What Is a Bivalve?

A bivalve is an animal that has two hinged shells, which are called valves. All bivalves are mollusks. Examples of bivalves are clams, mussels, oysters, and scallops. Bivalves are found in both freshwater and marine environments. 

Characteristics of Bivalves

There are about 10,000 species of bivalves.Bivalves range in size from less than a millimeter to close to 5 feet (e.g., the giant clam).

A bivalve's shell is formed of calcium carbonate that is secreted from the bivalve's mantle, which is the soft wall of the animal's body. The shell grows as the organism inside gets bigger. Not all bivalves have externally visible shells - some are small, some are not even visible. Shipworms are a bivalve that doesn't have a very visible shell - their shell is made up of two valves at the worm's anterior (back) end.

Bivalves have a foot, but not an obvious head. They also don't have a radula or jaws. Some bivalves move around (e.g., scallops), some burrow into the sediment (e.g., clams) or even rocks, and some attach to hard substrates (e.g, mussels).

Smallest and Largest Bivalves

The smallest bivalve is thought to be the saltwater clam Condylonucula maya. This species has a shell that is less than a millimeter in size.

The largest bivalve is the giant clam. The valves of the clam may be over 4 feet long, and the clam itself may weigh over 500 pounds.  

Bivalve Classification

Bivalves are found in the Phylum Mollusca, Class Bivalvia.

Where Are Bivalves Found?

Marine bivalves are found around the world, from polar regions to tropical waters and from shallow tide pools to deep-sea hydrothermal vents. 

Feeding - Them and You

Many bivalves feed by filter feeding, in which they draw water over their gills, and tiny organisms collect in the organism's gill mucus. The also breathe by drawing fresh oxygen from the water as it passes over their gills.

When you eat a shelled bivalve, you're eating the body or a muscle inside. When you're eating a scallop, for example, you're eating the adductor muscle. The adductor muscle is a round, meaty muscle that the scallop uses to open and close its shell.

Reproduction

Some bivalves have separate sexes, some are hermaphroditic (have male and female sex organs). In most cases, reproduction is sexual with external fertilization. The embryos develop in the water column and go through a larval stage before eventually developing their shell. 

Human Uses

Bivalves are some of the most important seafood species. Oysters, scallops, mussels, and clams are popular selections at just about every seafood restaurant. According to NOAA, the commercial value of bivalve harvests in 2011 was over $1 billion, just in the U.S.  This harvest weighed over 153 million pounds. 

Bivalves are organisms particularly vulnerable to climate change and ocean acidification. Increasing acidity in the ocean is affecting the ability for bivalves to effectively build their calcium carbonate shells. 

Bivalve Used in a Sentence

The blue mussel is a bivalve - it has two equally-sized, hinged shells that fit together and enclose the animal's soft body.

_{References and Further Information}

• _{Geller, J. B. 2007. "Bivalves." In Encyclopedia of Tidepools and Rocky Shores. University of California Press, p. 95-102.}
• _{Global Biodiversity Information Facility. Condylonucula maya D.R. Moore, 1977. Accessed December 30, 2015.}
• _{Lindberg, D.R. 2007. "Molluscs, Overview." In Encyclopedia of Tidepools and Rocky Shores. University of California Press, p. 374-376.}
• _{Martinez, Andrew J. 2003. Marine Life of the North Atlantic. Aqua Quest Publications, Inc.: New York.}
• _{NOAA, National Ocean Service. What Is a Bivalve Mollusk? Accessed December 30, 2015.}