The older species name, Tridacna mbalavuana Ladd, 1934, takes priority over the newer name, Tridacna tevoroa Lucas, Ledua & Braley, 1990 (Newman and Gomez 2000). Tridacna mbalavuana was first described from fossils in Viti Levu, Fiji (Ladd 1934) and was already commonly known to the locals as ‘tevoro’, the devil clam. After closer examination of their morphological characteristics, the two species are now considered synonymous, with T. tevoroa being the junior synonym of T. mbalavuana (Newman and Gomez 2000).

Tridacna mbalavuana has a highly restricted distribution within the Indo-Pacific, with only three confirmed localities (Fiji, New Caledonia, and Tonga). This species dwells deeper than other giant clam species (mainly at depths over 20–30 m depth), and the handful of surveys revealed small population sizes. Given the low abundance across its distribution, this species is vulnerable to overfishing and other anthropogenic activities such as habitat destruction. Also given that it is associated with coral reefs, there is evidence of declining quality of habitats. Therefore, a continuing decline in number of mature individuals and in number of subpopulations is projected for this species.

While there is some knowledge of the species’ reproductive ecology, this information is not currently used for conservation purposes. Also, the species’ general use, trade, and threats are known, but no management plans are in-place to monitor harvest and trade levels.

CITES provides important ongoing protection for this species. 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 species’ small AOO (< 500 km²), severely fragmented distribution across three distant locations, small number of individuals (likely < 2000 total with < 250 in each of the three subpopulations), and the projected declines, this species is currently listed as Endangered, B2ab(ii,iv,v); C2a(i).

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This species is known only and confirmed from Fiji, Tonga, and New Caledonia (Bouchet et al. 2001, Tiavouane and Fauvelot 2017). Lucas et al. (1991) mentioned that the known distribution of T. mbalavuana was limited to several island groups over about 300 km of distance in the southwestern Pacific. This species is likely distributed throughout the shallow waters surrounding these island groups. Surveys are needed to confirm its true distribution.

While there has been a single observation from Australia (A.M. Ayling, pers. comm. 2016, Newman and Gomez 2000), this record could not be verified (i.e., lack of photographic evidence). According to Wakum et al. (2017), Indonesia might be a locality for T. mbalavuana. However, no actual records were found in the study. Based on currently available information, the three known regions are interpreted as three separate localities that would each be impacted by relevant threats.

Tridacna mbalavuana appears to be generally rare throughout its known range (Lucas et al. 1991, Tiavouane and Fauvelot 2017). In the Vatoa area, Fiji, Lewis and Ledua (1988) estimated that its relative abundance compared to T. derasa (another giant clam species) was on the order of 1:50, suggesting that this species should be regarded as vulnerable to extinction. Ledua et al. (1993) also reported few live specimens (abundance, n = 20, 1989–1991) in the eastern Lau group of Fiji and a slightly higher abundance in Tonga (n = 50, 1989–1992). Despite exhaustive searches, only two living individuals have recently been reported from New Caledonia (Tiavouane and Fauvelot 2017). Although cryptic, evidence suggests that each of the subpopulations is likely to be small with < 250 individuals ever observed from each region.

The actual population decline is unknown, but approximately 76 individuals of T. mbalavuana were collected for spawning and culture between 1986 and 1992 at the Tonga Fisheries Department. There were also mentions of collections by fishermen (Ledua et al. 1993). Based on all of the above information, we can infer the species’ population size is very small.

In other giant clam species, the lowest observed population densities are 0.2–0.3 individuals per hectare (Neo et al 2017); for the area of occupancy (AOO) of T. mbalavuana, this would indicate a total living population of 1,680–2,520 individuals. Taking a precautionary approach, and since the known numbers of animals in each of the subpopulations is much lower, we are using the mid point of this range to estimate the total extant population for the species as around 2,100 mature individuals.

Contemporary Tridacna mbalavuana individuals inhabit relatively deep waters (>20 m) compared to other giant clam species and apparently might be intolerant of conditions in shallow water (Lucas et al. 1991). However, an individual appeared to be healthy after being moved from the deeper waters to shallow 1 m waters (J.W. Fatherree pers. obs. 2024), suggesting the species could acclimate to shallow conditions, but this needs to be further verified.

In Haápai, Tonga, individuals were seen on live coral habitat at >30 m depth in clear water, whilst in the eastern Lau group of Fiji, individuals were never found on live coral habitat but instead next to rocks on steep slopes (Ledua et al. 1993). Individuals were more recently found on the northeastern barrier reef of New Caledonia (Tiavouane and Fauvelot 2017).

All species of giant clams are known to be simultaneous hermaphrodites. There have been several mariculture attempts of T. mbalavuana, but there are no reports on its reproduction, size at maturity, or growth rate (Ledua et al. 1993, Mies et al. 2017).

The main threat to this species is fishing for domestic and commercial uses. In the late 1980s, it was collected for the commercial export of its meat in Fiji (Lewis and Ledua 1988). Large scale harvesting is now limited by previous population declines and protection from CITES. Another potential threat is the collection for sale in the aquarium trade and mariculture (Fatherree 2015). Past and ongoing local harvest for subsistence fishing is likely a major factor leading to the low population size of T. mbalavuana (Ledua et al. 1993; NOAA 2024). This threat is ongoing throughout the range of this species.

Climate change is a threat to this species. Studies have confirmed that increased irradiance and seawater temperature bleach other Tridacna spp., causing them to lose their photosymbionts. Tridacna mbalavuana lives in clear waters and is highly dependent on autotrophic energy (Klumpp and Lucas 1994). Since this species is associated with offshore tropical coral reefs, the projected declines in reef habitats are a threat to this species. Tridacna mbalavuana has an extremely restricted and fragmented distribution, with very low numbers outside of the largest subpopulations in Fiji and Tonga. This distribution means the species may have low resilience to vulnerable to future bleaching events or other impacts.

Tridacna mbalavuana previously was exploited for both domestic and commercial purposes. In Fiji, some individuals of this species were ‘accidentally’ collected along with T. derasa for commercial export of its meat (Lewis and Ledua 1988, Lewis et al. 1988). In Tonga, T. mbalavuana has been harvested for domestic markets either using SCUBA or traditional Pacific Islands fishing methods (Ledua et al. 1993). Even though the species’ preference for deeper water habitats may have offered some protection from harvesting (Lewis and Ledua 1988, Lucas et al. 1991), the development of SCUBA and hookah gear has facilitated access to previously inaccessible T. mbalavuana stocks.

There is little information regarding the mariculture of this species. A single successful spawning was reported in December 1991 at the Tonga Fisheries Hatchery, but the second attempt was unsuccessful (Ledua et al. 1993).

There is limited recent information on the use of species. On the other hand, the species appears to have made it into the aquarium trade (see attachment; G. Sew, Singapore, pers. comm. 2016). For additional information regarding the aquarium trade, see Adams 2015 and Fatherree 2015).

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).

No other known conservation actions are in-place due to a lack of data on the population size, distribution and trends, life history and ecology, use and trade, and threats.

While there is some knowledge of the species’ reproduction ecology, the information is not used for conservation purposes. The general use, trade, and threats of the species are known, but no management plans are in-place to monitor the trends of harvest and trade levels. Given the general rarity of this species, these anthropogenic activities could unknowingly push the species to extinction (partly due to the lack of baseline information on the status of wild populations).

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