Effect of air exposure on lysosomal tissues of Mytilus edulis L. from natural intertidal wild beds and submerged culture ropes.

Blue mussels collected from suspended culture ropes and from three natural intertidal wild beds from different areas of the German Bight were tested for their ability to cope with hypoxic conditions. During the experiment mussels were exposed to air from 0 to 72h. Mussels from all sampling sites displayed high tolerance to aerial exposure with moderate levels of mortality after 12 to 48h of exposure. Lysosomal membrane stability (LMS), a biomarker of general stress, changed notably between minimum values after 12h and maximum values after 24h of aerial exposure in intertidal mussels. In contrast, labilization times of mussels from the hanging culture increased continuously up to 48h of exposure. Intertidal mussels from the island of Heligoland exhibited significantly decreased membrane stability after 72h of air exposure, correlating to higher mortality rates. Intertidal mussels, although adapted to daily aerial exposure in their natural environment, showed a similar pattern of mortality and lower LMS values during the experiment than mussels from the suspended culture site. The increase of LMS values of mussels under hypoxic conditions at the beginning of the experiment at all sites was tested for the influence of macro-autophagic processes using immune labelling techniques. With this approach it could be demonstrated that high LMS values significantly correlate with low autophagic activity. However, hypoxic conditions do not enhance autophagic processes during the early periods of aerial exposure. Only at the end of the experiment, high values for autophagy were measured in mussels from an intertidal site accompanied with high mortalities. The results indicate that autophagic processes are not involved in the early adaptive processes that enable the mussel to cope with periods of aerial exposure.