazar et al. (1993) reported the life cycle of this species by using two six-year-old F1 individuals (female SL = 15.3 cm and male SL = 13.6 cm), yielding a successful F2 generation of juveniles.

The presence of this species can produce beneficial outcomes for coral reef ecosystems. For instance, an experimental study found that introduced H. hippopus can provide additional shelter via their shell surfaces to surrounding organisms (de Guzman et al. 2023). Furthermore, this species is a known host of cyclopoid copepod species (Anthessius amicalis, Anthessius discipedatus, Lichomolgus hippopi and Lichomolgus tridacnae), pea crabs (Xanthasia murigera), and pontoniinid shrimps (Anchistus australis, Anchistus gravieri and Anchistus miersi) (Neo et al. 2015).

Overexploitation of the species, for both consumption and shell trade, has been reported as a major cause of previous population declines (Hviding 1993, Poutiers 1998, Thaman et al. 2010, Purcell et al. 2020, Rehm et al. 2021). As the sub-adults/adults of this species are free-living (i.e., not attached to the substratum) they are easier to collect than other clam species, making them particularly vulnerable to over-extraction (Hviding 1993). Large scale harvesting is now limited by previous population declines and protection from CITES. The extent of fishing can vary depending on the local coastal communities. For instance, H. hippopus (with other large clam species) is opportunistically taken during fishing trips targeting other marine resources such as fish and lobsters (Purcell et al. 2020). On the other hand, in the Republic of Kiribati, all giant clams (including H. hippopus) are heavily exploited for subsistence purposes (Eurich et al. 2023).

Climate change could threaten this species. In Fiji, this species became locally extinct but not in other Pacific Island groups (Seeto et al. 2012). Through examining the shell middens, Seeto et al. (2012) found that the sea level drop of 55 cm from 1100 to 550 BC in the Fiji islands could have significantly contributed to local extinction by changing the coral-reef ecology to the disadvantage of this sensitive species. Changes in the sea surface temperatures (SST), either too low or too high, could also affect the survival of this species. Another study found a strong positive correlation between SST and shell growth in this species, where short-lived drops in sea-surface temperature led to significant and sustained reductions in shell growth (Aubert et al. 2009). It was thought that the cooling that accompanied the sea level fall around the Rove Peninsula during the period of its Lapita settlement contributed to the extirpation of this species at this locality and perhaps elsewhere in the Fiji archipelago (Seeto et al. 2012). Experimental research has shown that this species is more sensitive than other tridacnines when seawater temperatures become elevated (Blidberg et al. 2000), which causes bleaching in individuals. In a separate observation from mariculture of H. hippopus, the PT. Dinar Darum Lestari aquaculture facility, Bali (formerly the CV. Dinar facility) had cultured hundreds of H. hippopus in 2005 for the Japanese food markets (J.W. Fatherree pers. comms. 2024). As a follow-up in 2023, the facility lost most of their H. hippopus stock to severe bleaching events where individuals appeared more vulnerable to bleaching compared to other giant clam species (J.W. Fatherree pers. comms. 2024).

Other threats include the habitat destruction that contributed to localised declines; for example, development along the Singapore coastline is thought to have contributed to the presumed national extinction of this species, which has not been seen in the area since 1966 despite detailed studies (Neo and Todd 2013). Pollution from development and urban centres also increases turbidity in coastal areas, which may hamper the clams’ ability to feed and reproduce and therefore affect their survival; nevertheless, experiments have shown that turbidity does not often cause outright mortality (Neo and Todd 2012). In another study, tropical cyclones have been mentioned as a cause of giant clam species decline in the Solomon Islands (Thaman et al. 2010). Invasive diseases and predators may have also contributed to past and current declines. For example, Seeto et al. (2012) suggested that predators and diseases introduced by human groups in the process of colonisation could have affected population numbers. Norton et al. (1993) reported that an infection from a Rickettsiales-like organism could cause death when the cultivated clams (like H. hippopus) were overcrowded and in slow-moving water.

In the past, H. hippopus was a popular species for local harvesting and consumption (Juinio et al. 1987, Hviding 1993). Traditionally it is favoured as a delicacy, considered a ‘high-status food’ for use on special occasions or as a reserve food when times are difficult (Firdausy and Tisdell 1992, Hviding 1993, Rehm et al. 2021). For Pacific Island communities, the role of giant clam meat in providing high-quality protein was significant, as analysis showed that raw Tridacninae meat (H. hippopus and Tridacna crocea) contains high content of vitamin A, which is generally absent from the meat of other molluscs and some fish species (Hviding 1993). In contemporary times, this species (lumped together with other giant clams) continues to be harvested for subsistence purposes to varying extents (i.e., opportunistic to intensive harvesting) across its range (Purcell et al. 2020, Eurich et al. 2023).

In the 1990s, this species’ shells were used in various forms in the ornamental shell trade. In the Philippines, they were most widely used either as specimens, ornamental shells or materials for various shell crafts (e.g., ashtrays, lamps, vases, choker beads) (Juinio et al. 1987). The most marketable size range is 30–40 cm (Juinio et al. 1987). In Palau, the then Micronesian Mariculture Demonstration Center (MMDC, now known as PMDC) sold thousands of giant clam shells as ornaments in the early 1990s, primarily of H. hippopus and T. derasa (Heslinga 1996); they were extremely popular and three sizes were sold: 5-cm shells for USD$3, 10-cm shells for USD$5, and 12–14-cm shells for USD$8. Hippopus hippopus shells make attractive wasabi dishes and proved popular with Japanese tourists. In contemporary times, this species’ shells are heavily used in the handicraft carving industry located in Hainan Island, China (Larson 2016).

Another specific use/trade of the shells is the floor-tile industry. Together with those of other giant clam species, these shells were previously collected for the highly popular floor-tile industry in Central Java, Jakarta, and East Java (Firdausy and Tisdell 1992). Approximately 10–20 tonnes of processed clam shells per fortnight were sought for the floor-tile industry in Jakarta. Hippopus hippopus was listed as one of the species sold in the market.

Cultivated H. hippopus are marketed in the live aquarium trade (Mies et al. 2017, Vogel and Hoeksema 2024) and possibly for restocking (Militz and Southgate 2021). Elsewhere, such as in Palau, cultivated individuals are grown and sold for local consumption (M.L. Neo pers. obs. 2023).

All giant clams (subfamily Tridacninae) are listed in Appendix II of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) on the basis of so-called 'look-alike species', i.e., species whose specimens in trade look like those of species listed for conservation reasons (Wells 1997). Thus, CITES regulates the international trade in any of their parts (shells, tissues, alive or dead). This species suffered extreme declines prior to CITES listing in 1985, but the population has not declined as severely since that time.

In situ protection of stocks: This species has legal protection under the respective wildlife and fisheries laws in the following countries: Australia, China, Taiwan, India, Indonesia, Malaysia, Myanmar, Philippines, Singapore, New Caledonia, Solomon Islands, and Pitcairn Islands. In contrast, the South Pacific nations (such as Papua New Guinea, Federated States of Micronesia, Palau, Marshall Islands, Samoa, Tokelau, Tonga, and Vanuatu) protect their wild stocks by introducing restrictions on harvesting wild giant clams (such as using size, weight or bag limits, gear restrictions or permits) or introducing restrictions on individual uses, including recreational, tourism, and aquaculture. The levels of enforcement of laws, however, are unclear and underreported.

Stock enhancement through mariculture: During the 1980s, the efforts to re-establish or supplement depleted populations of giant clams (including H. hippopus) were mainly funded by the Australian Centre for International Agricultural Research (ACIAR) (Davila et al. 2017). The focus of the programme was to breed and release hatchery-reared giant clams back to the wild, at local and regional scales. Hippopus hippopus has been cultured in Palau, Australia (Orpheus Island Research Station, north Queensland), Malaysia, and the Philippines for purposes of translocation to other areas (e.g. from Palau to American Samoa, Yap, the Cook Islands, Samoa and Tonga) or restocking (Neo et al. 2017). Specifically, the areas that restocked their reefs with this species included the Marshall Islands, New Caledonia, Palau, Papua New Guinea, Solomon Islands, Vanuatu, Malaysia, and the Philippines (Teitelbaum and Friedman 2008, Kinch and Teitelbaum 2009, Neo et al. 2017). Areas that had translocated H. hippopus to their reefs included the Cook Islands, Fiji, Federated States of Micronesia, Northern Mariana Islands, Samoa, and Tonga (Teitelbaum and Friedman 2008, Kinch and Teitelbaum 2009). Unfortunately, there is little information regarding the restocking outcomes in these areas (i.e., no monitoring).

Wildlife trade: Most of the H. hippopus marketed in the trade are cultured, while the wild-sourced clams are traded in very low quantities (Vogel and Hoeksema 2024), and local capture for food does not involve international trade. Based on the CITES Trade Database, Micronesia, Palau, Indonesia, and the Marshall Islands were major exporting countries for cultured H. hippopus between 2011 and 2019 (Vogel and Hoeksema 2024). Notably, the trade numbers for this species were higher in recent years (2011–2019) compared to 2001–2010, suggesting demand for the species. Between 2001 and 2019, the import-export of live H. hippopus was greater than shells (Vogel and Hoeksema 2024). An example of a mariculture programme of H. hippopus is the PT. Dinar Darum Lestari aquaculture facility, Bali (formerly the CV. Dinar facility), the hundreds of cultured H. hippopus in 2005 were mostly destined for the Japanese food markets (J.W. Fatherree pers. comms. 2024). As a follow-up in 2023, the facility lost most of their H. hippopus stock to severe bleaching events where individuals appeared more vulnerable to bleaching compared to other giant clam species (J.W. Fatherree pers. comms. 2024). In general, it appears that several cultivation programmes are focused on growing giant clams for the global marine aquarium trade (Mies et al. 2017).

Presently, the global population genetics is not fully understood, and the potential presence of cryptic species suggests that additional precautions are necessary. With the recent availability of the H. hippopus mitochondrial genome (Ma et al. 2019), new avenues for studying its population genetics have opened up, emphasising the importance of conserving genetically diverse populations.

This species has been assessed as a proposed threatened species in a status review for the US Endangered Species Act (NOAA 2024).