Influence of incubation conditions on the anoxic survival of marine bivalves. Static and semi-static incubations

In a comparative study of 4 bivalve species we show that the apparent widely different tolerances in survival time observed in a closed system filled with N2-gassed seawater is mainly due to the experimental conditions. Both a high dose of cadmium and the antibiotic chloramphenicol increase survival time 2to 4-fold. Without precautions for bacterial growth, the survival time of the most tolerant species, Scapharca inaequivalvis, is about 4 times longer than that of the most sensitive species, Venus gallina. The other bivalves Tapes philippinarum and Mytilus galloprovincialis, have intermediate survival times. When proliferation of bacteria is prevented by chloramphenicol, the survival time of S. inaequivalvis remains the highest; however under these conditions the other 3 species show similar survival times. Bacteria cause a drop in pH of the incubation medium and the accumulation of sulphide. S. inaequivalvis exhibits a significant decrease in survival time in the presence of exogenous sulphide. The antibiotic postpones the accumulation of sulphide, but a significant population of sulphate-reducing bacteria is able to survive in its presence. In the case of V. gallina and M. galloprovincialis, exogenous sulphide (up to 400 μM) does not affect the improved survival time due to the antibiotic. This holds also for a decrease in pH. Bacterial toxins other than sulphide or direct action on the integrity of the organism must be responsible for the adverse effects on survival. The responsible micro-organisms are associated with the bivalves, and anoxia induces their proliferation. For this reason, accumulation of bacterial noxious products cannot be avoided by frequent renewal of the medium.