EFFECTS OF TOXIC DINOFLAGELLATES ON CLEARANCE RATES AND SURVIVAL IN JUVENILE BIVALVE MOLLUSKS

Feeding and survival experiments using unialgal cultures of the toxic dinoflagellates, Alexandrium (=Protogonyaulax) tamarense, and Gyrodini um aureolum, were conducted on several species of juvenile bivalve mol luscs. These experiments were designed to assess the potential impact of toxic algal blooms during the ''grow-out phase'' for the faster-gro wing juvenile stages. Mortality of juvenile bivalves after exposure to toxic dinoflagellates was dependent upon time after exposure and temp erature during exposure, suggesting species specific patterns and an o verall higher toxicity of Gyrodinium aurelum during both the winter an d summer experiments. Feeding rates on unialgal cultures of toxic dino flagellates during the winter of 1989 were uniformly low, and are corr elated with the lower mortality observed in the survival experiments. Preference for the non-toxic microalgae, Isocrysis sp. was significant during these experiments for all bivalves except Placopecten magellan icus, which probably reflects more on the size of Isochrysis sp. and t he functional morphology of the ctenidia of this species. Experiments conducted in the spring of 1990 reveal species-specific patterns which in some cases mirror the winter experiments. Other bivalve species sh ow a significant preference for toxic dinoflagellates that is not alwa ys correlated with the survival experiments suggesting that some speci es can ingest and utilize toxic dinoflagellates without short-term eff ects.