Crassostrea virginica (Gmelin, 1791) - eastern oyster encrusting a log in Florida, USA. (summer 2009)

Bivalves are bilaterally symmetrical molluscs having two calcareous, asymmetrical shells (valves) - they include the clams, oysters, and scallops. In most bivalves, the two shells are mirror images of each other (the major exception is the oysters). They occur in marine, estuarine, and freshwater environments. Bivalves are also known as pelecypods and lamellibranchiates.

Bivalves are sessile, benthic organisms - they occur on or below substrates. Most of them are filter-feeders, using siphons to bring in water, filter the water for tiny particles of food, then expel the used water. The majority of bivalves are infaunal - they burrow into unlithified sediments. In hard substrate environments, some forms make borings, in which the bivalve lives. Some groups are hard substrate encrusters, using a mineral cement to attach to rocks, shells, or wood.

The fossil record of bivalves is Cambrian to Recent. They are especially common in the post-Paleozoic fossil record.

The dead, bark-stripped log shown above has an encrusting lower valve of an eastern oyster, Crassostrea virginica. Shell shape varies from irregularly subcircular to elongated. The muscle scar has a distinctive purplish coloration. Oysters are sessile, benthic filter-feeders than encrust hard substrates - rocks, shells, or wood. The upper shell frequently detaches after death, while the lower shell often remains attached to the substrate. The cementing agent is the mineral calcium carbonate (CaCO3).

Classification: Animalia, Mollusca, Bivalvia, Pteriomorphia, Ostreoida, Ostreidae

Locality: southernmost tip of Cayo Costa Island, southwestern Florida, USA

More info. at: