The biology and functional morphology of the coral‐boring Jouannetia cumingii (Bivalvia: Pholadacea)

The coral-boring bivalve Jouannetia cumingii (Pholadacea) has been investigated from Phuket, Thailand. It only bores dead heads of Porites lutea and the siphons open, not on to the general coral surface, but into galleries created by other borers and inhabited by cryptobion. The bivalve may thus have a relationship not with the coral, except as a substratum, but with these other gallery occupants. The juvenile shell is anteroposteriorly reduced, the anterior face represented only by dorsally reflected shields between which extends the anterior adductor muscle. A considerable area of the anterior mantle is thus exposed to the burrow heading and there is a small pedal gape through which can be extended a large foot. This is absent, however, in the adult. The large posterior adductor is located between shell buttresses, one on the posterior face of each valve to reduce shear. Also attached here are small posterior pedal retractors. Anterior retractors are located on condyles projecting downwards from the hinge plate which is sunken and covered by the umbones and reflected anterior shell shields. The ligament is small, inequilateral, and covered by periostracum. A normal complement of mantle cavity organs is present but is reduced and posteroventrally-anterodorsally aligned. Gill ciliation is of type C(1) and the labial palps are large. Mechanical boring, despite anterior shell teeth, is impossible and a chemical boring process is proposed from a large gland located internal to, and discharging via, the pedal gape. Secretion cannot be from the outer surface of the exposed mantle as this, because of extensive mantle fusion, is covered by periostracum. In the adult, a callum is formed over the exposed anterior regions of the mantle. Callum formation and sexual maturity occur over a wide size range and is related, as in other pholads, to an inability to bore further. The callum is not an extension of the true shell and is thought to be secreted from pallial glands located on the internal surface of the mantle and discharging via the pedal gape. As with boring, secretion cannot be from the exposed general mantle surface. The callum of the left valve is much larger and external to that of the right. Left callum formation must take place first, against the heading of the boring. Right callum formation must take place subsequently, largely against the left. Dorsally, the callum covers the anterior shields and all areas of exposed mantle. The unequal callum is a consequence of the mode of shell adduction in which the valves rock about dorsal and ventral fulcra: an equal callum would not permit this, the unequal form does, the right callum sliding beneath that of the left. Adult J. cumingii are almost perfectly spherical except for a spiked siphonoplax developed on the posterior face of the right valve only, by enhanced growth of shell lamellae. The spherical shell form in Jouannetia may be a consequence of this animal's more intimate association with the coral community which, in providing protection, obviates the need for deeper burrowing and thus an elongate shell form.