Bioactive effects of Prorocentrum minimum on juvenile bay scallops (Argopecten irradians irradians) are dependent upon algal physiological status

The harmful dinoflagellate Prorocentrum minimum reportedly has variable toxicity to grazing animals. We used the bay scallop Argopecten irradians irradians as a bioassay organism to compare expression of harmful effects of P. minimum cultures in growth or senescent phases. The non-toxic alga Rhodomonas sp. was used as a control. Exposure to both types of P. minimum cultures decreased the degree of shell opening, amount of biodeposits produced, motility and byssal-thread attachment. As P. minimum cultures approached senescence, effects became more severe, and mortality increased to 15% in scallops exposed to senescent P. minimum . Pathological effects of P. minimum on scallops included derangements of scallop digestive tubules and the adductor muscles, and abnormal hemocyte distributions, which were more severe in scallops exposed to senescent cultures. These findings help to explain the variable toxicity of P. minimum to scallops and other bivalves reported in the literature. Further, these findings demonstrate that the definition of a “harmful algal bloom” should probably be expanded to include physiological status along with identity and abundance of a phytoplankton species.