The mechanism of boring inZirphaea crispata(L.) (Bivalvia: Pholadidae)

Abstract

The main activity during boring byZirphaea crispataconsists of the cyclical repetition of a group of movements, termed the boring cycle. Each boring cycle comprises the retraction of the shell to the base of the burrow, and the abrasion of the walls of the burrow by movements of the shell caused by the consecutive action of the posterior and anterior adductor muscles, supplemented by an accessory ventral adductor muscle. Each boring cycle is followed by slight anticlockwise and clockwise rotation of the animal in the burrow, while simultaneously the siphons are withdrawn and re-extended. A second type of rotational movement, resulting from changes in the position of the foot in the burrow, occurs over a longer time period, so that a circular, drop-shaped burrow is formed. The material abraded from the base of the burrow is collected into the mantle cavity and ejected as pseudofaeces from the inhalant siphon at intervals during boring. The pressures developed in the mantle cavity and haemocoele during boring are small compared with those generated by burrowing forms. During the boring cycle, low pressures (2 to 3 cm) serve to press the foot against the wall of the burrow where adhesion is aided by mucous secretion and by the action of a counter pressure from a pad of mantle tissue dorsally. Fluid is retained in the foot, and in the expanded mantle margins within the spaces of a loosely arranged connective tissue which fills these organs. The fluid filled mantle cavity and haemocoele allow the siphonal retractor muscles to act partly in antagonizing the adductor muscles, so that withdrawal of the siphons during boring restores the gape of the shell. Higher pressures (8 cm) are developed in the mantle cavity and haemocoele during the contraction of the adductor muscles and circular muscles of the siphons which is involved in the expulsion of pseudo-faeces. The tensions exerted by the pedal muscles during boring are small (2 to 2·5 g).