Researches on the Coast of Somalia. The Shore and the Dune of Sar Uanle

SUMMARY 1. Coenobita rugosus Milne Edwards, a land hermit crab, is common on the sandy beaches of southern Somalia. During the day, it hides in various types of refuges bordering, or near to, the dune and at night, it ventures on to the beach or into the bush in search of food and water. The orientational mechanisms which underlie these daily migrations differ according to the situation.2. In spontaneous transits over the dune, a positive anemotactic component appears where the visibility of the seaward horizon is minimal and disappears with an increased inclination of the dune towards the sea. In experimental launches in arenas Ø 1.5 m, anemotaxis is much more evident and disappears only at inclinations offering a maximum view of the seaward horizon. A geotactic effect due to inclination is to be excluded in both cases.3. Artificial ventilation, under cover or in the open, produces a positive anemotactic response which is not influenced by humidity or air velocity. Anemotaxis is not influenced by the position or direction of the home beach, the hour of testing or the direction of the migration of the free animals.4. In larger arenas, the initial response is anemotactic but once this is exhausted the animals show an orientation which differs in response to varying influences. On the beach, the animals tend to orient away from the sea and towards landmark cues such as rocks, trees or the dune and are (perhaps) aided by a form of negative geotaxis. In the bush, landmark cues have no importance and the animals orient directly towards the sea direction of their home beach (thus in the opposite direction from that taken on the beach) probably on the basis of celestial compass.5. Hypotheses are made on the function and biological value of these orientational mechanism in the light of the present knowledge on these phenomenon in other Invertebrates.