A Comparative Study of Bivalves Which Bore Mainly by Mechanical Means

Abstract

This account of the boring mechanisms of those bivalve groups which bore mainly by mechanical means attempts to show partly by reference to published accounts of boring and partly from our own recent observations of certain characteristics of the boring process in the Pholadidae and Petricolidae, that in contrast to the movements of burrowing forms from which originally all the boring movements derive, the process of boring makes few demands on the hydrodynamic system of the bivalve. The characteristics of the boring process are closely related to the movements in modern forms having epifaunal or infaunal habits, supporting the suggestions of Yonge (1963) concerning the origin of this habit in the Bivalvia. In all groups in which boringis mechanical, the shell forms the boring tool. However, in those groups in which boring has its origin in the epifaunal habit, the major force applied to the shell in abrading the burrow isprovided by contractions of the pedal or byssal retractor muscles. In the Adesmacea alone, where boring has been derived from a deep burrowing habit, the adductor muscles provide the major force in abrasion, and the basic digging cycle has become specialized by the addition of the rocking action of the valves which succeeds retraction. In the former group the ligament is retained and provides the strong outward force with which the shell is held against the wall of the burrow. In the latter group, the ligament is reduced, allowing the valves to rock, but here the reciprocal action of the adductors allows the valves to diverge anteriorly as the large posterior retractor muscle contracts. In the more specialized species, water pressure plays a minor role, the maximum pressures recorded being associated with actions subordinate to those involved primarily in abrasion, such as rotation in the burrow or expulsion of debris from the burrow aspseudofeces. The least specialized borers, such as Petricola, resemble burrowing forms in the importance of the hydrodynamic role of the body fluids. In all groups there is a tendency for hypertrophy to take place in the muscles which produce the main boring effect, and for their action to be applied with maximum mechanical advantage against a fulcrum provided in most cases by the foot.