Differentiation in radular and embryonic characters, and further comments on gene flow, between two sympatric morphs ofLittorina saxatilis(Olivi)

Abstract

On exposed rocky shores of Galicia, two morphs of the direct-developing snail Littorina saxatilis are found associated with different microhabitats. A ridged and banded (RB) morph is found in upper, less wave-exposed parts of the shore, while a smooth and unbanded (SU) morph is present in low, heavily wave-stressed parts. They overlap in a mid-shore zone where hybrids are found, usually at low frequencies (Nm) between morphs on a microgeographical scale (4). The results strongly support the suggestion that the two morphs are conspecific although steep gradients of selection over microenvironmental transitions promote a number of polymorphic traits.