First inventory of the shallow-water benthic hydrozoan assemblages of the Gökçeada Island (northern Aegean Sea)

The hydroid fauna of the Mediterranean Sea is considered one of the best known in the world, but the hydrozoans of the Aegean Sea remain poorly studied, hindering efforts to identify alien and invasive species in the region. The spatial and seasonal composition of the shallow-water (0–20 m depth) benthic hydrozoan assemblage from Gökçeada Island was investigated in summer 2012 and winter 2013. Overall, 48 hydrozoan taxa were identified, and their presence and ecological features are discussed herein. Twelve species are recorded for the first time in the Aegean Sea, and the same number for the Turkish coasts. Differences in species composition were detected between the northern and southern coasts of Gökçeada by cluster, multidimensional scaling (MDS) and permutational multivariate analysis of variance (PERMANOVA) analysis, whereas seasonal and vertical distribution patterns were not statistically significant. Differences in species richness and composition between the northern and southern coasts may be explained by the distinct geomorphological aspects of the shores, providing a spatial heterogeneity in the availability of substrates for the hydroid colonies. Observed differences are attributable to the occurrence and/or abundance of common species such as Sertularella ellisii, Aglaophenia tubiformis, Clytia hemisphaerica, Clytia linearis, Eudendrium racemosum, Plumularia obliqua, Eudendrium capillare, Turritopsis dorhnii and Dynamena disticha, rather than to the presence of rare, exclusive species.