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Specimens of four sympatric intertidal limpet species (Siphonaria dayi, S. tenuicostulata, S. anneae and S. nigerrima ) were collected from four localities on the east coast of South Africa and southern Mozambique. Their phylogenetic relationships were investigated using sequences of the mitochondrial COI gene and the intron-containing nuclear ATPSb gene. Two closely related lineages were recovered, which grouped specimens on the basis of geography rather than morphology. One lineage was associated with the subtropical coastline of South Africa’s east coast and the other with the tropical coastline of northeastern South Africa and southern Mozambique. This genetic discontinuity coincides with a biogeographic boundary located in the vicinity of Cape St Lucia. Combined genetic diversity of the four species was lower than that of three other southern African congeners, and fell within the range determined for single southern African marine mollusc species. We suggest that the four limpet species are in fact different morphotypes of a single species.

The validity of morphology-based species boundaries between the southern African representatives of the genus Tricolia Risso, 1826 was assessed using mitochondrial COI and 16S rRNA sequence data. Most phylogenies obtained from individual and combined genetic datasets recovered 10 of the southern African members of the genus as a monophyletic clade. No COI sequences of the 11th species (T. adusta) were available, but this species clustered among the other African species in the 16S rRNA phylogeny. Discrepancies between morphology and genetics were identified in two clades within which there was limited genetic variation and no differentiation between two groups of nominal species, comprising respectivelyT. africana (Bartsch, 1915) andT. capensis (Dunker, 1846), andT. bicarinata (Dunker, 1846),T. insignis (Turton, 1932) and T. kraussi (Smith, 1911). In both cases the distributions of the nominal taxa coincide with well-known biogeographic disjunctions, and there is evidence of overlapping and intergrading shell characters. We propose that both of these unresolved clades be recognized as single, phenotypically plastic species, for which the oldest available names are respectivelyT. capensis andT. bicarinata. Despite the resultant reduction in number of recognized species, the phasianellid fauna of southern Africa remains the most diverse in the world, with 10 endemic species and 3 tropical species extending south into KwaZulu-Natal.