Solar orientation of adult and laboratory-born juvenile sandhoppers: inter- and intra-population variation

Abstract

Abstract Sun orientation of the sandhopper, Talitrus saltator (Montagu, 1808) (Crustacea, Amphipoda), was studied in four populations from Mediterranean and Atlantic coasts. All the experiments were performed inland, far from the sea, under controlled conditions in an experimental arena. Variation at the population and individual level was studied by testing repeatedly (five times) adult individuals collected from the shore. Breeding pairs were selected, and the orientation of the first generation progeny of each breeding pair was analysed to check the inheritability of orientation behaviour. Marked differences among the populations were shown in the orientation of adults: the sandhoppers of Castiglione della Pescaia (Tuscany, Italy) oriented seawards, while Newborough (Anglesey, Wales, UK) adults moved towards the sun. In contrast, both the San Rossore (Tuscany, Italy) and Marina di Lesina (Apulia, Italy) adults did not show any directional tendency. In all four populations female orientation was more directed than males. The orientation of the progenies varied within each population: some families concentrated seawards, others moved in the direction of the sun, while most were random. These behavioural trends were shown in different proportions by each population. During the five subsequent trials of each individual, those juveniles belonging to the more directed families were more constant in their orientation than those from families which showed random orientation. Analysis of each family showed that, irrespective of the general population dispersion, the juveniles generally did not orient in a random manner, but individually oriented towards a precise direction. Consequently, a preferential direction towards the sea does not systematically emerge in juveniles, but the individual’s experience is necessary to develop a precise orientation strategy. Moreover, only in some populations did a phototactic tendency toward the sun seem to be inheritable as an underlying mechanism of sun compass. The differences between orientation of populations may be interpreted on the basis of different ecological pressures, such as tides, amount of detritus and human disturbance, prevailing at the localities of origin.