PLIO-PLEISTOCENE EXTINCTIONS, LESSEPSIAN INVASION AND THE FUTURE OF THE MEDITERRANEAN BIVALVE FAUNA

The fossil record may hold a key to understanding the origins of present-day macroecological patterns and their future fate in a human-impacted world. Following the opening of the Suez Canal hundreds of Red Sea species have entered the Mediterranean Sea making it a global hot spot of marine bioinvasion. With the ongoing influx of the subtropical and tropical alien species and increasing sea surface temperatures, the Mediterranean biota is currently gaining a more tropical character. This susceptibility to invasion was suggested to reflect the presence of an empty ecological space left after decimation of the incumbent warm-water fauna during Plio-Pleistocene climate fluctuations. As mollusks are among the most prolific immigrants, we test this hypothesis using data on taxonomic composition and body size of Pliocene Mediterranean bivalves derived from literature sources and museum collections.

The Pliocene inter-specific size-frequency distribution (SFD) is strikingly similar to the SFDs of the Recent Red Sea bivalve fauna, in spite of different biogeographic affinities. In contrast, body-size patterns in both assemblages are significantly different from the present-day Mediterranean fauna, which is characterized by smaller median and modal size. This distinct shape of the modern Mediterranean SFD may reflect the selective nature of the late Pliocene-Pleistocene extinction pulses related to the onset of the Northern Hemisphere glaciations. Subsequent re-immigration of warm-water species from the tropical Atlantic was hindered by the cold upwelling along the NW coasts of Africa. The resulting invasion credit in the Mediterranean Sea is currently being paid by the Red Sea bivalves. Species crossing the Suez Canal tend to be larger than native ones, not due to size-selective invasion process, but because of the gross differences in the body-size distributions of the source and recipient species pools. We suggest that the continuing inflow of tropical invaders will restore the Pliocene body-size patterns in the Mediterranean bivalve fauna.