Atrina pen shell bivalve (Algiers Beach, Sanibel Island, F…

Atrina sp. - pen shell in Florida, USA (December 2012). 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 brown shell shown above is a pen shell, Atrina sp. This is the interior surface of a left valve (anterior to the right, posterior to the left, dorsal at top, ventral at bottom). Atrina bivalves are known in the fossil record - their reported age range is Triassic to Holocene. Atrina shells are thin, moderately fragile, and relatively organic-rich (Atrina shells can be partially dissolved by the dilute bleach that is used to clean these and eliminate foul odors). The interior surface of individual Atrina shells has some attractive, rainbow-iridescent nacreous aragonite. Classification: Animalia, Mollusca, Bivalvia, Pteriomorphia, Pterioida, Pinnidae Locality: Algiers Beach, southern shore of Sanibel Island, Gulf of Mexico coast of southwestern Florida, USA (vicinity of 26° 25