A biomechanical model of rock drilling in the piddock Barnea candida (Bivalvia; Mollusca)

The bivalve Barnea candida (Pholadacea) makes its burrow in clay, soft rock and peat. Barnea has developed a number of adaptations to accommodate this lifestyle. Four muscles enable burrowing. These are situated around a dorsal pivot in such a way that the piddock is able to rotate the shells around two approximate orthogonal axes. The anterior adductor muscle anterior (AAM-A) and the posterior adductor muscle rotate the shells around a dorso-ventral axis; the anterior adductor muscle posterior (AAM-P) and the ventral adductor muscle rotate the shells around an antero-posterior axis. The AAM-A and the AAM-P have evolved from a single anterior adductor muscle and are attached to a piece of the shell that is folded inside out, the umbonal reflection. At the dorsal side of the piddock, the shell margins are reduced. This prevents collision of these margins during movement. Electrical stimulation experiments revealed that the opening of the antero-ventral side of the piddock is faster than its closure. These results were incorporated into a computer model that could simulate shell movements. The computer model allowed predictions about the shapes of burrows and scrape marks. As in Nature, simulated burrows had a long droplet shape with straight scrape marks.