geoduck, (species Panopea generosa), marine invertebrate of the class Bivalvia (phylum Mollusca) that inhabits the sandy muds of the intertidal and shallow sublittoral zones of the Pacific coast of North America from southern Alaska to Baja California. The geoduck is the largest known burrowing bivalve. Its gaping shell reaches about 180–230 mm (7–9 inches) in length, but the siphons, which cannot be retracted into the shell, may extend up to 1.3 m (about 4 feet). The clam, including the shell, may weigh as much as 3.6 kg (8 pounds). The geoduck is highly prized for food, but inhabits deep burrows and is difficult to dig out. The name is probably derived from a Nisqually Indian phrase meaning “dig deep.” Several species similar to the Pacific geoduck occur in other parts of the world. Panopea bitruncata, for example, is found from the coast of North Carolina to the Gulf of Mexico.