No need to hide in caves: shelter-seeking behavior of surface and cave ecomorphs of Asellus aquaticus (Isopoda: Crustacea).

Shelter-seeking is a vital behavior for stress reduction and survival in a range of animals. It comes at the cost of reduced foraging, mate finding, dispersal and territoriality, and is expected to reflect the trade-off between fitness costs and benefits. One way to test this hypothesis is to compare shelter-seeking behavior in surface habitats and in caves where external threat factors are largely reduced. We did so using the freshwater isopod Asellus aquaticus from the Postojna-Planina Cave System and surrounding surface waters. Animals from two distinct, replicated pairs of surface and cave populations were individually offered a choice between shelter and open area. The thigmotatic sensation of a transparent plastic plate was the only stimulus that could trigger the sheltering behavior. Video recordings showed a clear reduction of shelter-seeking behavior in the cave ecomorph in one population pair (Pivka). There were no changes in the other population pair (Rak), where the behavioral response had a much higher variance. Our results were partly in agreement with the hypothesis that shelter-seeking behavior should be selected against in an environment with reduced external threats. It is nevertheless too early for generalizations as the results in the second population pair were inconclusive. Additionally, we showed that for benthic walkers like A. aquaticus the use of rough substrate is crucial to obtain unbiased behavioral responses. Results of some previous studies using smooth glass or plastic substratum could be affected by unnatural behavior of animals constantly trying to find firm contact with the ground.