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This enormous shellfish is the largest species of bivalve mollusc in the fossil record, and the heaviest of all the living molluscs. Like all bivalve molluscs, the shell consists of two valves, although in the larger giant clams these cannot close completely. The shell is extremely thick and lacks bony plates; when viewed from above, each valve has four to five inward facing triangular projections. The mantle of the clam is visible between the two shells, and is a golden brown, yellow or green, although there may be such an abundance of small blue-green circles that the overwhelming impression is of a beautiful iridescent colour. A number of pale or clear spots on the mantle, known as 'windows', allow sunlight to filter in through the mantle. The mantle is completely fused with the exception of two holes (or 'siphons'). The gills are visible through the inhalant siphon, while the exhalent siphon is tube-like and is capable of expelling a large volume of water during spawning or if the clam's shells close suddenly.
Giant clams are found throughout the Tropical Indo-Pacific oceanic region, from the south China seas in the north to the northern coasts of Australia and from the Nicobar Islands in the west to Fiji in the east.
Giant clams occupy coral reef habitats, typically within 20 meters of the surface. They are most common found in shallow lagoons and reef flats, and are typically embedded in sandy substrates or those composed of coral rubble.
Giant clams are listed as vulnerable by the IUCN because of extensive collecting for food, aquaculture, and the aquarium trade. Numbers in the wild have been greatly reduced.
Giant clams have been extensively harvested for their meat and to supply the aquarium trade with such exotic specimens. Unable to sustain this exploitation, populations are now showing signs of decline; Tridacna gigas have not been seen in Fiji for over 50 years, primarily as a result of past over-collection for food.
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