Biotic Affinities in a Transitional Zone Between the Atlantic and the Mediterranean: A Biogeographical Approach Based on Sponges

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Abstract
This study characterizes the demosponge assemblages established on the continental shelf of Alboran Island, a site where the faunistic influences of three biogeographical regions overlap (i.e. Mauritanian, Lusitanian and western Mediterranean regions). The patterns of species composition and abundance of Alboranian fauna are compared with those of five other Atlantic-Mediterranean islands. This comparison provides a general idea about the faunistic affinities of the Alboranian assemblages, as well as an estimate of the general faunistic heterogeneity in the biogeographical province. An important biogeographical regionalism was found in both the quality and quantity of the demosponges. The general pattern of biotic affinity is determined predominantly by horizontal distances (geographical distances) and marine currents. The role of vertical distance (bathymetry) was much less significant. However, it is an important determinant for Alboranian assemblages. The infralittoral assemblages of this island are strongly affected by Lusitanian fauna. Circalittoral assemblages, however, harbour fauna with a peculiar species composition, lacking affinities with all other communities considered in this study. This fact suggests strongly that the circalittoral level of the Alboranian shelf is an ecotone established in the boundary layer between the Atlantic and Mediterranean water masses. The pattern of present-day biotic affinities among all the archipelagos was also used to test, by parsimony analysis, various hypotheses on the historical origin of the Mediterranean benthos. Results suggest that sublittoral Mediterranean fauna have a major Lusitanian-Mauritanian origin, although a few Tethyan or Paleomediterranean elements still persist in some circalittoral communities of the Alboranian shelf.

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