BIOTIC AFFINITIES IN A TRANSITIONAL ZONE BETWEEN THE ATLANTIC AND THEMEDITERRANEAN - A BIOGEOGRAPHICAL APPROACH BASED ON SPONGES

This study characterizes the demosponge assemblages established on the continental shelf of Alboran Island, a site where the faunistic influ ences of three biogeographical regions overlap (i.e. Mauritanian, Lusi tanian and western Mediterranean regions). The patterns of species com position 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 assembla ges, 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 ge neral pattern of biotic affinity is determined predominantly by horizo ntal 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 infrali ttoral assemblages of this island are strongly affected by Lusitanian fauna. Circalittoral assemblages, however, harbour fauna with a peculi ar species composition, lacking affinities with all other communities considered in this study. This fact suggests strongly that the circali ttoral level of the Alboranian shelf is an ecotone established in the boundary layer between the Atlantic and Mediterranean water masses. Th e pattern of present-day biotic affinities among all the archipelagos was also used to test, by parsimony analysis, various hypotheses on th e historical origin of the Mediterranean benthos. Results suggest that sublittoral Mediterranean fauna have a major Lusitanian-Mauritanian o rigin, although a few Tethyan or Paleomediterranean elements still per sist in some circalittoral communities of the Alboranian shelf.