HABITAT CONSTRAINTS ON SHELL-COLOUR VARIATION OF A DESERT LANDSNAIL, TROCHOIDEA SIMULATA

Trochoidea simulata, a landsnail widespread in desert regionsof Israel, varies greatly in shell colour. We have previouslyshown that predator and thermoegulatory mediated selection cannotexplain the maintenance of this colour variation. In this paper,we test the hypothesis that observed morph frequencies are aneffect of habitat variation, and that the colour variation hasno functional significance. Because white snails have thickershells than striped or brown snails, we propose that the colourof an individual snail depends on its access to CaCO₃. We testedthis hypothesis by comparing the frequency of white, striped,and brown snails at 9 locations. Each location consisted ofa paired hillside and loess wadi-bed site with high and lowCaCO₃ content respectively. There were significantly more whitesnails at hillside than loess sites. In addition, these habitatsvaried mostly in abiotic characteristics, and not in terms ofsnail population parameters or vegetation characteristics. Hillsidesites had significantly more CaCO₃ and limestone rocks thanloess sites. Soil CaCO₃ content was the strongest predictorof morph frequency among paired sites. Shell-colour variationin this species may be an effect of individual access to resources,mainly CaCO₃ that is ingested with food. (Received 5 August 1996; accepted 25 October 1996)