The mechanisms of boring in Martesia striata Linné (Bivalvia: Pholadidae) and Xylophaga dorsalis Turton (Bivalvia: Xylophaginidae)

Penetration of timber by the wood-boring bivalves Martesia striata and Xylophaga dorsalis is effected by means of the cyclical repetition of a group of movements termed the boring cycle. In Martesia the boring cycle comprises first the retraction of the shell to the base of the burrow, followed by the abrasion of the wall of the burrow by movements of the shell caused by a single consecutive contraction of each of the adductor muscles. In Xylophaga similar movements are involved, but the boring cycle in this species has become elaborated by repetition of the contractions of the adductor muscles which may be repeated to give a series of up to 24 rocking movements of the shell about a dorso-ventral axis. In both species the boring cycle may be followed by movements involving anti-clockwise and clockwise rotation in the burrow, while simultaneously the siphons are partially withdrawn and re-extended; in both, longer term rotations in the burrow result in the production of a drop-shaped burrow of circular cross-section. In both species the material abraded from the base of the burrow is collected into the mantle cavity; in Martesia it is then ejected as pseudofaeces through the inhalant siphon at intervals during boring, while in Xylophaga a larger proportion passes into the gut and eventually collects in the form of faecal pellets to form a plug to the burrow. The pressures developed in the mantle cavity and haemocoele during boring are small compared to those in burrowing forms, but of the same order as those recorded from the related rock-boring pholad Zirphaea crispata, and it is concluded that the body fluids play a decreasing hydraulic role as specialization for boring increases.