TRANSFERS OF BRYOZOAN SPECIES BETWEEN THE ATLANTIC-OCEAN AND THE MEDITERRANEAN-SEA VIA THE STRAIT OF GIBRALTAR

The deep-water benthos (115-2100 m) was sampled in the Atlantic-Medite rranean exchange area (Balgim cruise, N.O. Cryos: lberian-Moroccan Gul f, Strait of Gibraltar, Alboran Sea). The species richness of the bryo zoan fauna (146 spp.) showed a marked between-site variability. Peaks of species richness occurred on both sides and in the Strait of Gibral tar under various conditions of temperature and salinity. 'rhe common features of the corresponding sites were the abundance of suitable sub strates and the presence of currents facilitating supply of food and p ropagules. In the Alboran Sea, the species richness decreased with dep th more rapidly than in the Atlantic and the depth distribution of sev eral deep-water species showed an upward shift. There is an eastward d ecrease in the stock of Atlantic species: 15 % of them are not found b eyond the Strait sill and only 79 % of the remaining species extend to the whole Mediterranean. Several endemic Atlantic species or morphoty pes can live in conditions of typical Mediterranean water (high temper ature and salinity), but do not spread beyond the eastern limits of ei ther the Gibraltar Strait or the Alboran Sea. These occurrences are hy pothesized to be signatures of local increases of the trophic resource s, which could mask the barrier effect of the Mediterranean oligotroph y. The organic sedimentation originating from the high-productivity ar ea localized in the north-west of the Alboran Sea appears to create th is distribution pattern.