Derasa Giant Clam Tridacna derasa

The Derasa Clam is one of several Giant Clams, bivavle molluscs in the genus Tridacna. It is also called the Bluegreen, the Southern and the Smooth Giant Clam. One of the larger giant clams, it can grow to 60cm wide; the biggest, T. gigas, grows to over a metre wide. It is native to outer edges of coral reefs in the southern Indo-Pacific from Indonesia and the Philippines to Australia and Fiji.

Their mantle, which can be closed in the shell for protection, can be brown with white/cream/pink streaks, but can also be blues and greens. Despite their alternative name, the mantle is not as vibrant blue-green as some other members of their genus. The Derasa is called the Smooth as its shell is often smooth without jutting "scutes", but, as seen above, it can have horizontal jutting ridges and it can sometimes be difficult to see the shell obscured by mantle and attached anemones. It has some 4-7 gentle vertical ribs or folds. The larger T. gigas has 4-5 ribs with much more pronouned folds and a rougher exterior (also without scutes).

Giant Clams feed by sucking water in and filtering out plankton (as well as taking the oxygen through gills) and also hosting algae which can photosythesise to provide food. The clams hatch from eggs as free-swimming larvae. After their first few years as a juvenile clam they become male as adults and later hermaphrodite. They release sperm and eggs in the water to breed (not simultaneously, to avoid self fertilisation). They are vulnerable to over-collection for food and the aquarium trade.