Zoogeography of the southern African ascidian fauna

Aim To describe the biogeography of the ascidian fauna of southern Africa, to compare the results obtained with those reported for other fauna and flora of the same region, and to speculate about the origin of ascidians in the region. Location Southern Africa extending over 4000 km from Mossâmedes (15° S–12° E) to Inhaca Island (26°30′ S–33° E), including Vema Seamount (31°40′ S–8 °20′ E), Amsterdam‐Saint Paul Islands (38° S–77°30′ E) and the Tristan‐Gough Islands (38° S–12°20′ W). Methods We constructed a presence/absence matrix of 168 species for 26 biogeographical divisions, 21 classical biogeographical regions described by Briggs (Marine zoogeography, McGraw‐Hill, New York, 1974) and five provinces within the southern African region. We considered the following limits and divisions into provinces for the southern African region: Namibia, Namaqua, Agulhas and Natal as proposed by Branch et al. (Two oceans. A guide to the marine life of southern Africa, David Philip Publishers, 1994), and the West Wind Drift Islands province (WWD) according to Briggs (Global biogeography, Elsevier Health Sciences, Amsterdam, 1995). To examine the biogeographical structure, species and divisions were classified using cluster analysis (based on UPGMA as the aggregation algorithm) with the Bray–Curtis index of similarity. This classification was combined with MDS ordination. Main conclusions Four main groups were obtained from the analysis of affinities among species: (1) species present in the WWD, separated by a high percentage of endemisms and a low number of species with a southern African distribution. Moreover, in the light of the species distribution and the results of further analysis, which revealed that they are completely separated and not at all related to the southern African region, it appears that there are no close relationships among the different islands and seamounts of the West Wind Drift Island province. This province was therefore removed from the remaining analyses; (2) species with a wide distribution; (3) species of colder waters present in Namaqua and Agulhas provinces, a transitional temperate area in which gradual mixing and replacement of species negate previous hypotheses on the existence of a marked distributional break at Cape of Good Hope; (4) species of warmer waters related to Natal province. The classification into biogeographical components was dominated by the endemic (47%), Indo‐Pacific (25%) and cosmopolitan (13%) components. The analysis of affinities among biogeographical areas separated Namibia from the rest of the southern African provinces and showed that it was related to some extent to the Antarctic region because of the cold‐temperate character of the province and the low sampling effort; Namaqua, Agulhas and Natal were grouped together and found to be closely related to the Indo‐West Pacific region. In general, our results were consistent with those obtained for other southern African marine invertebrates. The frequency distribution of solitary/colonial strategies among provinces confirmed the domination of colonial organisms in tropical regions and solitary organisms in colder regions. Finally, we speculate that the southern African ascidian fauna mainly comprises Indo‐Pacific, Antarctic and eastern Atlantic ascidians.