Boring and burrowing mechanisms in Petricola pholadiformis Lamarck

Abstract

Both burrowing into sand and boring into clay or chalk by Petricolapholadiformis involves the same series of movements in a digging cycle, repeated over an extended period. During burrowing, digging cycles at first occur at intervals of about 30 sec, but as greater resistance is encountered this time interval increases. Initially, during each cycle, pressure recordings from the mantle cavity show a single pressure pulse of up to 10 cm water associated with shell adduction and ejection of water from the mantle cavity. Later there is a second pulse of up to 20 cm water, associated with a secondary cycle of withdrawal of the foot and siphons following each downward movement. Exactly similar events occur during burrowing in the related species Venerupis pultastra. The forces causing the secondary pressure pulse serve to assist the ligament in opening the shell against the resistance of the surrounding sand. Similar pressure pulses occur during boring into clay, when the higher pressures exerted during the secondary cycle serve to consolidate the walls of the burrow. The main abrasive action during boring occurs when the posterior pedal retractor muscle contracts, forcing the anterior margins of the shell, which bear chisel-like projections, downwards and across the base of the burrow.