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The effects of the toxic dinoflagellate, karenia brevis (Wilson clone), on larval survival and development of the northern quahog (=hard clam, Mercenaria mercenaria), eastern oyster (Crassostrea virginica) and bay scallop (Argopecten irradians) were studied in the laboratory. The effects of K. brevis on feeding activities of juveniles from these species plus the green mussel (Perna viridis) were also examined. Finally, adult bay scallops were exposed to K. brevis for two weeks to investigate possible cytotoxic effects. Survival of 3-day-old larvae was generally > 85% for all shellfish species at Karenia brevis densities of 100 cells. ml-1 or less, and not significantly different between whole and lysed culture. At 1,000 cells. ml-1, survival was significantly less in lysed culture than whole culture for both M. mercenaria and C. virginica. Survival of 7-day-old larvae in all species was not significantly affected at densities up to 1,000 cells. ml-1. At 5,000 cells. ml-1, however, survival was reduced to 37, 26 and 19% for A. irradians, M. mercenaria and C. virginica, respectively. Development of C. virginica and M. mercenaria larvae was protracted at K. brevis densities of 1,000 cells. ml-1. Clearance rates of juveniles were determined under static and flow-through conditions using whole and lysed cultures of K. brevis. The bay scallop was most sensitive, exhibiting a 79% reduction in clearance rate at 1,000 cells. ml-1 of whole culture. The eastern oyster was least responsive, showing a 38% reduction in clearance rate between the same treatments. The green mussel and the northern quahog displayed intermediate responses. Similar results were observed during longer (2 day) exposures to a continuous supply of K. brevis. Bay scallops showed a significant decline in clearance rate at 100 cells. ml-1 after 24 hr 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. Adult bay scallops exposed to K. brevis for two weeks showed degenerative and inflammatory changes in the digestive gland, including reduced thickness of the epithelium, increased size of digestive tubule lumens and hemocytic infiltration. The prospect for recovery of bay scallop populations in Florida may be hampered by recurring blooms of K. brevis. |
