Tridacna costata is a species of giant clam. The small giant clam has the widest range of all giant clam species. His men brought back many giant clam shells. This subfamily contains the largest living bivalve species, including "Tridacna gigas", the giant clam. The attractive colours of the small giant clam are the result of crystalline pigment cells. As a result, the two scientists were fed to the Giant Clam. It is a naturally occurring, non-nacreous, calcareous concretion (pearl) from a giant clam. Today the giant clam is considered neither aggressive nor particularly dangerous. The rare giant clam and the seahorse can sometimes be seen. Located in this island is the Giant Clam Ocean Nursery and Marine Protected Areas (MPAs). Species of mollusc include cowry, Strombidae, "Lambis" species, "Charonia tritonis", and giant clam. The latter include the giant clam and various nudibranchs and cone snails. On the black market, giant clam shells are sold as decorative accoutrements. Other species in danger from overharvesting include the giant clam and the dugong. On the bottom, the giant clam "Inoceramus" left common fossilized shells in the Pierre Shale.