ORIENTATION DURING SHORT-RANGE FEEDING IN THE CRAB DOTILLA-WICHMANNI

The ocypodid crab Dotilla wichmanni is a common inhabitant of tropical sandy shores, where it feeds at low tide by sorting the material depo sited by the ebbing tide. Feeding occurs in the immediate vicinity of the burrow by means of systematically sampling the sand surface. While feeding, the crabs move along trenches radiating from the burrow and produce pseudofaecal pellets which are amassed over the already excava ted area. When disturbed, the crabs rapidly vanish into the burrow, wh ere they remain hidden for a while. Upon re-emerging, they recommence feeding moving along the same trench as before retreat. The crabs, how ever, were found to assume the same feeding direction held before retr eating even if any sign that could be derived from their previous acti vity was removed experimentally. To uncover the orientation cues used in this behaviour, the area near the burrow was manipulated in particu lar ways. The crabs were found to rely mainly on the skylight polariza tion pattern, while visual landmarks near the feeding area may play a role when astronomic cues provide no useful information, such as under overcast skies. Both cues were used by the crabs as references to ass ume the same feeding direction as that used before retreat.