Ontogenetic changes in boring behavior by the rock-boring bivalve, Barnea manilensis (Pholadidae)

Ontogenetic changes in morphology related to boring behavior by Barnea manilensis (Philippi, 1847) were observed. The shell outline changes from round to elongate during the transition from pediveliger to juvenile stages. Along with such a morphological change, the boring style gradually changes from anterior boring in early round-shelled juveniles, where the opening of the anterior valve margin, with rotation around a dorso-ventral axis, abrades the burrow wall, to ventral boring in older and larger long-shelled individuals, which open the ventral margin with rotation around a longitudinal axis (hinge line) for abrasion. This observation, with an examination of the literature, leads to the suggestion that early juveniles of all pholads employ anterior boring. Later, many pholads continue anterior boring throughout life, whereas others gradually shift toward ventral boring. In addition, anterior boring is thought to be a primary character of pholads, and ventral boring a specialized character derived from anterior boring.