Comparative effects of the toxic dinoflagellate Karenia brevis on clearance rates in juveniles of four bivalve molluscs from Florida, USA.

The effects of Karenia brevis (Gymnodiniales, Gymnodiniaceae) on the feeding activity of juveniles of four species of bivalve mollusc were examined in the laboratory to assess the potential impacts on these important shellfish populations from Florida. Clearance rates were determined under short-term (one hour) static and long-term (two days) flow-through conditions using both whole and lysed cultures of K. brevis. Under short-term conditions, the bay scallop, Argopecten irradians, was the most sensitive species, exhibiting a 79% reduction in clearance rate at 1000 cells ml(-1) of whole K. brevis culture compared to the control (no K. brevis). The eastern oyster, Crassostrea virginica, was the least responsive, showing a 38% reduction in clearance rate between the same treatments. The green mussel, Perna viridis, and the northern quahog, Mercenaria mercenaria, displayed intermediate responses. Similar results were also observed during long-term exposures to a continuous supply of K. brevis. Bay scallops showed a significant decline in clearance rate at 100 cells ml(-1) after 24h exposure; clearance rate of oysters was not affected by K. brevis at this concentration. No mortality was observed for any species during these brief exposures. The prospect for recovery of bay scallop populations in Florida estuaries where they were once abundant may be hampered by recurring blooms of K. brevis. Reduced clearance rates in M. mercenaria at high K. brevis densities could translate into poor growth of cultured Florida hard clams. On the other hand, P. viridis, which also showed reduced clearance rates at high K. brevis concentrations, might be negatively impacted by K. brevis blooms, thereby affecting their ability to spread into estuaries hampered by recurring toxic algal blooms.