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

Abstract

Trochoidea simulata, a landsnail widespread in desert regions of Israel, varies greatly in shell colour. We have previously shown that predator and thermoegulatory mediated selection cannot explain the maintenance of this colour variation. In this paper, we test the hypothesis that observed morph frequencies are an effect of habitat variation, and that the colour variation has no functional significance. Because white snails have thicker shells than striped or brown snails, we propose that the colour of an individual snail depends on its access to CaCO3. We tested this hypothesis by comparing the frequency of white, striped, and brown snails at 9 locations. Each location consisted of a paired hillside and loess wadi-bed site with high and low CaCO3 content respectively. There were significantly more white snails at hillside than loess sites. In addition, these habitats varied mostly in abiotic characteristics, and not in terms of snail population parameters or vegetation characteristics. Hillside sites had significantly more CaCO3 and limestone rocks than loess sites. Soil CaCO3 content was the strongest predictor of morph frequency among paired sites. Shell-colour variation in this species may be an effect of individual access to resources, mainly CaCO3 that is ingested with food.