Tridacna gigas, the largest species among the giant clams, is primarily distributed in the Central Indo-Pacific region. Despite extensive surveys, it is currently found in extremely low numbers (i.e., presence limited to one individual in the Pacific Islands) or is locally extinct in many regions where it has been previously reported, while other remaining populations have also declined; this represents an estimated total population decline of approximately 84% over three generation lengths (= ca 100 years, since 1924). Given that it is associated with coral reefs, there is evidence of declining quality of habitats in this period. Moreover, its large size and high value for meat and ornamental purposes make it particularly susceptible to overfishing. It is also vulnerable to the impacts of ocean warming that affect its symbiotic relationship with photosynthetic algae and its overall habitat stability. Poor water quality and habitat destruction further threaten its survival.

This species continues to face fishing pressures where individuals are collected for domestic consumption or shell trade. Notably, the ornamental trade of T. gigas is mostly of cultured origins, suggesting success in mariculture. While the mariculture knowledge on 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 these cultured clams for conservation are not well understood or underreported. Further work on the potential for mariculture to protect or enhance wild populations is needed for future conservation. The population decline is projected to continue in the future as the causes of reduction may not have ceased.

The information gathered in this assessment indicates that the status of the species has continued to decline since the previous assessment. CITES provides important ongoing protection for this species. Giant clams are vulnerable to climate change, as rising temperatures can cause bleaching of their photosymbionts. Ongoing monitoring is important to provide early warning of future declines.

On the basis of the observed population decline (>80% over three generation lengths), ongoing exploitation, and decline in area of occupancy (AOO) over the last 100 years, Tridacna gigas is assessed as Critically Endangered (CR A2acd).

Tridacna gigas has been found throughout the Indo-Pacific, from Myanmar (Burma) to the Republic of Kiribati (but not the Cook Islands) and the Ryukyus (southern Japan) to Queensland (Australia) (Neo et al. 2017). There are at least 31 reported regions with natural wild populations of this species, but it is severely depleted, locally extinct, or lacking data in the vast majority of regions (see Neo et al. 2017).

Based on anecdotal accounts, the previous range of this species possibly extended to southeast Africa (Kenya: Accordi et al. 2010), Madagascar (Hopkins 2009), and Mauritius (Michel et al. 1985), but this information could not be verified.

New information has better resolved the full range of this species. Notably some extant populations were previously considered possibly extinct—in Vanuatu, Fiji, and Micronesia—at the time of the last Red List assessment (Wells 1996); however, far more regional populations have declined and are possibly extinct or locally extinct—in Tuvalu, Thailand, Taiwan, Singapore, Papua New Guinea, New Caledonia, and Kiribati. These apparent fluctuations highlight a long pattern of ongoing decline for the species and reduction in population and range.

Broadly, the paucity of contemporary information on Tridacna gigas could reflect the species' rarity on reefs today and the local extinctions. Recent survey efforts on this species generally concurred that it is rare throughout its geographic range (Yusuf et al. 2009, Gonzales et al. 2014, Harahap et al. 2018, Purcell et al. 2020, Rehm et al. 2021, Dolorosa et al. 2024), being found in low numbers (i.e., fewer than 10 individuals per subpopulation) or only in a few patches. Moreover, extensive surveys in the Pacific Islands indicated that the presence of this species is sometimes limited to one individual (C.C.C. Wabnitz pers. obs. 2016). An exception is the Great Barrier Reef in Australia, which is the most extensive area within the natural distribution of T. gigas that still supports relatively undisturbed and stable populations (Braley 1984, 1986, 2023) and exhibits evidence of natural recruitment (Braley and Muir 1995, Braley 2023). The largest known subpopulation comprises 277 individuals, including 208 mature ones (Braley 2023, data from 2017). Threats to the Australian population could result in a much higher extinction risk of the species. 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 at 431.9 individuals per hectare at Michaelmas Reef, Australia (Pearson and Munro 1991). Furthermore, the uncertain presence or local extinction of this species has been reported from 26 of the 31 localities (Neo et al. 2017), strongly suggesting that the global population has declined significantly over the decades.

The age of Tridacna gigas has been measured at 76 years (Mecha and Dolorosa 2020), or estimated as at least 60 years (Watanabe et al. 2004). According to Mingoa-Licuanan and Gomez (2007), this species exhibits male maturity at 42 cm and females at 53 cm, but no age was provided. 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. Under mariculture conditions, male-phase maturity is at 3.5 years (i.e., a mean size of 18.7 cm, Heslinga and Perron, 1983, Heslinga et al. 1984) or at 5 years (Syafyudin Yusuf pers. comm. 2024); the mean value of 4 years is considered. This provides: Age of first reproduction + [ z * (length of the reproductive period) ] 4 + (0.5 * 56) = 32 years (generation length for the male phase). Female-phase maturity is at around 10 years (H.A. Roa-Quiaoit pers. comm. 2024), so: 10 + (0.5 * 50) = 35 years (generation length for the female phase). Mean value = 33.5 years. This assessment considers 100 years as representing three generations for this species.

Recent research on the population genetics of this species is sparse, but studies from the 1990s indicate discrepancies between current ocean currents and the observed genetic differences among populations in the Central and West Pacific groups, the Great Barrier Reef, and the Solomon Islands (Benzie and Williams 1992, 1995). This discrepancy suggests that despite the large size of T. gigas, its dispersal capabilities might be more limited than those of its relatives, potentially increasing its risk of local extinction due to reduced genetic exchange and diversity.

While the knowledge on the mariculture of this species is well-established and the raising of cultured individuals has been successful, the outcomes of translocating these cultured clams for conservation are not well understood or under reported (i.e., it is unclear whether efforts have led to an increase in stock numbers). The exception is restocking efforts in the Philippines that have yielded success in recruitment from their cultured T. gigas clams (Gomez and Mingoa-Licuanan 2006, Cabaitan and Conaco 2017, Requilme et al. 2021).

This species is usually free-living on either sand or hard reef substrata and typically lives in coral reefs with good light penetration. All species of giant clams are known to be simultaneous hermaphrodites. Tridacna gigas has one of the highest fecundity rates, producing up to 500 million eggs in a single spawning event. The reproductive periodicity of the Great Barrier Reef population of this species showed diel periodicity to spawning, generally coinciding with incoming tides near the second (full), third, and fourth (new) quarters of the moon phase, occurring over the austral summer (Braley 1984). The mariculture of T. gigas is well known and published (Crawford et al. 1986; Braley et al. 1988), and the reproductive potential is known to some extent. A piece of indirect evidence for the natural reproductive potential of this species is the successful recruitment of juveniles in the Philippines arising from restocked cultured individuals transplanted in 1985, which took approximately 30 years (Cabaitan and Conaco 2017).

Tridacna gigas can produce beneficial outcomes for coral reef ecosystems. For instance, a population of cultured T. gigas can produce substantial biomass (e.g., 29 tonnes ha^-1 yr^-1 of wet tissue biomass, estimated 1,682 kg dry weight ha^-1 yr^-1) (Barker et al. 1988). Giant clams filter large quantities of seawater; even a sparse population of mature T. gigas (0.04 clams m²) on the Great Barrier Reef is capable of filtering over 28,000 L ha^-1 h^-1 (Neo et al. 2015). An experimental study found that T. gigas introduced onto degraded reefs significantly improved fish diversity and abundance compared to control plots with no clams (Cabaitan et al. 2008) and their shells provided additional shelter to surrounding organisms (Cabaitan et al. 2018, de Guzman et al. 2023). An opportunistic observation also discovered that the T. gigas gametes were a source of food for the blue sprat (Spratelloides delicatulus) (Maboloc and Mingoa-Licuanan 2011). Furthermore, this species is a known host of cyclopoid copepod species (Anthessius alatus and Lichomolgus tridacnae), pea crabs (Tridacnatheres whitei and Xanthasia murigera), and pontoniinid shrimps (Anchistus australis, Anchistus demani, Anchistus miersi, Marygrande mirabilis, and Paranchistus armatus) (Neo et al. 2015).

Overexploitation of the species, both for consumption and the shell trade, has been reported as a major cause of population declines (Dawson and Philipson 1989, Hviding 1993, Gomez 2015, Purcell et al. 2020, Rehm et al. 2021). As the sub-adults/adults of T. gigas are free-living (i.e., not attached to the substratum) they are particularly vulnerable to over-extraction (Hviding 1993). The extent of fishing can vary depending on the local coastal communities. For instance, T. gigas (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 T. gigas) are heavily exploited for subsistence purposes (Eurich et al. 2023). Present-day populations of T. gigas continue to face high levels of exploitation pressure and habitat deterioration (Gomez 2015). This species remains a valuable coastal resource for both domestic and commercial markets, as it is highly favoured for its meat as food and large shells for the ornamental trade (Larson 2016).

Poaching of T. gigas (and other large giant clam species) for food was widespread throughout the region. Prior to the 1980s, commercial exports of giant clam adductor muscles to Asian markets and poaching by long-range foreign vessels were responsible for the severe stock reductions occurring in the Indo-Pacific (Dawson and Philipson 1989). Between 2012 and 2017, coastal resource authorities from various countries (Australia, Cambodia, Malaysia, and the Philippines) reported an increase in the number of fishing boats harvesting giant clams illegally (see Neo et al. 2017). Large-scale poaching also poses a major and persistent threat to wild populations.

Climate change has been mentioned as a threat to this species. Studies have confirmed that increased irradiance and seawater temperature bleach T. gigas. Specifically, Buck et al.'s (2002) experiment found that there is a loss of symbionts, reduced chlorophyll content in symbionts, retention of small zooxanthellae in the tissue, and release of ammonium into the water column. Bleaching of T. gigas can further reduce the fitness of host clams, resulting in reduced growth, fecundity/reproduction, and survival (Leggat et al. 2003, Sayco et al. 2023). In extreme cases, mortality of T. gigas has been associated with high seawater temperatures in Australia (Norton et al. 1995) and the Philippines (Gomez and Mingoa-Licuanan 1998). The early life stages of this species could also become vulnerable to elevated temperatures, where higher water temperatures promote rapid progression through early larval development but result in lower overall survival as a consequence of abnormal development and reduced post-settlement survival (Enricuso et al. 2019).

Other threats include anthropogenic influences such as effluent outfalls and agricultural run-offs that could affect calcified reef invertebrates. For example, T. gigas supplemented for three months with elevated ammonium and phosphate had significantly enhanced growth rates, but compromised shell formation (i.e., weaker shells) (Belda et al. 1993). The survival rates of T. gigas larvae were also found to decrease considerably with reduced salinities and the presence of copper (Blidberg 2003, Sayco et al. 2019). In juvenile T. gigas, reduced salinities showed effects of reduced heterotrophic feeding (Maboloc et al. 2014), and reduced rates of respiration and photosynthesis (Maboloc and Villanueva 2017). When cultured in masses, this species may be threatened by ectoparasite pyramidellids (Turbonilla spp.) that have been shown to impact the growth, survival, and physiology of juvenile T. gigas (Boglio and Lucas 1997). Apart from the various predators (see Neo et al. 2015), bacterial disease could cause mortality in juvenile T. gigas (Norton et al. 1993).

In the past, T. gigas was highly regarded as a primary species of interest for food and its shells because of the large body sizes. It is traditionally used as a reserve food when times are difficult (Firdausy and Tisdell 1992, Rehm et al. 2021). 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, the shells of this species were exported internationally for sale (to Japan, the US, and Europe) (Tisdell 1994). In the Philippines, the large T. gigas shells were made into bird baths, wash basins, and garden ornaments (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 (Heslinga 1996). This species was particularly fashioned as large bowls and soap dishes (Heslinga 1996). In contemporary times, T. gigas shells are heavily used in the handicraft carving industry located in Hainan Island, China (Larson 2016). One of the largest Tridacna shell markets today is China. Many of the local fishermen from Tanmen, Hainan, have converted from traditional fishing to the more lucrative tridacnine fishing as their main livelihood (Zhang 2014). Shells of giant clams may have become a substitute for ivory, the import of which is now regulated strictly (Gomez 2015, Larson 2016).

Another specific use/trade of T. gigas shells is the floor-tile industry. The shells of this species (lumped together with other giant clams) 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 were sought every two weeks for the floor-tile industry in Jakarta. Tridacna gigas was listed as one of the species sold in the market.

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

In 1983, T. gigas was formally listed in Appendix II of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES). This is unlike the other giant clam species of subfamily Tridacninae, which are listed 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).

In situ protection of stocks: T. gigas has legal protection under the respective wildlife and fisheries laws in the following countries: Australia, China, Taiwan, Indonesia, Malaysia, Myanmar, Philippines, Singapore, New Caledonia, Solomon Islands, and Pitcairn Islands. In contrast, the South Pacific nations (such as Papua New Guinea, FSM, Palau, Marshall Islands, Republic of Kiribati, 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 T. gigas) 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. Tridacna gigas has been extensively cultured and reintroduced (albeit sometimes limited to a couple of individuals in some areas) to Peninsular Malaysia, Sabah, the Philippines, Fiji, Northern Mariana Islands, Vanuatu, and Tonga, as well as introduced to American Samoa, the Cook Islands, Hawaii (USA), and Samoa (Neo et al. 2017). Some of the oldest maricultured T. gigas individuals were 34 years old (produced at the Palau’s Micronesian Mariculture Demonstration Center in 1982), as well as two others around 40–45 years old. All individuals were previously displayed at the Waikiki Aquarium in Honolulu (Carlson 2012, Heslinga 2012). While, in general, there is little information available regarding the outcomes of restocking in these areas, there has been a notable exception in the Philippines. According to Neo et al. (2017), for >20 years, the Marine Science Institute, University of the Philippines, has been culturing giant clam species for the restoration of depleted populations in the Philippines. Several species were initially restocked, but later efforts focused on T. gigas (Gomez and Mingoa-Licuanan 2006). Recruits of this species were first observed in the vicinity of Bolinao, Pangasinan, where the broodstock was placed (Cabitan and Conaco 2017). Additional reports on T. gigas recruitment around the Philippines have been reported by Requilme et al. (2021).

Wildlife trade: Most of the T. gigas marketed in the trade are cultured, while the wild-sourced clams are traded in very low quantities (Vogel and Hoeksema 2024). Based on the CITES Trade Database, Tonga was a major exporting country for both wild-sourced and cultured T. gigas between 2001 and 2010, while the Philippines was a major exporting country for both wild-sourced and cultured individuals between 2011 and 2019 (Vogel and Hoeksema 2024). Notably, the trade numbers for this species were much lower in recent years (2011–2019) compared to 2001–2010, suggesting a reduced demand for the species. Between 2001 and 2019, the import-export of T. gigas shells was greater than that of live specimens (Vogel and Hoeksema 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).

As the largest extant bivalve species, T. gigas holds substantial scientific interest. For instance, its significant size and longevity make it an important model for studying bivalve growth patterns and longevity (Li et al. 2024). In another example, subfossil T. gigas shells have the potential to yield reliable records of past changes in seasonality and ENSO variability, and mean climate conditions (Welsh et al. 2011). This makes its conservation not only an ecological priority but also a significant research imperative.

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