FACTORS AFFECTING VARIABILITY OF FORAGING EXCURSIONS IN A POPULATION OF THE LIMPET PATELLA-VULGATA (MOLLUSCA, GASTROPODA)

Paths moved by the limpet Patella vulgata L. were monitored on a shelt ered vertical rocky shore in North Wales using time-lapse photography throughout nocturnal low tides in April and November 1992, roughly cor responding to periods of minimum and maximum gonad ripeness, respectiv ely. Various motion parameters, including total duration, total length , and maximum distance reached from home were computed from 124 comple te foraging routes obtained from 18 higher-zoned and 18 lower-zoned li mpets. P. vulgata typically performed a single loop per night (average total length, ca. 70 cm), moving from the individual home scar to gra ze the surrounding algal grounds (average maximum distance, ca. 25 cm) , then following its own trail back home. On the average, limpets move d from the home scar during three of four available nocturnal low tide s and exploited about half the emersion time. Evidence for size-relate d variability in behaviour was found, as a positive correlation was as sessed between both total duration and length of the excursions, and s hell length. In addition, zonation was proved to affect the limpets' f oraging behaviour, since low-shore limpets moved faster and covered gr eater distances than high-shore ones. Moreover, consistent seasonal va riation in foraging behaviour emerged, since in April the excursions w ere longer and longer-lasting than in November, but limpets exploited a larger fraction of potential activity phases in November than in Apr il. The within-population variability in the temporal and spatial char acteristics of the foraging excursions is discussed in relation to the available data on zonal and seasonal variation of food resources and in relation to physiological changes due to reproductive cycle.