Temporal variation of lysosomal capacities in relation to susceptibility of mussels, Mytilus edulis, to summer mortality

We examined the temporal variation of lysosomal enzyme activities and lysos omal membrane stability in two stocks of blue mussels (Mytilus edulis L., 1 758) from the Magdalen Islands which differ in their susceptibility to summ er mass-mortality. The activity of lysosomal enzymes reflects the autolytic capacity which could be used during the transfer of reserves between stora ge and reproductive tissues or for mobilisation of reserves for energy requ irements. The peak of lysosomal enzyme activities in mantle tissue in June 1992 probably was related with autolysis of storage cells to support matura tion of the developing gametes. A second peak of lysosomal enzyme activity which occurred in mid-August of 1992 and 1993 may help to cover energetic r equirements of maintenance metabolism during this period of high temperatur es and reduced food quality. Measures of the destabilization of lysosomal m embranes in the digestive gland confirmed that mussels showed a significant stress response in mid-August. Mussels from the stock with a higher suscep tibility to summer mortality underwent a longer period of stress and accumu lated less glycogen between spawning and the stressful period in mid-August than mussels from the more resistant stock. In conclusion, our field obser vations demonstrate high activities of lysosomes not only in spawning perio ds but also during stressful periods of high temperatures and reduced food quality.