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.

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