Behavioural and neurohistological changes in aging Sepia.

The life cycle of the cuttlefish in the English Channel is characterized by a succession of homogeneous population cohorts. These conditions provide an excellent opportunity for the study of aging in this cephalopod. In a first longitudinal study, we considered the oldest animals and compared their success rates at the first capture attempt. During the first weeks of the study, the results remained constant and then, during the weeks immediately preceding the natural death, a dramatic drop was observed. This deterioration may be due to defects of visuomotor coordination. In a second study, we used an associative learning protocol with negative reinforcement and the performances of young and old animals were compared. The most striking results showed that the performances of the oldest animals during the retention test were very mediocre. Such results suggest that the long-term memory process is affected. Finally, a modification of the Fink-Heimer silver stain enabled us to draw a map of spontaneous terminal degeneration in the central nervous system of the oldest animals. The structures which are characterized by the presence of multimodal inputs (the spine of the peduncle lobe and the basal lobes) present the most obvious signs of degeneration.