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

Trochoidea simulata, a landsnail widespread in desert regions of Israe l, varies greatly in shell colour. We have previously shown that preda tor and thermoegulatory mediated selection cannot explain the maintena nce of this colour variation. In this paper, we test the hypothesis th at observed morph frequencies are an effect of habitat variation, and that the colour variation has no functional significance. Because whit e 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, stripe d, and brown snails at 9 locations. Each location consisted of a paire d hillside and loess wadi-bed site with high and low CaCO3 content res pectively. There were significantly more white snails at hillside than loess sites. In addition, these habitats varied mostly in abiotic cha racteristics, and not in terms of snail population parameters or veget ation characteristics. Hillside sites had sig nicantly more CaCO3 and limestone rocks than loess sites. Soil CaCO3 content was the strongest predictor of morph frequency among paired sites. Shell-colour variati on in this species may be an effect of individual access to resources, mainly CaCO3 that is ingested with food.