The geoduck is a species of very large saltwater clam, a marine bivalve mollusk in the family Hiatellidae. The shell of this clam is large, about 15 centimetres (5.9 in) to over 20 centimetres (7.9 in) in length, but the extremely long siphons make the clam itself very much longer than this: the “neck” or siphons alone can be 1 metre (3.3 ft) in length.