One foot, no head, and a shell that can snap shut — meet the bivalves!
What has one foot, no head, and rarely comes unhinged? Clams, mussels, scallops, and oysters! Collectively, they're known as bivalves, because they're encased in two valves, or shells, that are connected by a hinge. Oysters, mussels, and many other bivalves remain fixed in one place for all or most of their lives; others, including clams, use a muscular "foot" to burrow into sand or mud. Scallops are the gadabouts of this group, propelling themselves from place to place by flapping their hinged shells.
Bivalves have two symmetrical shells, which they open and close with powerful muscles. Large gills inside these shells siphon and expel water, extracting food and oxygen. Stories abound of pearl divers who have drowned when a giant clam's shell has snapped shut on one of their legs, but there have been no documented fatalities. The giant clam reaches more than 1 m (3.3 ft) wide and can weigh more than 250 kg (550 lb).
It may be hard to believe, but pearls are actually made by irritated oysters! A pearl forms when an oyster secretes calcium carbonate around a grain of sand or some other irritant that has become lodged inside the animal's shell. Although all bivalves can produce pearls, the finest come from inedible oysters. Some people grow (or "culture") pearls by implanting a substance into oysters.
Because so many larger creatures feed on them, mollusks need to use all the tricks they can to avoid being devoured. The eyes of scallops can detect motion, while clam eyes are able to pick up light changes. Both close their shells like the shutter on a camera lens when they sense the presence of a predator. Look at the dozens of blue eyes this scallop has around the edges of its shell!
The farming of fish, shellfish, and seaweed, known as mariculture, probably began centuries ago in China. Today Southeast Asia still produces the most aquatically-farmed products, but with dwindling supplies of marine life everywhere, mariculture is becoming popular throughout the world.
Swimming scallops — A flame scallop (Lima scabra) extends its vivid red tentacles, showing how these bivalves jet-propel themselves through the water.
Source: Microsoft Oceans (1995) CD-ROM. Text liberated from original screen art; images, audio & clips restored from disc. Original media is Microsoft/supplier copyright — non-commercial educational preservation. Credits & Acknowledgements