ECOLOGICAL SEGREGATION, POLYMORPHISM AND NATURAL SELECTION IN TWO INTERTIDAL GASTROPODS OF THE GENUS NERITA AT ELAT (RED SEA, ISRAEL)

ABSTRACT

A preliminary ecological survey of two sympatric intertidal species of Nerita at Elat showed that beach rock slabs are inhabited by N. forskalii whereas on pebble beaches it is excluded from the upper levels by N. polita. Activity patterns of both species are determined by their vulnerability to predation and desiccation; to evade their marine predators they are active when the tide is out, and, to avoid desiccation, they are active mostly at night. When inactive, N. forskalii hides in cracks and crevices of rock and N. polita hides by burrowing into the sand. Both species are variable in shell coloration, which matches their background. The ratio in which colour “morphs” are taken by predators suggests that, at least in N. polita, variability per se is adaptive by impairing the predator's hunting efficiency.