FAUNAL ZONATION OF THE DEEP-WATER BRACHYURAN CRABS IN THE STRAITS OF FLORIDA

The pattern of species replacement with depth (zonation) of deep-water brachyuran crabs across the continental slope of the Straits of Florida is described. Rather than an abrupt faunal break between the Shelf Province (SP) and the Archibenthal Zone of Transition (AZT), a gradual change in species composition is observed along the vertical gradient from 183 to 500 m of depth. Within the AZT two different faunal zones are distinguished, Upper and Lower Archibenthal Zones (UAZ/LAZ), delimited by the 10-degrees-C isotherm. A significant rate of species change occurs at depths at which this isotherm intercepts either the continental or the insular margins of the Straits (150 to 400 m-600 m). Maximum rate of faunal change was recorded between 500 and 700 m. UAZ is characterized by a heterogeneous species assemblage which includes 65% of the brachyuran population studied, while LAZ is more homogeneous in composition encompassing few species (6.5%) which are indigenous to the deep slope habitat of the Straits. The scheme of faunal zonation derived from this study is consistent with previous depth boundaries defined for the AZT in the western North Atlantic and the north-west Mediterranean. Similarly, temperature and substrate are invoked to explain the zonal distribution observed, though the existence of biological interactions particularly among congeneric species is not ruled out as a possible cause for the faunal gradients recognized in the Straits of Florida.