Reduced survivorship of Himasthla (Trematoda, Digenea)-infected cockles (Cerastoderma edule) exposed to oxygen depletion

Bivalve populations from inshore waters often accommodate a diverse tremato de fauna that may have a variety of effects on host specimens. In particula r, larval trematodes that grow or reproduce within their host are known to be severe pathogens, whereas trematodes utilising bivalves only for encystm ent are thought to be relatively benign. Yet this may depend on the environ mental conditions, and it can be expected that such trematodes in concert w ith other stress agents can be detrimental to host organisms. To examine th e impact of such larval trematodes on hosts subjected to stress, we studied the digenetic trematode Himasthla elongata and one of its second intermedi ate hosts, the bivalve Cerastoderma edule. Experimentally infected cockles and non-infected cockles were exposed to oxygen depletion, whereupon we mea sured their burrowing ability and survivorship. After 30 h of hypoxia, the survival of infected cockles was significantly reduced compared to non-infe cted cockles, whereas no effect of parasites on cockles under normoxic cond itions was found. In addition, parasites tended to reduce the burrowing abi lity of cockles exposed to hypoxia but the effect was not clear. The effect of parasites and possible ecological consequences are discussed and it is suggested that the combined effects of parasites and oxygen deficiency may explain some hitherto unexplained cases of mass mortalities in bivalve popu lations. (C) 1999 Elsevier Science B.V. All rights reserved.