Derasa Clam 3"Tridacna derasa is known as a sand clam. It prefers clear water, and after reaching sexual maturity, it will release the attaching threads and move to a sandy area. They are usually colored with orange yellow and black stripes and have a blue rim. It is one of the largest clams, growing to 16 inches in length. This is a very easy clam to keep in aquariums and should be placed on the bottom directly on the sand.