Small Giant Clam, Maxima Clam

The small giant clam is less than a third of the size of the true giant clam (Tridacna gigas). As an adult, it has a large shell that adheres to a rock by its byssus, a tuft of long, tough filaments that protrude from a hole next to the hinge of the shell. When open, the bright blue, green or brown mantle is exposed and obscures the edges of the shell which have prominent distinctive furrows.

Source: http://en.wikipedia.org/wiki/Maxima_clam