Bivalvia - Behaviour

Behaviour

Most bivalves adopt a sedentary or even sessile life style, often spending their whole lives in the area in which they first settled as juveniles. The majority of bivalves are infaunal, living under the seabed, buried in soft substrates such as sand, silt, mud, gravel or coral fragments. Many of these live in the intertidal zone where the sediment remains damp even when the tide is out. When buried in the sediment, burrowing bivalves are protected from the pounding of waves, desiccation and overheating during low tide, and variations in salinity caused by rainwater. They are also out of the reach of many predators. Their general strategy is to extend their siphons to the surface for feeding and respiration during high tide, but to descend to greater depths or keep their shell tightly shut when the tide goes out. They use their muscular foot to dig into the substrate. To do this, the animal relaxes its adductor muscles and opens its shell wide to anchor itself in position while it extends its foot downwards into the substrate. Then it dilates the tip of its foot, retracts the adductor muscles to close the shell, shortens its foot and draws itself downwards. This series of actions is repeated to dig deeper.

Other bivalves, such as mussels, attach themselves to hard surfaces using tough byssus threads made of keratin and proteins. They are more exposed to attack by predators than the burrowing bivalves. Certain carnivorous gastropod snails such as whelks (Buccinidae) and murex snails (Muricidae) feed on bivalves by boring into their shells. The dog whelk (Nucella lamellosa) drills a hole with its radula assisted by a shell-dissolving secretion. The dog whelk then inserts its extendible proboscis and sucks out the body contents of the victim, which is typically a blue mussel. The whelk needs a few hours to penetrate the shell and thus living in the littoral zone is an advantage to the bivalve because the gastropod can attempt to bore through the shell only when the tide is in.

Some bivalves, including the true oysters, the jewel boxes, the jingle shells, the thorny oysters and the kitten's paws, cement themselves to stones, rock or larger dead shells. In oysters the lower valve may be almost flat while the upper valve develops layer upon layer of thin horny material reinforced with calcium carbonate. Oysters sometimes occur in dense beds in the neritic zone and, like most bivalves, are filter feeders.

Although many non-sessile bivalves use their muscular foot to move around, or to dig, members of the freshwater family Sphaeriidae are exceptional in that these small clams climb about quite nimbly on weeds using their long and flexible foot. The European fingernail clam (Sphaerium corneum), for example, climbs around on water weeds at the edges of lakes and ponds; this enables the clam to find the best position for filter feeding.