The smallest of the giant clams, wedged into coral with a vividly patterned mantle along the shell margin.
Shell to about 35 cm, commonly under 20; sources disagree. An elongate, asymmetric shell with four to six low, closely set radial ribs bearing small scales, bored into the coral rock so that only the gape and the mantle show.
The mantle is the field mark: electric blue, green, gold or brown with fine iridescent speckling, spread flat in sunlight. The cells that produce those colours probably also manage the light reaching the algae living inside the mantle.
The fluted giant clam is the confusion and has tall, widely spaced flutes on the shell and sits free rather than embedded. There is also a rare Red Sea endemic giant clam with a documented population bottleneck, which is the more interesting animal to find.
Most of its food comes from single-celled algae living in its own tissue, topped up by filter feeding, so it lives in shallow sunlit water — reef flats and reef tops between the surface and about 20 m. It bores itself into the rock over years by rocking and grinding, and once settled it does not move. It snaps shut when a shadow passes.
It is listed on Appendix II of CITES. A second species was separated out of it in 2013 and is found from Taiwan and Japan west to Christmas Island, so many Pacific photographs labelled with this name are now the other animal; the Red Sea record is not affected.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.