There is now substancial evidence that the climate of our planet is in a phase of change and that, for instance, the average surface temperature of the sea is increasing. While the long-term effects of this warming are still debated, the responses of the marine biota are already visible. The Mediterranean Sea, the widest marine warm-temperate region in the world, forms a sort of climatic and biogeographic hinge between tropical and temperate seas. Present-day Mediterranean biodiversity is undergoing rapid alteration: because of the increased occurence of warm-water biota, it has been said that the Mediterranean is under a process of "tropicalization".