Taxonomic, Ecological and Historical Considerations on the Deep-Water Benthic Mollusc Fauna of the Red Sea

The semi-enclosed and narrow Red Sea basin is characterized by bathyal zones in its axial sectors. It is determined by extreme hydrological parameters regarding its deep-water salinity and temperature which are a serious challenge to be coped with by deep-water benthos. Besides, it is separated from the adjacent Indian Ocean (Gulf of Aden) by a remarkably shallow sill that not only opposes easy transit for deep-water benthos but also exerts a strong control on the basin's hydrology budget during sea-level fluctuations, likely causing pulsing basin-wide extinctions at times of low stands. Among the relevant macrobenthic groups inhabiting the deep Red Sea, Mollusca stand out as the more diverse phylum. Although the full taxonomic appreciation of the Red Sea deep-water molluscs is still unresolved, as many as 262 species are recorded to date from depths below 400 m (163 Gastropoda, 94 Bivalvia, 4 Scaphopoda and 1 Polyplacophora). Part of this fauna is represented by eurybathic species with a wide bathymetric range. A substantial aliquot is equipped with a larval strategy (planktotrophy) in principle enabling the crossing of the shallow sill from the Gulf of Aden. Various taxa occur also in the Indo-West Pacific, and only a few are putatively considered as Red Sea endemics.