Marine Bioinvasions in the Mediterranean Sea – History, Distribution and Ecology

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The Mediterranean Sea is in many ways a unique body of water. It is small, but deep compared to other bodies of water of its size, and for its size (0.82 % in surface area of the world oceans and 0.32 % in volume) it encompasses an impressive variety of ecosystems. From a biodiversity perspective, it can be considered relatively rich in species. Bianchi and Morri (2000) estimate that more than 8500 macroscopic marine species should live in the Mediterranean Sea, which is 4–18 % of the world ' s marine species (depending on different estimates of global diversity). This means that it has high species density for its size (Bianchi and Morri 2000). The body of water that is now the Mediterranean Sea went through dramatic changes in its biota through most of its existence. It is a vestige of the Tethys Ocean, meaning that in prehistoric times it was inhabited by tropical biota. After it was squeezed between Eurasia and Africa and cut off from the rest of the Indo-Pacific at the end of the Miocene (ca. 10 million years ago) it slowly lost its tropical characteristics. It was also cut off from the Atlantic Ocean several times throughout its history, eventually becoming a warm-temperate to subtropical body of water once the Straits of Gibraltar opened at the late Pleistocene (ca. 5 million years ago). These changes in its environmental conditions, that were followed by changes in its biota (as evident from its fossil record; Ruggieri 1967; Sorbini 1988; Zaccaria 1968), naturally occurred over timescales of thousands to millions of years. But lately the rate of biotic change has been increasing dramatically. The biodiversity in the Mediterranean Sea has been altering at an alarmingly high rate for the past two centuries due to human-mediated arrival of new species, with an apparent acceleration in the rate of recorded invasions in the last four decades of the twentieth century. In this chapter we review the current status of the invasion process in the Mediterranean, examine spatio-temporal patterns of species from three major taxo-nomic groups of invaders, and explore the ecological and conservation implications of some of the most infamous invasions. Special emphasis is given to the major vector of invasion into the Mediterranean Sea — the Suez Canal, and to lagoons as important hotspots of invasion in the western Mediterranean.