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Bivalve mollusks (e.g., clams, oysters, mussels, scallops) have an external covering that is a two-part hinged shell that contains a soft-bodied invertebrate

spiny oyster

Spondylus or spiny oyster from North Rock Cave at 200 feet depth. Image courtesy Bermuda Deep Water Caves 2011, NOAA Ocean Explorer.

Like fish, bivalve mollusks breathe through their gills. As filter feeders, bivalves gather food through their gills. Some bivalves have a pointed, retractable "foot" that protrudes from the shell and digs into the surrounding sediment, effectively enabling the creature to move or burrow.

Bivalves even make their own shells. An internal organ called the mantle secretes calcium carbonate so that as the inner invertebrate grows, the outer shell provides a roomier home.

Many bivalve species play important roles in aquatic and marine ecosystems by filtering the water and serving as habitat and prey for a variety of sea life. This diverse group of species, estimated at about 9,200, inhabits virtually the entire world ocean, from the balmy tropics to the sub-zero Arctic, and from the deep ocean to sandy and rocky shorelines. A few have even taken up residence around hydrothermal vents found deep in the Pacific Ocean, below 13,000 feet.

If the bivalve's biological brilliance alone fails to impress, then consider that NOAA estimated the 2011 economic value of commercial bivalve mollusk harvesting at about $1 billion annually in the U.S., and the weight of the harvest was estimated at 153.6 million pounds.