Visual versus non-visual selection of shell colour in an Israeli freshwater snail.

The Israeli freshwater snail Theodoxus jordani exhibits high variability both in shell colour and shell pattern.In most localities in Israel, more than 80% of the snails are black regardless of the colour of their background. Within Lake Kinneret, however, there is a significant association between the relative frequency of dark patterned shells and the distribution of a black (basalt) background.Predators of jordani include the fishes Barbus longiceps, Blennius fluviatilis, and the crab Potamon potamon. In Israel, the former two are limited in their distribution to the Kinneret. Thus, the distribution of cryptic variants of the snail is positively correlated to the distribution of some of its predators.Beyond the Kinneret, shells are black probably as a result of selection by the injurious effects of ultraviolet radiation. Within the Kinneret, snails can manage without an anti-radiation shield probably because the penetration of UV light into the water is considerably reduced, due to the daily summer storms and to the seasonal bloom of unicellular algae.Apparently, then, outside the Kinneret selective forces for crypsis are partly relaxed and their effects overidden by selective pressures for solar radiation devices; within the Kinnertt, selective pressures for anti-radiation devices are partly reduced and their effects overridden by selective pressures for crypsis.