Abstract
Strong gradients in physico‐chemical properties between abutting water masses create prominent transition zones in the marine environment. The S trait of G ibraltar forms the well defined boundary between the M editerranean and A tlantic, and this paper examines spatial variation of hydroid assemblages in this transition zone. Although several studies highlighted the transitional character of the Strait and defined it as an ecotone, the benthic hydroid assemblages did not show differences between the G ulf of C ádiz and the A lboran S ea. However, there is an asymmetrical influence of the Atlantic waters on the coastal benthic ecosystems of the A lboran S ea, which maintains a more Mediterranean character in the hydroid assemblages of the northern coast, whereas a more A tlantic character was found in the rest of the studied sites. The transition zone between A tlantic and M editerranean benthic communities could be associated with an A tlantic I nfluence Z one rather than with the S trait of G ibraltar itself.