Variation and ontogeny of the response to visual stimuli in Mediterranean populations of Talitrus saltator (Montagu) (Crustacea-Amphipoda)

The responses to a white directional light and a black stripe covering 60 degrees of the horizon were studied throughout the 24 h cycle in constant conditions in three Mediterranean populations of Talitrus saltator (Montagu): a Ligurian (San Rossore), a Tyrrhenian (Burano) and an Adriatic one (Lesina). Both adult individuals caught in the field and young born in the laboratory were tested in the same conditions in order to investigate the inherited behavioural patterns and the effect of experience in the natural environment. The results are: (1) In sandhoppers, phototaxis and scototaxis are separate behaviours, not different aspects of the same behavioural response. This is demonstrated by the fact that Mediterranean Talitrus saltator shows positive scototaxis during the same phase of the cycle when it is also highly photopositive to a punctiform light. (2) The two taxes are inherited because they are more marked in laboratory-born young individuals than in adults. (3) Young individuals from different populations show differences in their rhythmic responsiveness indicating that there is an intraspecific genetic variability even in such a basic behaviour. (4) Mediterranean adult specimens of T. saltator show similar behaviours indicating a general similarity in the lifestyle and epigenetic adaptation of adult talitrids along Mediterranean coasts. (5) Mediterranean sandhoppers show an almost reversed pattern of change in taxes compared with Atlantic ones. The difference is probably due both to genetic distance and adaptation to markedly different environments. It is suggested that the reversal of scototaxis shown by the young is a basic pattern which may be modified by ecological constraints that vary on different beaches, such as the tidal regime and the amount and zonation of stranded detritus, both of which determine the migration pattern. (C) 1997 Elsevier Science B.V.