The clam whose mantle is the blue-green thing everyone photographs on a shallow reef flat, wedged into the rock so deep that only the opening shows. Thirty-five centimeters (14 in) of shell, most of it out of sight.
The name misleads: maxima is the smallest of the giant clams, not the largest. It earns its place another way - it is the most widespread of them, and the one that colonizes hard reef rock by grinding itself in. A young clam settles on the surface and rasps and rocks its way down over years until the shell sits in a socket of its own making, safe from anything that would pry it out.
Being embedded costs it something. It cannot close fully around a mantle that has to stay spread in the light, and the animals living inside that mantle are the reason: symbiotic algae that supply most of its food. In return the clam gives them a lit, protected greenhouse and pumps water past them all day.
Because they are permanently open, they are also easy to take, and this is the clam that vanished first around inhabited coasts. CITES Appendix II covers the whole family. On the reef the rule is simple: they respond to a shadow by closing, and repeatedly making one close costs the animal energy it spent all day collecting.
How to recognise it
shell sunk into the rock, only the mantle edge visible
mantle in electric blue, green or gold with fine speckling
shell ribs low, without the tall flutes of its fluted relative
The extent of fishing of T. maxima can vary depending on the local coastal communities. For instance, T. maxima (and other large clam species) is opportunistically taken during fishing trips targeting other marine resources such as fish and lobsters (Purcell et al. 2020). In the Republic of Kiribati, all giant clams (including T. maxima) are heavily exploited for subsistence purposes (Eurich et al. 2023). On the other hand, it is relatively untargeted by fishers in areas where larger species occur, and high densities of the species are still observed on some isolated and enclosed reefs of the Central Pacific (Van Wynsberge et al. 2016). As this species is highly popular in the aquarium trade, numerous South Pacific nations (such as Fiji, Solomon Islands, Vanuatu, and the Federated States of Micronesia) were extracting their wild T. maxima for live exports in the early 1990s and 2000s, which appears to have impacted populations (Neo et al. 2017).
Climate change could threaten this species. Massive mortalities due to climate variability (i.e., high water temperatures) have been reported from the isolated populations of this species in the atolls of French Polynesia (Andréfouët et al. 2013, Van Wynsberge and Andréfouët 2017) and Lakshadweep Reefs (Apte et al. 2019). In addition, thermal stress alone can cause the degradation and death of Symbiodiniaceae cells (Dubousquet et al. 2016) and reduce fertilisation success in this species (Armstrong et al. 2020). Experimental studies combining the effects of elevated temperatures and _p_CO2 levels found that elevated temperatures exert a stronger impact on this species’ physiology than acidification (Armstrong et al. 2020, Brahmi et al. 2021). These collectively suggest that variability in thermal conditions could be detrimental to this species in the future.
Threat classification from the IUCN Red List.
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