Borers in the Shell of the Sea Scallop, Placopecten magellnnicus

Shells of the sea scallop, Placopecten magellanictis Gmelinfrom Newfoundland waters were examined for borers by directstereomicroscopic and X-ray radio-graphic means. Young shells are first attacked by the boring sponge, Clionavastifica, and the spionid polychaetes, Polydora websteri andPolydora concharum. The former settles almost exclusively onthe lower valve, but as it grows it often spreads to the uppervalve via the hinge region. The spionids settle on the uppervalve or occasionally on the periphery of the lower valve. Older shells are bored by the cirratulid polychaete, Dodecaceriaconcharum, which usually settles in empty Polydora burrows andenlarges them as they grow. The bivalve, Hiatella arctica, settlesin Cliona holes. The burrows formed by the borers can be recognized on the radiographs.However, the identity of the present inhabitant cannot be predictedwith accuracy because the original borer is often replaced bynestlers. The rale of growth was documented by making sequential radiographsat monthly intervals from May to October 1968. Rate of growthin all forms appears to be temperature-dependent. The watertemperature increased from 1°C in May up to 18°C inAugust. Polydora concharum and P. websteri grew more rapidlyfrom July to October than in Mayand June, while Cliona and Dodecaceriaonly grew during the months of July to October.