Analysis of the blennioid assemblages associated with different rocky shore habitats in the Ligurian Sea

The differences among blennioid assemblages (families Blenniidae and Tripterygiidae) on different habitats were assessed at two localities of the Ligurian Sea. namely Arenzano and Riva Trigoso. The assemblage composition and species relative density were evaluated visually on four different habitats of diverse wave exposure and substratum orientation (macro-habitat characteristics): two vertical intertidal and subtidal habitats (exposed and sheltered rockwalls) and two horizontal subtidal habitats (semi-exposed flat rock and boulders and pebbles). Each habitat was also characterized in relation to micro-habitat features, such as substratum complexity, heterogeneity and amount of algae cover. Patterns of differences among habitats in assemblage variables and fish density, and the influences of macro- and micro-habitat features on these patterns were studied at small (within localities) and large (across localities) spatial scales. Higher values of species richness (S), diversity (H' sp ) and evenness (J) were generally associated with vertical habitats, as a result of a positive correlation with substratum orientation. The presence of an intertidal zone in the rockwall habitats may partially explain the observed differences in assemblage variables between vertical and horizontal habitats. The strength of relationships between S. H' sp and J and the other investigated habitat variables (exposure, complexity, heterogeneity and algae cover) varied greatly depending on spatial scale. All these relationships were positive, except for complexity. Significant variation in the assemblage total density among habitats was recorded only at Arenzano, where a larger number of fishes were counted on rockwalls rather than on the horizontal habitats. The positive effect of orientation on fish total density was strictly dependent on spatial scale. Fish total density showed a negative correlation with complexity and a positive correlation with heterogeneity, both relationships being unaffected by spatial scale. The unexpected relationship with complexity was probably due to the fact that, in the most complex habitat (i.e. boulder and pebbles), the potential positive effect of high complexity on fish density might be overcome by the negative influence of other environmental features, such as horizontal orientation and low wave exposure. Complexity and heterogeneity thus seemed good predictors of fish total density, but their role needs to be carefully interpreted. The most marked differences in species composition and relative density were found between rockwalls and the other habitats, mostly due to an unbalanced distribution of some stenoecious species. Variations in species relative density were related to different combinations of both macro- and micro-habitat features, and these relationships usually changed depending on spatial scale.