How adverse are Mediterranean waters to the deep-sea fauna? A study of the Gibraltar exchange based on Mollusca from the “BALGIM” expedition

The BALGIM expedition was designed to assess the distribution of marine organisms in the transition from Gulf of Cadiz to Mediterranean Sea across the Strait of Gibraltar. There were 99 hauls below 200 m depth, down to 2110 m. Two matrixes with abundance data were constructed using the data of gastropods and scaphopods from this study and those of bivalves studied earlier. Species were scored according to their bathymetric and geographic distribution as (1) deep-sea species occurring exclusively in the Atlantic, or (2) deep-sea species reported as living both in the Atlantic and the Mediterranean. A total of 4641 live-taken individuals corresponding to 154 species of molluscs collected alive were identified, and almost twice as many (243 species) including those collected as shells only. The samples do not form clearly defined clusters based on their faunal content. More than half of the species (84) occur both in the Mediterranean Sea and the Atlantic Ocean. Most of the 62 species with an Atlantic-only distribution are associated to the cool or cold waters below 600 m depth along the Moroccan margin, and are barred by the Mediterranean Outflow Water in the northern part of Gulf of Cadiz. Four species are preferent of the warm but low-saline North Atlantic Central Water, also off the Moroccan margin. The Mediterranean outflow largely shares a set of species also occurring in the Mediterranean, whereas there are no Mediterranean-only deep-sea species. Therefore, variation in salinity in a range of 1–2 ‰ is not critical compared to temperature which explains most of the restricted distributions. • Mediterranean Outflow Water supports species with Atlanto-Mediterranean distribution. • Species living in cold water masses in Gulf of Cadiz are barred from the MOW pathway. • There are deep-sea molluscs restricted to the Atlantic, but none to the Mediterranean. • Temperature is probably the most important factor for deep-sea mollusc distributions.