ORIGINAL ARTICLE: Zoogeography of the shallow-water holothuroids ofthe western Indian Ocean

摘要

Aim: To analyse the zoogeography of the shallow-water holothuroids of the western Indian Ocean (WIO). Based on this analysis we ask to what extent differences in species' ability to disperse across potential barriers provide an explanation for holothuroid zoogeography. Location: Shallow-waters (50 m isobaths) of the WIO, extending from Suez to Cape Town and from the coastline of East Africa upward to 65 degree E. Methods: Data for the analysis were obtained from monograph on the shallow-water sea cucumbers of the WIO. A species presence/absence matrix with a resolution of 1 degree latitude/longitude was constructed. These cells were assigned to eight coarser operational geographical units, which were delimited on the basis of published faunistic and geological borders. The analytical zoogeographical methods employed were cluster analysis on several beta -diversity coefficients and parsimony analyses of endemicity. The influence of life-history strategies on the distribution pattern was analysed through examination of latitudinal and longitudinal gradients, and by plotting cumulative curves for species number against range size. Results: The shallow-water holothuroid fauna of the WIO can be split into several biogeographical units. To the north, we found evidence that the northern Red Sea holothuroid fauna differs from that of the southern Red Sea. The latter has closest affinity with south-east Arabia and the Persian Gulf, and thus the biogeographical barrier of Bab-el-Mandab nowadays seems to be of minor importance. The cold upwelling at the east coast of Somalia forms an effective barrier for holothuroids and especially those with lecitothrophic (short-lived) larvae. Even though the circumtropical biogeographical pattern is not well resolved, important taxonomic turnovers suggest that it is composed of several distinct subprovinces. Taxonomic turnover is at least partially dictated by the dispersion capacity of the different orders. Main conclusions: This study concludes that the WIO is best split into at least three biogeographical realms: (1) the Red Sea and associated Arab Basin, (2) the asymmetrical circumtropical region stretching from the horn of Africa to southern Mozambique, and (3) southern Africa. Conspicuous differences in dispersal abilities of the three dominant orders are identified. The biogeography of the WIO is best explained by: (1) species' dispersion ability, (2) the prevalent current patterns, and (3) to a lesser, geographically limited extent, recent geological history. As a serendipitous discovery, we found that Rapoport's rule does not hold in the WIO.