File:Tridacna sp. clam.jpg

English: Tridacna sp. clam

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 live Tridacna sp. The genus includes the largest bivalve species on Earth. Tridacnid bivalves sit on seafloors with their two shells open to expose colorful mantle tissues. Within the clam's mantle, small microbial organisms (zooxanthellae) live in a symbiotic relationship with the host. In tridacnid bivalves, the zooxanthellae are photosynthesizing dinoflagellates (www.daviddarling.info/images/dinoflagellate.gif). The dinoflagellates provide food for the clam, and the clam also obtains food by filter feeding.

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Classification: Animalia, Mollusca, Bivalvia, Veneroida, Cardiidae

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More info. at: en.wikipedia.org/wiki/Tridacna and

en.wikipedia.org/wiki/Giant_clam