Testing the Habitat Safety Hypothesis with behavioural field experiments: amphipod orientation on sandy beaches with contrasting morhodynamics

Abstract: Understanding behavioural adaptations in sandy beach macrofauna has been a major step in theoretical development of sandy beach ecology. In particular, behavioural studies in supralittoral amphipods have been useful in understanding macrofaunal responses and adaptations to environmental changes in these dynamic ecosystems. The predictions of the Habitat Safety Hypothesis (HSH) were tested for the first time on Atlantorchestoidea brasiliensis through orientation experiments carried out on 2 sandy beaches, one dissipative and the other reflective. Orientation patterns of A. brasiliensis followed the trend predicted by the HSH: even though sandhoppers oriented seawards on both beaches using a sun compass, the orientation on the reflective beach was more precise than on the dissipative one. Thus, stability conditions in the supralittoral zone of the reflective beach favoured a higher precision of the sun compass. The precision of seawards orientation on the dissipative beach increased when landscape references were added to the sun compass mechanism. The response to an unexpected object was also experimentally tested by use of a visual pattern, and a higher individual variability in the orientation of the reflective beach population was found. Thus, orientation strategies in sandy beach environments seem to respond to the stability of the supralittoral zone, widening the general pattern predicted by the HSH for the supralittoral fauna to the behavioural traits of the population.