16. The Making of the Mediterranean Molluscan Biodiversity

The present Mediterranean molluscan biodiversity, comprising more than 2,000 species, was shaped by the multiple events connected with the geodynamic and climatic evolution of this basin in the last million years coupled with the process of biological evolutionary processes on the global scale. Climate, hydrology, basin physiography, and connections via seaways were and are the main forcing factors tuning the type of molluscs making the Mediterranean diversity at any geological instant, resulting in (often cyclical) taxonomic additions and subtractions. However, this basic motif operating in the Mediterranean since its formation as an independent basin over millions of years, has been seriously perturbed by the deliberate or accidental introduction of non-indigenous (alien or non native) species in the last decades. The taxonomic addition of mostly warm-water species is taking place at an unprecedented pace with respect to even the dramatic changes that have marked glacial-interglacial transitions, thus making the present day Mediterranean Sea a planetary biogeographic unicum.