Impacts of the Toxic Dinoflagellate Alexadrium monilatum on Three Ecologically Important Shellfish Species

Abstract

Little is known about interactions between shellfish and Alexandrium monilatum (Howell) Balech, a toxigenic dinoflagellate that forms blooms mostly in the Gulf of Mexico. Toxic A. monilatum produces endotoxins with hemolytic and neurotoxic properties, and has been linked to major fish and invertebrate kills. The responses of three ecologically important shellfish species to A. monilatum (toxic strain AMO3) were experimentally assessed. In the first set of experiments, grazing studies were conducted with adult and juvenile eastern oysters (Crassostrea virginica Gmelin), northern quahogs (Mercenaria mercenaria Linnaeus), and green mussels (Perna viridis Linnaeus), which inhabit areas where A. monilatum blooms occur. Clearance rates of each shellfish species were depressed when exposed to toxic A. monilatum (bloom density of ˜550 cells ml-1) alone or with nontoxic Instant Algae® Pavlova, in comparison to clearance rates of control animals fed benign cryptophyte algae. There was also a reduction in the clearance rate of adult and juvenile C. virginica and P. viridis, as well as juvenile M. mercenaria exposed to A. monilatum, in comparison to control animals that were exposed to a nontoxic strain of a dinoflagellate of similar size, Alexandrium tamarense (clone CCMP115). Exposure to toxic A. monilatum significantly decreased shellfish valve gape in adult P. viridis and C. virginica. Intact A. monilatum cells were found within shellfish feces, but A. monilatum cells did not divide following passage through the gut. In the second set of experiments, survival of larval M. mercenaria and C. virginica was tested when the larvae were exposed to A. monilatum as intact cells, cells held in dialysis tubing, or sonicated cells. Survival of larvae was significantly less when exposed to sonicated A. monilatum, in comparison to survival of control larvae that were tested with nontoxic A. tamarense. Overall, these data indicate that A. monilatum blooms can adversely affect survival of some shellfish species by reducing clearance rate and valve gape, affecting food intake, and inducing larval mortality.