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 dominan t bivalves (the edible cockle Cerastoderma edule, the blue mussel Mytilus e dulis, and the Manila clam Tapes philippinarum) on an intertidal sandflat i n the Bay of Arcachon (French Atlantic coast) revealed the occurrence of a rich community of macroparasites. These parasites belong to digenean tremat odes (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 musse ls (max. = 5 per ind.) and only 1.7 in clams (max. = 4 per ind.). Total par asite prevalence was higher ill cockles and mussels (84.9 and 72.3 %, respe ctively) than in clams (49.7 %). Host specificity was much higher for non-e ncysted larvae than for encysted metacercariae or non digenes. The occurren ce of sporocysts of the digene Labratrema minimus in cockles seems to induc e 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/paras ite intensity' relationship, the macroparasites are highly suspected to inc rease the mortality of recruits and adult bivalves. (C) 2000 Ifremer/CNRS/I RD/Editions scientifiques et medicales Elsevier SAS.