PREDATION OF INFAUNAL BIVALVES BY Carcinus maenas IN THE BURRY INLET, SOUTH WALES

Abstract

Abstract : Laboratory experiments and field studies in the Burry Inlet were designed to investigate the impact of crab predation on the population of burrowing bivalves. Sampling of Carcinus maenas in the inlet indicated that significant numbers of adult crabs move onto the sand banks at every flood tide for all but the coldest months of the year, but it proved difficult to estimate actual numbers. Settlement of first-stage crabs occurs in July. These young crabs live in nursery areas near HWNT until about one year old. Stomach contents analysis demonstrated that large C. maenas feed mainly on infaunal bivalves. The food priority varied from one season to another and between juveniles and adults. Laboratory experiments were conducted on the predatory behavior of crabs on four infaunal bivalve species. Methods used by crabs to open the bivalve shells are described and shown to be related to shell strength. For any given crab size, there was an optimal prey size where the prey value was at a maximum. Other experiments showed that crabs selected prey of a size close to the predicted optimum for both Cerastoderma edule and Macoma balthica, even when they were buried in the sand. Burrowing behavior of both M. balthica and Scrobicularia plana was found to significantly reduce crab predation, but it had little effect on the shallow burrower C. edule. Crabs preyed heavily on those bivalves that occurred close to the sand surface. When crabs of similar size, small specimens of three species of bivalves buried in sand, they actively selected cockles. Protection of small cockles from various predators by the use of cages in the field decreased their mortality It was concluded that crabs must be considered serious predators of first and second summer cockles