Neuroenzymatic activity and physiological energetics in Manila clam, Ruditapes philippinarum, during short-term sublethal exposure to harmful alga, Heterocapsa circularisquama.

The harmful alga, Heterocapsa circularisquama, causes recurrent mortalities of bivalve molluscs in Japan, with demonstrated hemolysis and cytotoxicity in rabbit erythrocytes, HeLa cells, and bivalve tissues. Nonetheless, the effects of exposure to sublethal cell densities on the physiological energetics of bivalves have not been investigated, nor the potential involvement of neurotoxicity. In the present study, two sets of experiments were conducted with adult clams, Ruditapes philippinarum. In the first set, the clearance rate (CR), respiration rate (RR), absorption efficiency (AE), ingestion rate (IR), and absorption rate (AR) were examined in clams exposed to H. circularisquama to quantify the scope for growth (SFG) as an indicative of the bioenergetic status of clams (5, 50, 2.5×10(2), and 5×10(2)cellsml(-1); under 15°C and 20°C). In the second set, the activity of the biomarker of neurotoxic exposure, acetylcholinesterase (AChE), was monitored following 3, 6, 24, and 48h of exposure (5, 50, 5×10(2), and 10(3)cellsml(-1), at 20°C) in gills of R. philippinarum, and compared to that in Mediterranean mussels, Mytilus galloprovincialis; a species also affected by H. circualrisquama and in which AChE activity was more extensively studied. At 15°C, CR, IR, and AR were decreased for exposures to 50-5×10(2) cells ml(-1) resulting in a significant decrease in the absorbed energy (A), and a significant decrease in SFG at 5×10(2)cellsml(-1). At 20°C, AE was null for exposures to 2.5×10(2) and 5×10(2)cellsml(-1). RR was decreased at 2.5×10(2) and 5×10(2)cellsml(-1), CR, IR, and AR were decreased at 5-5×10(2)cellsml(-1), and the AE was null for 2.5×10(2)-5×10(2)cellsml(-1) resulting in a significant decrease in the respired energy (R), but mainly in (A) especially at 2.5×10(2) and 5×10(2)cellsml(-1) decreasing the SFG over the entire range of cell density with negative values for 2.5×10(2) and 5×10(2)cellsml(-1). The activity of AChE in both clams and mussels was significantly reduced following 3-48h of exposure to 5-5×10(2)cellsml(-1). Reduction in the energy available for clams to grow and reproduce was induced mainly via decreased energy acquisition, and delayed and/or reduced functions of the digestive organs. In addition, this is the first report of decreased neuroenzymatic activity in two bivalve molluscs induced by exposure to H. circularisquama potentially via either neurotoxic compounds affecting the activity of gill cilia and/or gill muscles, and/or potentially via either neurotoxic compounds affecting the activity of gill cilia and/or gill muscles, and/or non-cholinergic affects associated with other functions; both resulting in decreased SFG. The relationship between the decreased AChE and decreased SFG should be corroborated in future research.