Coupling motographic and sonographic recording to assess foraging behaviour of Patella vulgata

Motographic recording of locomotor activity and sonographic recording of rasping were simultaneously obtained in the field from a group of Patella vulgata L. inhabiting a sheltered shore (Menai Bridge, North Wales). Each limpet was equipped with a piezo-electric transducer, a small amplifier, and a microlighter encased in dental acrylic and glued to the shell. A remote filter-amplifier-recorder device continuously recorded rasping noise through the activity periods (nocturnal low tide), when individual movements were also recorded using an automatic camera. Movements of experimental limpets did not differ from those of specimens equipped with the micro-lighter only. Locomotor activity was definitely concentrated during the first (outgoing) and last (homing) parts of each activity phase. Rasping started when limpets remained exposed to air at ebb tide, 1-2 hours before leaving home, and occurred during the whole period spent away from home. Rasping was fairly variable in rate, consisting in non-rhythmic bouts separated by recovery phases. No significant correlation was observed between rasping rate and speed variation during different parts of each excursion. However, the number of rasps performed in each spatial unit of the path was strongly correlated with the time spent by the limpet in that unit. The data suggest that in Patella vulgata the spatial organisation of foraging within each excursion is based on changes in travel speed related to local food density but not on modulation of rasping rate in time.