Differentiation in radular and embryonic characters, and further comments on gene flow, between two sympatric morphs of<i>Littorina saxatilis</i>(Olivi)

Abstract

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 (<20%). We compared shells of embryos from upper-shore RB females with those from low-shore SU females. We also compared embryonic shells from mid-shore RB and SU females, at two sites. The size of the nuclear whorl, the aperture width, the shell diameter and the number of whorls differed between embryos of the two morphs both in allopatry and in sympatry. Populations of the same morph from different microhabitats differed only in 1–2 of the charaeters. The number of cusps on the outer marginal tooth of the radula of adult females differed between both allopatric and sympatric populations of the two morphs. We used allozyme data from Johannesson etal. (1993) to estimate gene flow (Nm) between morphs on a microgeographical scale (<50 m). There was less gene flow between morphs than within morphs at similar distances. Gene flow between morphs was, however, large enough to prevent stochastic differentiation over tens of metres (Nm>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.