Sometimes they come back: the re-colonization of the alien seagrass Halophila stipulacea (Forssk?l) Ascherson, 1867 (Hydrocharitaceae) in the Palinuro Harbor (Tyrrhenian Sea, Italy)

摘要

We report the return of the alien seagrass Halophila stipulacea (Forssk?l) Ascherson, 1867 (Hydrocharitaceae) in the harbor of Palinuro (Salerno, Tyrrhenian Sea, Italy), six years after the local extinction of this species in this area in 2011. The record of this new colonization event of H. stipulacea in Palinuro represents the northernmost location of this tropical alien seagrass in the Western Mediterranean. The species, when first studied in June 2007, had a bottom cover of 16 m2, which was reduced to 4 m2 in 2009, and to 2 m2 in 2010. In 2011, the species was no longer detectable and was considered extinct in the area. In September 2017, the species was recorded again in the area in a few zones, close to where it was originally observed, and covering approx. 5.5 m2. Seven patches were monitored, occurring between 1.5 and 4 m depth, and with a mean shoot density of 6,300 shoots/m2, mean leaf length of 33.8 mm, and mean width of 5.3 m, while flowers were absent. Evidence of strong grazing (bite-marks) was observed on 57% of the H. stipulacea leaves by native fish species.