Diel variation of visual response in Talitrus saltator and Talorchestia deshayesii (Crustacea: Amphipoda) from high latitude beaches of low tidal amplitude

The responses to a white directional light and a black stripe covering 60° of the horizon were studied in two Swedish populations of the amphipod species Talitrus saltator (Montagu, 1808) and Talorchesia deshayesii (Audoin, 1826) from two beaches on the southern Baltic Sea. Adult individuals were caught in the field and tested in constant laboratory conditions in order to investigate the possible variation of the responses during the 24 h cycle, and then to compare their behaviour with that of the Atlantic and Mediterranean populations, already known in literature. Previous studies explained the observed differences between Atlantic and Mediterranean populations with the differences in tidal excursions and did not take into account the different climatic conditions. The Baltic coasts, like those of the Italian Mediterranean, are not tidal and present prevalent low mean temperatures and high humidity throughout the year like those of Britain. The response to the artificial white light remained photopositive throughout the 24 h cycle in both the species sampled, although its intensity was lower during the day than at night in Talitrus saltator. The response to the black stripe showed an inversion from positive to negative during the 24 h cycle, the animals being directed towards the black stripe during the day and away from it at night; this response was more intense and clear cut in Talorchestia deshayesii than inTalitrus saltator. The pattern of response to the black stripe shown by the Swedish talitrids, differed from both that of the British and Italian populations, and was interpreted as a basic response related to the diel rhythm of emergence and burrowing.