Detection of two coexisting species of Oxystele (Gastropoda: Trochidae) by morphological and electrophoretic analyses

Abstract

Oxystele variegata (Anton, 1838) is one of the commonest intertidal topshells on the coast of South Africa. In this study we show that the so‐called O. variegata actually consists of two sympatric species, which differ in shell colour patterns, in the proportions of the central tooth of the radula, and in the electrophoretic mobilities of some enzymes: O. impervia (Menke, 1843) (which tends to occur higher up the shore), and O. variegata (Anton, 1838) (occurring further down the shore).The shell of O. impervia has a pattern of dark red, orange, yellow or brown maculations, on a ground colour that is red, orange, yellow or brown, respectively; that of O. variegata has a pattern of red maculations or lines on a ground colour that is either off‐white, or greenish‐grey. Both O. impervia and O. variegata exhibit an immense variety of shell colours and pattern and in both, shell colour variation is an interplay between two factors: ground colour and maculated spiral bands. By expanding the maculated spiral bands in breadth, at the expense of the ground colour, an original shell pattern of spiral bands may be transformed into one of axial or diagonal stripes.The radula of both species resemble the typical trochid pattern. They are, however, consistently different: the central tooth of the radula of O. imperial usually has a shallow base and a cusp that is slightly indented, whereas that of O. variegata usually has a deep base, and a well indented cusp.Further differences between the species were revealed by electrophoretic studies. Of six loci examined, two were found to be taxonomically diagnostic. Another two had at least one species‐specific allele at a frequency of 0·1 or greater, and significant heterogeneity in allelic frequency between the two species was found in the four non‐diagnostic loci.The differences in shell colour patterns, radular tooth structure and enzyme mobilities support the conclusion that O. impervia and O. variegata are separate but closely related species, with similar geographic distributions and an overlapping zonation.