AN AUTOMATIC TECHNIQUE FOR RECORDING GRAZING ACTIVITY OF LIMPETS

The foraging activity of intertidal chitons and gastropods livingon rocky shores has been investigated by many authors by usingmicrophones or piezoelectric transducers to obtain audio-recordingsof radular grazing on various algae. However, the lack of automaticdevices for recording the noise associated with radular activityhas made the assessment of the spatial and temporal organizationof foraging of these animals difficult and labour intensive.A new technique is described that permits automatic recordingon computer and the analysis of grazing noises produced by limpetswhile foraging. The apparatus consists of three parts: a transducingpre-amplifying section (on limpets, or free in the field asa reference); an amplifying, filtering and digital-encodingsection (a free standing device) and a data acquiring and storingsection (on computer). Recordings of Patella caendta Linnaeusobtained in the laboratory during spring and summer 1993–1994,were analysed using a standard procedure to investigate grazingactivity. Each grazing bout is recorded on computer as a sequenceof 2–6 unit events, lasting about 0.4–0.8 sec, separatedby 1–3 sec. Each foraging phase consisted of a sequenceof several grazing bouts (maximum frequency about 1500–2000grazing bouts per hour), concentrated during high tides, witha distinct preference for nocturnal periods. The results ofthe automatic analysis were consistent with those obtained bydirect audiblescoring of the corresponding audio-recordings.The software used also permitted discrimination of true grazingactivity from other noises produced by the limpets, such asshell rubbing on the substrate while moving. This new techniquehasa number of advantages with respect to previous methods. Themost important characteristic is that the apparatus is ableto resolve automatically the grazing activity of shelled molluscsat different time scales, from msec to hours and days, accordingto an integrating constant that can be set on the apparatus. (Received 27 October 1994; accepted 28 December 1994)     

RELATIVE IMPORTANCE OF AIRBORNE ODOURS AND TRAILS IN THE GROUP HOMING OF LIMACUS FLAVUS (LINNAEUS) (GASTROPODA, PULMONATA)

The relative importance of airborne odours and con-specifictrails in the group homing of Limacus flavus has been studiedin the laboratory using an Y-olfac-tometer and trail followingtests. The slugs showed positive anemotaxis along air flowspassing through shelters inhabited or recently inhabited byconspecifics. In the choice between single and group shelters,the animals headed preferentially towards the latter ones. L.flavus is able to discriminate between conspecific and heterospecific(Limax maximus) shelters. Trail following tests failed to showsignificant trail superimposition, but did show a significantdeflection of the test slug path towards the marker after crossing.In direct comparison, distant chemoreception prevailed overtrail following in the orientation towards conspecifics (Received 10 August 1987;     

COUPLING MOTOGRAPHIC AND SONOGRAPHIC RECORDING TO ASSESS FORAGING BEHAVIOUR OF PATELLA VULGATA

Motographic recording of locomotor activity and sonographicrecording of rasping were simultaneously obtained in the fieldfrom a group of Patella vulgata L. inhabiting a sheltered shore(Menai Bridge, North Wales). Each limpet was equipped with apiezoelectric transducer, a small amplifier, and a micro-lighterencased in dental acrylic and glued to the shell. A remote filter-amplifier-recorderdevice continuously recorded rasping noise through the activityperiods (nocturnal low tide), when individual movements werealso recorded using an automatic camera. Movements of experimentallimpets did not differ from those of specimens equipped withthe micro-lighter only. Locomotor activity was definitely concentratedduring the first (outgoing) and last (homing) parts of eachactivity phase. Rasping started when limpets remained exposedto air at ebb tide, 1–2 hours before leaving home, andoccurred during the whole period spent away from home. Raspingwas fairly variable in rate, consisting in non-rhythmic boutsseparated by recovery phases. No significant correlation wasobserved between rasping rate and speed variation during differentparts of each excursion. However, the number of rasps performedin each spatial unit of the path was strongly correlated withthe time spent by the limpet in that unit. The data suggestthat in Patella vulgata the spatial organisation of foragingwithin each excursion is based on changes in travel speed relatedto local food density but not on modulation of rasping ratein time. (Received 4 July 1993; accepted 2 August 1993)