Marine mussels Mytilus edulis (L.) of different ages were exposed to copper (50 pg I-') for up to 6 d and then transferred back to clean seawater. Copper exposure induced lysosomal destabilisation in the digestive cells of mussels of all ages. However, the mussel's ability to restore lysosomal integrity declined with increasing age. Lipid in the digestive cells of all mussels was associated with lysosomes after 6 d of copper exposure but its distribution returned to normal in all but the oldest mussels following reimmersion in clean seawater. The lipofuscin content of the digestive glands of all the mussels remained unchanged during copper exposure but increased in the oldest mussels following reimmersion in seawater. These findings indicate that mussels of all ages display a similar susceptibility to stress, but their capacity to recover and hence adapt to environmental changes declines with Increasing age.

Marine mussels Mytilus edulis (L.) from 3 different age/size groups were exposed for 24 h to combined hypoxia and hyperthermia (28 k 1°C) and subsequently reimmersed in seawater at 10 k 1°C. Exposure to the physical stressor induced lysosomal destabilisation and enlargement in the digestive cells of mussels of all ages. Recovery of lysosomal integrity was demonstrated after 24 h in seawater by the youngest mussels (2-4 yr) only. Levels of unsaturated neutral lipids increased in all mussels followlng exposure and was associated with enlarged lysosomes No consistent pattern or significant change was observed in lipid levels in mussels of any age during the recovery period. Levels of lipofuscin (stress pigment) in mussels exposed to hypoxia and hyperthermia and in those subsequently reunmersed in seawater did not follow any consistent pattern, but were generally much higher in the youngest n~ussels than in the older age groups. Recovery of metabolic rates to control levels was observed 12 h after reimmersion in seawater in the youngest mussels only. Metabolic rates of the 6-8 and 210 yr mussels remained elevated relative to the controls after this time. These findings indicate that the stress reaction is independent of age but that the potential for recovery of lysosomal integrity and metabolic rate is age-related.