Borers in the Shell of the Sea Scallop, Placopecten magellnnicus

Abstract

Shells of the sea scallop, Placopecten magellanictis Gmelin from Newfoundland waters were examined for borers by direct stereomicroscopic and X-ray radio-graphic means. Young shells are first attacked by the boring sponge, Cliona vastifica, and the spionid polychaetes, Polydora websteri and Polydora concharum. The former settles almost exclusively on the lower valve, but as it grows it often spreads to the upper valve via the hinge region. The spionids settle on the upper valve or occasionally on the periphery of the lower valve. Older shells are bored by the cirratulid polychaete, Dodecaceria concharum, which usually settles in empty Polydora burrows and enlarges them as they grow. The bivalve, Hiatella arctica, settles in Cliona holes. The burrows formed by the borers can be recognized on the radiographs. However, the identity of the present inhabitant cannot be predicted with accuracy because the original borer is often replaced by nestlers. The rale of growth was documented by making sequential radiographs at monthly intervals from May to October 1968. Rate of growth in all forms appears to be temperature-dependent. The water temperature increased from 1°C in May up to 18°C in August. Polydora concharum and P. websteri grew more rapidly from July to October than in Mayand June, while Cliona and Dodecaceria only grew during the months of July to October.