A giant clam with tall fluted scales rising off the shell in ranks, up to 40 cm (16 in), sitting loose on the reef rather than sunk into it. The flutes are the field mark: its common neighbor, the Small Giant Clam, bores itself into coral rock and shows only a slot of mantle.
The mantle is a farm. Inside it live symbiotic algae, and the clam has evolved a lens system to feed them - iridescent cells that scatter incoming light sideways and downward so it reaches the algae at survivable intensities instead of burning them. The colors a diver photographs are the optics of that arrangement, and no two clams carry the same pattern.
The algae supply most of the clam's food, which is why giant clams live in clear shallow water and cannot be moved deeper. Filtering is a supplement rather than a livelihood.
It starts male and becomes female with size, so the large individuals in a population are the ones producing eggs - the ones fisheries take first. Every giant clam is listed on CITES Appendix II, and populations here are thin compared to what the reefs once held.
How to recognise it
shell with four to six ranks of tall projecting flutes
sits on the bottom or in a hollow, not embedded in rock
The extent of fishing of T. squamosa 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, in the Republic of Kiribati, all giant clams (including T. squamosa) are heavily exploited for subsistence purposes (Eurich et al. 2023). Large scale harvesting is now limited by protection from CITES.
Climate change could threaten this species. There were some reports of bleaching incidences during past global bleaching events (Junchompoo et al. 2010, Mies et al. 2019). Elevated temperatures were also found to be detrimental to the photosynthetic performance of juveniles (Elfwing et al. 2001) and to impact the development and survival of veligers (Eckman et al. 2019). Furthermore, experiments combining the effects of elevated temperatures and high _p_CO2 levels revealed strong synergistic effects that significantly reduced the survival and growth of juveniles (Watson et al. 2012, Syazili et al. 2020). Notably, ocean acidification alone has sublethal effects on this species, with individuals exhibiting reduced shell growth rates (i.e., lower calcification rates) (Watson 2015, Li et al. 2022).
Other threats mentioned include anthropogenic influences such as coastal development and habitat loss (Neo and Todd 2012). Notably, there have been several studies examining the effects of varying light levels or irradiance on the photosymbiosis process in T. squamosa. Under lower light conditions (i.e., increased sediment output), the photosynthetic performance appeared to be reduced (Blidberg et al. 1999, Tedengren et al. 2000). These studies suggest that the maximum depth distribution of this species could be largely restricted by reduced light availability in areas with higher levels of sediment input.
Threat classification from the IUCN Red List.
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