Leaping and other movements in some cardiid bivalves

Abstract

Burrowing in Cardium echinatum, Laevicardium crassum and Cardium edule involves similar integrated cyclic movements, in which the active stage of downward movement is that of contraction of the pedal retractor muscles. Final position in the sand is reached only after several repetitions of the digging cycle comprised in a digging period. Digging cycles in the three species differ in the timing of the various stages. The species also differ in the number of cycles comprised in a digging period. These differences may be related to shell form. Recovery from deep burial occurs by similar cyclic movements, but modifications in anchorage and in the time and duration of the forces acting on the shell cause upward movement. The stage of protraction and straightening of the foot is the active stage of recovery. Leaping in C. echinatum and in L. crassum involves also cyclic movements, the leaping cycle, and occurs in response to starfish contact. The species differ in the number of repetitions comprising a single response and in the duration of various stages of the cycle. In burrowing, recovery and leaping succeeding cycles in a single period occupy progressively longer time intervals.