Tridacna derasa is primarily distributed in the Central Indo-Pacific region. Despite extensive surveys, it is currently found in moderately low numbers or locally extinct in most of the regions where it was previously known, indicating an estimated population decline of approximately 75% over three generation lengths (ca 100 years, 1924–2024). Given that it is associated with coral reefs, there is evidence of declining quality of habitats in this period. Moreover, this species is particularly susceptible to overfishing, given its large size and the high value placed on its meat. It is also vulnerable to the impacts of ocean warming, which affects its symbiotic relationship with photosynthetic algae. Poor water quality and habitat destruction further threaten its survival.

This species continues to face fishing pressures where individuals are collected for domestic consumption. Notably, the ornamental trade of T. derasa is mostly of cultured origins, suggesting success in mariculture. While the mariculture knowledge of T. derasa 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. In addition, the cultured T. derasa likely involved only a few populations, which does not adequately safeguard its genetic diversity. Therefore, the population decline is projected to continue in the future as the causes of reduction in wild populations may not have ceased.

CITES provides important ongoing protection for this species. The use of limited gene pools in aquaculture, coupled with the overall vulnerability of this species, necessitates urgent conservation measures to preserve their genetic diversity and prevent further decline. Further work on the potential for mariculture to protect or enhance wild populations is needed for future conservation. 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 potential future declines.

On the basis of the observed population decline (75% over three generation lengths), due to ongoing exploitation, along with decline in area of occupancy (AOO) over the last 100 years, Tridacna derasa is assessed as Endangered (EN A2acd).

Tridacna derasa is found from the Cocos (Keeling) Islands to Tonga and from China to Queensland (Australia) (Neo et al. 2017). Of the over 20 regions in which the presence of this species has been recorded, several have origins from managed reintroduction, which do not necessarily contribute to the self-sustaining viable global population; many wild populations are severely exploited, and native populations in five geographic regions are extinct or possibly extinct, impacting perhaps 75% of recent historical range (as reviewed in Neo et al. 2017).

Broadly, the paucity of contemporary information on T. derasa could reflect the species' rarity on reefs today and local extinctions. Some of the more recent survey efforts on this species generally concurred that it is rare throughout its geographic range (Hernawan 2010, Dumas et al. 2011, Harahap et al. 2018, Purcell et al. 2020, Rehm et al. 2021, Dolorosa et al. 2024). Most studies generally conclude that this species is found in low numbers (i.e., fewer than 50 individuals within a subpopulation) or only in a few patches. An exception is the Great Barrier Reef in Australia, which is the most extensive area within the natural distribution of this species that still supports relatively undisturbed and stable populations (Braley 1984, 1986, 2023) and exhibits evidence of natural recruitment (Braley 2023). As reviewed by Neo et al. (2017), the lowest density reported was 0.3 individuals per hectare at Milne Bay Province, Papua New Guinea (Kinch 2002), while the highest density reported was 250 individuals per hectare at Meara Island, Philippines (Gonzales et al. 2014). Furthermore, 12 of the 16 previously known regions report the uncertain presence or local extinction of this species (Neo et al. 2017), strongly suggesting that the global population has declined significantly over the decades.

Tridacna derasa is believed to reach 100 years of age, but this information cannot be verified (Ungvari et al. 2013). In another study, this species was estimated to reach 30 years of age (i.e., with shell length of ca 44 cm; Pearson and Munro 1991). Another similar species, T. gigas, is known to reach at least 60 years (Watanabe et al. 2004). 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. Male-phase maturity is at sizes of 15.5–29 cm (Heslinga et al. 1984, Mingoa-Licuanan and Gomez 2007) or at age 3 or 4–5 years (Heslinga and Perron 1983, Heslinga et al. 1984, Syafyudin Yusuf pers. comm. 2024)—the mean value of 4 years is considered appropriate. 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 8 years (i.e., a mean size of 32 cm, Murakoshi et al. 2002), therefore: 8 + (0.5 * 52) = 34 years (generation length for the female phase). Mean value = 33 years. This assessment uses 100 years for the 3-generation time period.

Recent studies on the genetic connectivity of this species are scarce, but a 1992 study highlighted distinct genetic divisions between populations in the Great Barrier Reef, Fiji, and the Philippines. Within the Great Barrier Reef, low genetic diversity suggests strong internal connectivity. Like other giant clam species, T. derasa reproduces by spawning, and larval dispersal is significantly influenced by ocean currents. Despite this, the study indicated low genetic connectivity between distant populations, which could increase the risk of local extinction due to reduced genetic resilience and adaptability (Macaranas et al. 1992).

While 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 (i.e., it is unclear whether efforts have led to an increase in stock numbers). The exception to the latter is the restocking efforts in Yap (Federated States of Micronesia) that have yielded success in recruitment from their native giant clams (Neo et al. 2017).

This species is mostly free-living as adults. It inhabits a wide range of habitats, including reef flats, fore reefs, barrier reefs, and atoll lagoons down to depths of 20 m. All species of giant clams are known to be simultaneous hermaphrodites. The reproductive periodicity of the Great Barrier Reef population of T. derasa showed a potential austral late spring to early summer spawning for this species (Braley 1984). The mariculture of this species is known and published (Heslinga and Perron 1983, Heslinga et al. 1984), and the reproductive potential is known to some extent. A piece of indirect evidence for the natural reproductive potential of this species is the recruitment of juveniles arising from restocked cultured individuals transplanted in 1984 (i.e., 30 years; Neo et al. 2017).

The presence of this species has beneficial outcomes for coral reef ecosystems. For instance, a population of cultured individuals produced substantial biomass (16 tonnes ha^-1 yr^-1 of wet tissue biomass, an estimated 928 kg dry weight ha^-1 yr^-1) (Heslinga et al. 1984). As a potential reservoir of endosymbionts, this species can discharge 4.9 x 105 cells clam^-1 d^-1 of intact zooxanthellae (Maruyama and Heslinga 1997). The substantial quantities and (possibly) types of zooxanthellae released from giant clams then become available for other zooxanthellate-dependent species to ‘take up’, hence contributing to the wider coral reef ecosystem. An experimental study found that introduced T. derasa can provide additional shelter (i.e., via their shell surfaces) to surrounding organisms (de Guzman et al. 2023). Furthermore, this species is a known host of pontoniinid shrimps (Anchistus australis, Conchodytes tridacnae) (Neo et al. 2015).

Overexploitation of the species, for both consumption and shell trade, has been reported as a major cause of population declines (Pearson 1977, Dawson and Philipson 1989, Hviding 1993, Purcell et al. 2020, Rehm et al. 2021). As the sub-adults/adults of T. derasa 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). The extent of fishing can vary depending on the local coastal communities. For instance, this species (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, giant clams (including T. derasa) are regularly exploited for subsistence purposes (Rehm et al. 2021). Present-day populations of this species likely face high levels of exploitation pressure as it remains a valuable coastal resource for domestic markets and commercial markets, because it is highly favoured for its meat as food and large shells for the ornamental trade (Larson 2016).

Poaching of this species (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.

While the threat information for this species is generally limited, climate change has been mentioned to pose risks to their physiological functions. A single study by Blidberg et al. (2000) found that when seawater temperature was raised by +3°C over 24-hour periods, T. derasa reduced its photosynthetic activity and respiration rate. Like other giant clam species, T. derasa is likely vulnerable to anthropogenic pressures such as habitat destruction, poor water quality, and diseases.

In the past, large individuals of this species were highly valued for their meat and shells, which were used as food and curios (Pearson 1977). 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 Philippines, T. derasa shells were commonly not distinguished by shell dealers as a separate species but rather as a “heavier variety” of T. gigas or H. porcellanus (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). The T. derasa shells, most of which were 15–30 cm in length, were sun-dried, chemically bleached, and sold for USD$5 to USD$10 per matched pair (Heslinga 1996). Soap dishes are one of the easiest types of giant clam craft ware to make. In 1994, the MMDC gift shop retailed hundreds of T. derasa soap dishes for $5 each (Heslinga 1996). In contemporary times, shells of this species are heavily used in the handicraft carving industry located in Hainan Island, China (Larson 2016).

Another specific use/trade of this species’ shells is the floor-tile industry. These shells (lumped together with shells of 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 derasa was listed as one of the species sold in the market.

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

In 1983, Tridacna derasa 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 (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). Thus, CITES regulates the international trade in any of their parts (shells, tissues, alive or dead).

In situ protection of stocks: This species has legal protection under the respective wildlife and fisheries laws in the following countries: Australia, China, Taiwan, Indonesia, Malaysia, Philippines, New Caledonia, Solomon Islands, and Pitcairn Islands. In contrast, the South Pacific nations (such as Papua New Guinea, Palau, Fiji, 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. derasa) 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. Mariculture of this species has been highly successful (e.g., Palau, Marshall Islands, Federated States of Micronesia, the Cook Islands, and Solomon Islands). Spats tend to be produced for local enhancement, occasionally for translocation programmes to other countries. While, in general, there is little information available on the outcomes of restocking in these areas, there has been a notable exception in the Yap, Federated States of Micronesia. According to Neo et al. (2017), T. derasa juveniles were found by local fishermen and international experts from the Secretariat of the Pacific Community (SPC) in the early 1990s (J.O. Fagolimul and P. Dor pers. comm. 2016) after an extensive reintroduction program initiated in the mid-1980s undertaken with clams cultured at Palau’s MMDC (Teitelbaum and Friedman 2008). In 2013–2014, some of these Yapese T. derasa recruits reached full maturity and were used with replicated success as breeding stock in a local hatchery managed by Mr. Philip Dor (P. Dor pers. comm. 2016). Mr. Dor has successfully produced commercial numbers (hundreds of thousands) of macroscopic juveniles of this species in the Yap hatchery, as verified by international experts.

Wildlife trade: The aquarium trade continues to sell both wild-sourced and cultured specimens of this species, but there is more trade of cultivated ones (Vogel and Hoeksema 2024). Based on the CITES Trade Database, Marshall Island, Micronesia, and Palau were the top three exporting countries for cultured T. derasa between 2011 and 2019 (Vogel and Hoeksema 2024). Notably, the trade numbers for this species were slightly higher in 2011–2019 compared to 2001–2010, suggesting that the demand for the species may be plateauing. Between 2001 and 2019, the import-export of live individuals was greater than that of shells (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).

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