 Most bivalves adopt a sedentary or even sessile lifestyle , 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 . 
