Introduction

Bivalves are aquatic mollusks (Phylum Mollusca) that belong to the Class Pelecypoda (or Bivalvia), meaning "hatchet-foot." Mussels, clams, scallops and oysters are examples of bivalves, which have two shells (valves) covering the left and right sides of the body. There are many different types of both freshwater and marine bivalves, and their shells come in a wide variety of sizes, shapes, and colors.  They may range in size from 2 millimeters to over four feet (the giant clam of the South Pacific).

Mussels attach themselves to rocks with bundles of tough threads called "byssal threads" so that they can remain stable in intertidal areas. The narrow end of the mussel points toward the breaking waves to protect the animal. Some examples of mussels are the Blue mussel (Mytilus edulis), which is common along the east coast, and the California mussel (Mytilus californianus), which is found on the west coast in exposed rocky intertidal zones.

Clams are another type of bivalve and include the Northern quahogs (Mercenaria mercenaria), also called "littlenecks" or "cherrystones," which make popular meals for seafood lovers. An example of a West Coast clam is the Razor clam (Siliqua patula) named for its sharp shell and resemblance to an old-fashioned straight razor.

Scallops have a variety of shell colors, with 17-20 ribs on the exterior.  Scallops lie on the seabed and move by rapidly opening and closing their shells, ejecting water out.  Most kinds have a series of brightly colored eyes that are sensitive to minor changes in light intensity. The

Atlantic Deep-sea Scallop (Placopecten magellanicus) forms the basis of the largest scallop fishery in the United States and one of the most important fisheries in the New England area.
 

Oysters are reef-forming animals, and these reefs or "beds" often provide structured habitats for many other species.  Shell size usually ranges from four to eight inches, and is rough and irregularly shaped.  Oysters are born as males, but may change into females if environmental conditions are favorable and food is plentiful.  Some species of oyster are known for their pearls, which are formed if a particle becomes lodged inside the shell.  The Eastern oyster (Crassostrea virginica) is most likely the species served if you are enjoying seafood on the East Coast.  The Pacific Oyster (Crassostrea gigas) is the most common oyster grown on the west coast.

Anatomy

Different types of bivalves will vary somewhat in their anatomy, so the figure below should be used only as a general reference. The two halves (valves) of a bivalve's shell are hinged at the top by an elastic ligament, and are opened and closed by two adductor muscles on the inner surfaces of the valves.  The top portion of the shell (where it is hinged) is the oldest part of the shell, called the umbo, and it is from this area that the animal extends as it grows. Some bivalves must be able to close their shells tightly when exposed at low tide so that they can retain water and keep their gills moist.  Within the shell is a soft layer of tissue called the mantle, which secretes the three layers of the shell:  the outer layer (organic periostracum), the middle (calcareous prismatic) layer, and the pearly, inner (nacreous) layer.  Pearls are formed when a foreign particle finds its way into the shell, and the animal deposits nacreous layers around it.

Bivalves have a muscular "foot" at their front end that is used for movement such as burrowing, or in some bivalves like the razor clams, for swimming.  In the blue mussels mentioned above, the foot is primarily used to create byssal threads. Bivalves have two siphons at the rear of the body which are used for the inflow (inhalant) of water and outflow (exhalant) of water and wastes.  Sometimes the two siphons are joined into one "neck."  The animal can extend these siphons if it needs to hide under the sediment to avoid predators.

Feeding

Bivalves consume the phytoplankton and other tiny organic matter that flows through their gills.  Gills function both in respiration and in straining out food particles. Two small organs called palps help carry food to the mouth. Mussels are extremely efficient feeders, and can filter 10-15 gallons of water per day.

Reproduction

 Bivalve reproduction takes place externally when eggs and sperm from the gonads are released into the water, and a female may spawn millions of eggs in her lifetime.  Once fertilized, tiny, free-swimming larvae are formed. This planktonic stage usually lasts a few days, and the larvae settle and attach themselves to a hard surface, where they then begin to form a shell.