RHYTHMICAL TIMING AND SPATIAL SCATTERING OF FORAGING IN A HOMER LIMPET (PATELLA-RUSTICA)

Homing to a more or less permanent scar after each foraging excursion is a common movement pattern among intertidal gastropods and chitons; however, details of the timing and spacing of foraging activity have b een investigated only in a few species. The present study analyzes the short-term behavior of the limpet Patella rustica along the Tyrrhenia n coast, Italy, using a motographic technique to assess the fine organ ization of its foraging during favorable periods of sea roughness. P. rustica becomes active once the upper midlittoral is well splashed. It alternates foraging excursions and resting at home with a periodicity slightly longer than 12 h, suggesting a tidal-diel pattern. However, periodogram analysis of the sea level oscillations during the study pe riods revealed no such rhythmicity because tidal oscillations were hid den by irregular variations caused by waves. As a result of this the p artitioning, limpets move, on average, less than 50% of their potentia l activity time. Time partitioning may be highly adaptive in reducing potential risks. Nevertheless, in the absence of clear external drivin g cues, the significance of a very regular and apparently tidal patter n, fairly synchronous among the different specimens, remains to be exp lained. The activity of P. rustica during each excursion is organized into three parts: the outgoing journey during which grazing activity p rogressively increases, a central part characterized by intense grazin g, and the return characterized by fast displacement and a more or les s consistent trail following. Limpets head for random directions to re ach foraging grounds in successive excursions, showing only a slight a voidance of the direction taken during the previous outward journey. T his pattern produces a spatial scattering of grazing activity, allowin g efficient exploitation of grazing areas distributed radially around home during subsequent excursions.