Rhythmical timing and spatial scattering of foraging in a homer limpet {Patella rustica)

Abstract

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 been investigated only in a few species. The present study analyzes the short-term behavior of the limpet Patella rustica along the Tyrrhenian coast, Italy, using a motographic technique to assess the fine organization 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 periods revealed no such rhythmicity because tidal oscillations were hidden by irregular variations caused by waves. As a result of this time partitioning, limpets move, on average, less than 50% of their potential activity time. Time partitioning may be highly adaptive in reducing potential risks. Nevertheless, in the absence of clear external driving cues, the significance of a very regular and apparently tidal pattern, fairly synchronous among the different specimens, remains to be explained. The activity of P. rustica during each excursion is organized into three parts: the outgoing journey during which grazing activity progressively increases, a central part characterized by intense grazing, and the return characterized by fast displacement and a more or less consistent trail following. Limpets head for random directions to reach foraging grounds in successive excursions, showing only a slight avoidance of the direction taken during the previous outward journey. This pattern produces a spatial scattering of grazing activity, allowing efficient exploitation of grazing areas distributed radially around home during subsequent excursions.