giant clam

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of various very large bivalve mollusks of the subfamily Tridacninae, found in tropical waters of the Indian and Pacific Oceans and including the largest living mollusks.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a large clam inhabiting reefs in the southern Pacific and weighing up to 500 pounds