RESEARCHES ON THE COAST OF SOMALIA. THE SHORE AND THE DUNE OF SAR UANLE

Open Access
Publisher Website
Google Scholar
Abstract
At Sar Uanle (southern Somalia, 0°29′48''S-42°25′30''E), Nerita textilis Dillwyn (Gastropoda Prosobranchia) inhabits zones which are exposed to wave motion. During their rest periods, some animals are found scattered about 100 cm above the MHWS, while others are found clustered below. The lower limit of the feeding range is the base of the cliff. The specimens occasionally seen heading back towards the coast over the rocky platform extending from the base of the cliffs were, in our opinion, probably washed off the cliff during a preceding high water. According to our observations, N. textilis is capable of returning to the coast after being passively displaced. It orients actively towards the coast and is not simply carried passively by the waves. A homeward component is not present in either the horizontal or vertical movement. Orientation occurs exclusively along the perpendicular to the coast on the rocky platform and consists of an upward movement plus random diffusion on the cliff. The animal orients toward the cliff under varying conditions of illumination. It relies directly on cues from the cliff when on the platform and not on indirect cues from other sources (sun position, magnetic field, etc.). The biological significance of this behaviour is discussed and hypotheses proposed for the type of stimulation involved.