Population dynamics of the non-indigenous hydrozoan Clytia hummelincki (Hydrozoa: Campanulariidae) in two contrasting Mediterranean habitats

The non-indigenous hydrozoan Clytia hummelincki, recorded for the first time in the Mediterranean Sea in 1996, has now become established on barren grounds where conditions of low diversity, high grazing pressure and low levels of competition occur. The mechanisms of persistence of this species, as well as its population dynamics in this habitat and in species-rich, algae-dominated shallow rocky communities were investigated in situ at four sampling sites in the Northern Ionian Sea from October 2012 to September 2014. Multivariate analyses were conducted to assess the differences in population dynamics of this species in both contrasting habitats. Clytia hummelincki reached peak abundance during summer, with numerous reproductive structures observed in August and September. A remarkable regression period was evident in the cold season, when colonies were largely reduced to dormant tissue in the coenosarc of the hydrorhiza and were often covered by encrusting algae. Both regeneration of colonies from resting hydrorhizae and recruitment of new planulae are involved in the maintenance of the populations. Colonies regrew from hydrorhizae less than half of the time. Population dynamics in sea-urchin barren grounds and algae-dominated communities did not differ significantly, suggesting that populations of this hydrozoan are now well established in the central Mediterranean Sea. This species is no longer confined to low diversity, highly grazed, low competition environments.