Abstract
The South African coastline can be divided into at least four temperature-defined marine bioregions, including the tropical north-east coast, the subtropical east coast, the warm-temperate south coast, and the cool-temperate west coast. There are also two biogeographical transition zones, the south-west coast and the south-east coast (or Wild Coast). The former is sometimes considered a distinct marine bioregion, but no such status has yet been suggested for the Wild Coast. Previous data on the distribution of a recently described but very common coastal crab, Hymenosoma longicrure, indicated that this species could be a Wild Coast endemic. If confirmed, this would be a first indication that this region harbours unique fauna, and that additional research is required to determine whether the Wild Coast constitutes a distinct bioregion that needs to be managed separately from other coastal regions. In the present study, we generated novel genetic data for H. longicrure and compared the species’ range with that of its southern African congeners. We found that H. longicrure occurs north of the Wild Coast, where its range overlaps with that of H. projectum. This finding rejects the idea that the Wild Coast harbours endemic fauna and suggests that the ranges of the two species may be linked to the subtropical and tropical bioregions, respectively, with some southward dispersal facilitated by the southward-flowing Agulhas Current. We conclude that there is as yet no compelling evidence that the Wild Coast is a distinct marine bioregion, and concur with previous biogeographical studies which have suggested that the Wild Coast is an area in which species from the subtropical and warm-temperate bioregions have overlapping ranges. Nonetheless, that fact that no biological information is available for the majority of the region’s estuaries highlights the necessity of comprehensively documenting the biodiversity of this understudied region to fully resolve this issue.
Highlights
The coastline of South Africa is exceptionally diverse
In the haplotype network (Fig. 2) sequences of H. longicrure, H. projectum, and H. orbiculare were recovered as three distinct clusters
Samples of H. projectum were found in the tropical and subtropical bioregions and on the Wild Coast, samples of H. longicrure were present in the subtropical region as well as along the Wild Coast, and samples of H. orbiculare were not found north of the warm-temperate bioregion (Fig. 3)
Summary
The coastline of South Africa is exceptionally diverse. Located at the contact area of the Atlantic and Indian Ocean biomes, it comprises biotic elements from two oceans [1], but there are large numbers of endemics [2,3,4]. Environmental conditions change considerably from west to east and based on species assemblage data, most authors have accepted three major temperature-defined coastal marine biogeographical regions, including the cool-temperate Namaqua bioregion in the west, the warm-temperate Agulhas bioregion. Several recent studies have suggested that there is a tropical region in the north-east (the Delagoa bioregion [3, 9]; Fig. 1), and genetic data strongly support this [8, 10,11,12,13]. There are two major biogeographical transition areas whose status as bioregions is less clear. The region between the two temperate bioregions in the south-west has been treated as a transition zone in the older literature [14,15,16], but more recently it has been referred to as the South-western Cape bioregion [3].
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