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Spreading of the alien seagrass Halophila stipulacea (Hydrocharitaceae) along the sicilian coast (western Mediterranean Sea)


Halophila stipulacea (Forsskål) Ascherson is a tropical seagrass distributed along the western coasts of the Indian Ocean and in the Red Sea (den Hartog, 1977). This species was previously considered a paleomediterranean element, survived as a relict in the eastern Mediterranean Sea, but later on Por (1971) hypothesized that it was a Lessepsian immigrant entered the Mediterranean Sea after the opening of the Suez Canal (1869). H. stipulacea remained in the eastern Mediterranean for several decades (Lipkin, 1975), and only recently it spreads towards the western basin through Malta and the Ionian coast of Sicily (Lanfranco, 1970; Van der Velte and Den Hartog, 1989; Alongi et al., 1993). The first record in the western part of the Mediterranean basin was in Vulcano Island (Acunto et al., 1997). Numerous records have been yet registered along the northern coast of Sicily till now (Vulcano Island, Oliveri-Tindari coastal lakes, harbour of Catania, Giardini Naxos coasts). The spreading of H. stipulacea along the Tyrrhenian coast of Sicily is actually in progress as proved by this new record off the coast of Termini Imerese (Palermo). The seagrass could have reached yet the Gulf of Palermo, where numerous plant fragments have been recently observed. Although settlement of H. stipulacea is still spatially limited, its expansion along the coasts of Sicily should be monitored; indeed, since it out-competes the native Mediterranean seagrasses, it could induce severe changes in the sublittoral communities.