Tridacna gigas clam
The T. gigas clam can grow to be several feet in size. The one in this picture was acquired at 23" wide by a local fish store for their 1000g display. A big tank is required by anyone wanting to care for this species of clam.
Fish may host in a giant clam.
This clam is sitting on the lid of a salt bucket, and Banggai Cardinal fish are hosting in it.