Impacts of harmful algal blooms on physiological and cellular processes of bivalve molluscs

Harmful algal blooms (HABs) are increasingly recognized as having profound effects upon ecology of coastal seas and upon economics of fisheries and aquaculture. Further, HABs are included in a list of concerns about changes in marine ecosystems that increase impacts of diseases and parasites on important resource species and the food webs that support them. This research assessed the interactions between HABs and bivalve molluscs. A number of specific HAB-bivalve interactions were studied, measuring fundamental physiological processes such as clearance and filtration rates of bivalves when exposed to pure cultures of HAB species or mixed benign and HAB cultures. Results indicate that clearance and filtration rates, as well as biodeposit production, were species specific. Further studies demonstrated the presence of intact HAB cells in the biodeposits with the ability to recover, suggesting potential risks for bivalves to be vectors of introduction of HABs into new environments. A simple method for mitigating this risk was demonstrated: keeping shellfish out of water for 24 hr, or depurating at least 24 hr in quarantined seawater renders cells non-viable. ^ Physiological responses of bivalve molluscs to HABs varied according to the algal/mollusc combination. This study further investigated whether these physiological interactions could be related to hemocytes and their involvement in a response of the bivalve to toxic algae. Specific bivalve-algal pairs were selected according to the type of physiological interaction observed. In-vitro experiments exposing bivalve hemocytes to cultured harmful algal cells, followed by in-vivo exposures of the whole animal to cultures of harmful algae showed that these algae cause immuno-modulation in bivalves. Several harmful algal species caused immuno-suppression; whereas, others activated protective, hemocyte immune functions. ^ Combined effects of HABs and parasites on physiological and immunological parameters of bivalves were demonstrated; exposure to a HAB can change the susceptibility of shellfish to diseases and modulate the host-parasite interaction, favoring the host or the parasite. These findings highlight the importance of considering multiple environmental factors when assessing the immunological and histopathological status of bivalve molluscs. ^ Interactions between harmful algae and bivalve molluscs are complex, species specific and can have major impacts on the interactions of bivalves with their environment.