File:Spondylus tenellus (scarlet thorny oyster) (offshore southern Australia) (24543235886).jpg

Spondylus tenellus (Reeve, 1856) - scarlet thorny oyster from Australia. (public display, Bailey-Matthews Shell Museum, Sanibel Island, Florida, USA)

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 bivalve shown above is a scarlet thorny oyster, Spondylus tenellus. From museum signage: "A beautiful thorny oyster with a geographic distribution restricted to Australia, Spondylus tenellus has very highly variable shells, with color ranging from orange, pink, mauve or white and spines of darker color. This world record sized shell, found by scuba diving off southern Australia, measures 111 mm in length."

Classification: Animalia, Mollusca, Bivalvia, Pteriomorphia, Pectinoida, Spondylidae

Locality: offshore southern Australia

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