Role of the cephalic tentacles in trail following by Littoraria irrorata (Say)

Abstract

Gastropod molluscs deposit mucous trails on surfaces over which they crawl Littorinids, as well as certain other gastropods, follow conspeafic mucous trails in the direction in which they were laid, that is '• The stimuli and sense organs used by snails to determine mucous trail polarity are not known; however, six mechanisms of trail polarity detection have been suggested in the literature' Of these, the hypothesis which has received the most attention is that trails possess a concentration macrogradient of a volatile chemical which is detectable across the points of contact of a pair of sensory structures UJJi The assumption that polarity detection involves a concentration macro-gradient has been tested by experimentally reversing the assumed concentration macro-gradient in trails of Littoraria irrorata} Snails still travelled against the assumed gradient in favour of the actual direction of trail deposition. Furthermore, snails exhibited random directional choices when trail polarity was rendered ambiguous for all potential cues except for a hypothetical concentration macro-gradient Thus, it was concluded that a concentration macro-gradient is not involved in trail polarity detection in L. irrorata Further evaluation of hypotheses for the mechanism underlying trail polarity detection would be aided by knowledge of the sensory basis for this behaviour. The paired lips of Euglandmd and the anterior lateral folds of Navanax' are reportedly involved in detecting the trails of prey and mates, but neither species can apparently detect trail polarity.' The oral lappets are supposedly involved in detecting mucous trails in Onchidium, which can detect trail polarity.' As a Littoraria irrorata progresses along the substratum, the cephalic tentacles alternately move up and down, contacting the substratum with the ventral surface of the tentacle tips an average of 16-20 times per minute under normal conditions. The paired cephalic tentacles of littorinids are known to be tactile and one of the sites where chemical detection of predator mucus is possible.' Thus, the cephalic tentacles are likely locations for sensory receptors involved in trail polarity detection. This report presents an experiment designed to examine the role of the cephalic tentacles of L. irrorata in