Ultrastructural changes in the lysosomal-vacuolar system in digestive cells of Mytilus edulis as a response to increased salinity

The effects of increasing salinity on the ultrastructural morphology of the lysosomal-vacuolar system in digestive cells of the common mussel Mytilus edulis were investigated in order to relate structural changes to previous biochemical and cytochemical observations. After 3 h of increased salinity, from 21 to 35%., the digestive cells showed an increase in numbers of heterolysosomes. There was some evidence of digestive cell breakdown, the contents forming membrane-bound bodies and being released into the tubule lumen. After 12 h of increased salinity, heterolysosomes were prevalent in the digestive cells. There was increased evidence for digestive-cell breakdown, many of the tubule lumina being packed with membrane-bound bodies. It is concluded that increasing salinity from 21 to 35%. stimulates the lysosomal-vacuolar system, as a result of autophagocytosis or apoptosis; this is consistent with the hypothesis that intracellular, lysosomally-mediated, catabolism of proteins is a source for free amino acids during the adaptation of mussels to increased salinity.