A census of macroparasites in an intertidal bivalve community, Arcachon Bay, France

Amongst the potential biotic factors affecting bivalve population dynamics, parasitism has received little attention. Examination of the three dominant bivalves (the edible cockle Cerastoderma edule, the blue mussel Mytilus edulis, and the Manila clam Tapes philippinarum) on an intertidal sandflat in the Bay of Arcachon (French Atlantic coast) revealed the occurrence of a rich community of macroparasites. These parasites belong to digenean trematodes (ten species), turbellarians tone species), cestodes tone species) and copepods tone species). The mean number of parasite species increased with shell length and reached 4 in cockles (maximum = 7 per ind.), 2.6 in mussels (max. = 5 per ind.) and only 1.7 in clams (max. = 4 per ind.). Total parasite prevalence was higher ill cockles and mussels (84.9 and 72.3 %, respectively) than in clams (49.7 %). Host specificity was much higher for non-encysted larvae than for encysted metacercariae or non digenes. The occurrence of sporocysts of the digene Labratrema minimus in cockles seems to induce a lower infection by other parasite. The youngest bivalves (less than 10 mm in shell size) were nearly free of parasites. From the 'shell size/parasite intensity' relationship, the macroparasites are highly suspected to increase the mortality of recruits and adult bivalves. (C) 2000 Ifremer/CNRS/IRD/Editions scientifiques et medicales Elsevier SAS.