Functional Morphology of Perihelia conradi Relative to Shell-Penetration

The pholad, Penitella conradi, is found along the California coast in the calcareous shell of the abalone, Haliotis rufescens. These pholads penetrate the abalone shell, and when they break through the inside of the shell they cease boring, secrete a callum, and then become sexually mature. The normal adult is a stenomorphic form,defined by Bartsch as an animal whose sexual maturity is induced by over-crowding or insufficient substratum in which to bore. In the case of P. conradi, sexual maturity is always induced by the spatial limits of the substratum, that is, the relatively thin abalone shell.

The role of mechanical abrasion by the valves of P. conradi is minor. Experiments indicate that the teeth of P. conradi are worn at a greater rate than the polished shell of the abalone.

The boring process in P. conradi proceeds mainly by chemical dissolution of the calcareous substrate. The pathway of the solvents is unknown. It may be through the organic matrix, or the solvent may react directly with the crystals. Mechanical abrasion helps to remove loosened crystals and/or organic matrix which are then carried to the exterior by the ciliary currents flowing in through the pedal gape and out through the exhalent siphon.