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The Mediterranean Sea is a somewhat depauperate part of the eastern Atlantic Ocean, with about 540 species of fishes. Drying out during the Messinian Salinity Crisis millions of years ago eliminated most fishes from the Mediterranean, and cooler temperatures in the Straits of Gibraltar prevented warm water fishes found in the Gulf of Guinea from moving into the warm waters of the eastern Mediterranean when the Atlantic and Mediterranean were reconnected at the end of the Messinian Salinity Crisis about 5.3 million years ago (Patarnello et al. 2007).
In 1869, a sea-level route, the Suez Canal, was opened, connecting the warm but depauperate eastern Mediterranean with the Red Sea, the latter being part of the rich Indo-West Pacific region. For some time after construction, faunal transfers between the Red Sea and the Mediterranean were inhibited by the saline waters of the Bitter Lakes in the middle of the canal. In the first edition of A history of fishes in 1931, Norman reported that 16 species of Red Sea marine fishes had moved through the Suez Canal and established themselves in the eastern Mediterranean Sea. In the ensuing decades, Adam Ben-Tuvia, Daniel Golani, and others raised the number to 24, 27, 31, 46, and most recently to 68 (Golani 2006). All but one of the species (a seabass,Dicentrarchus) are what are termed Lessepsian migrants (named after Ferdinand Lesseps who was in charge of constructing the canal), having moved in one direction, from the Red Sea into the eastern Mediterranean. At least 24 species of Red Sea origin have reached as far west as the southeastern Aegean Sea. As an example of how successful a migrant can be, the Brushtooth Lizardfish (Saurida undosquamis) was first taken in the Mediterranean in 1952. By 1955, 266 tons of this lizardfish were landed by local trawlers, constituting close to 20% of the trawler catch in Israeli waters (Golani 1993).
Why have these movements been virtually onedirectional? First, the diversity of inshore fishes is greater in the Red Sea, part of the Indo-West Pacific fish fauna, than in the Mediterranean, suggesting that niches are more completely filled in the Red Sea, which means fewer ecological opportunities for new immigrants. Second, there appears to be an “empty niche” in the eastern Mediterranean, associated with water temperatures, with temperatures again being warm enough for warm water fishes.
Finally, many of the species that penetrated the canal are widespread species, adapted to a wide variety of living conditions. Consider the distributions of three of the invading species, a halfbeak and two mackerels. Hemiramphus far is the most widespread member of its genus, known from South Africa across the Indian Ocean north to Okinawa, south to Australia, and east to Tonga and Fiji. It, rather than the Red Sea-Persian Gulf He. marginatus, successfully moved through the canal and established populations that have now spread west and north as far as Albania. The two mackerels, the Narrow-barred Spanish Mackerel (Scomberomorus commerson) and the Indian Mackerel (Rastrelliger kanagurta), are the most widespread members of their genera, occurring from South Africa north to the Red Sea, east to China and Japan, and south to Australia and Fiji (Collette & Nauen 1983, maps on pp. 49 and 63). Such generally successful colonist species could have been predicted as the most likely taxa to take advantage of the opportunities in the eastern Mediterranean because they are adapted to a wide range of ecological conditions.
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