Structure of cnidarian populations in Mediterranean sublittoral benthic communities as a result of adaptation to different environmental conditions

The presence and abundance of Cnidarians, extremely cornmon in Mediterranean sublittoral benthic communities, exerts a profound effect on the entire structure of such communities, although the roles of Anthozoans and Hydrozoans differ in accordance with their different biological characteristics. Structural differences in benthic cnidarian populations were observed in three sublittoral cornmunities located within 100 m of each other yet differing considerably in light intensity and water movement. Species-area and Shannon's diversity-area curves were computed for each community from reticulate samples constituted by 18 subsamples of 289 cm² . Species-area curves were fitted to semilogarithmic functions, while diversity-area curves were fitted to Michaelis-Menten functions by the method of least squares. Species richness, species distribution, alpha-diversity, and pattem diversity were estimated from the fitted curves. Patch size for Anthozoans was smaller than that for Hydrozoans in all three communities. Diversity was greatest in the communities with the lowest and with the highest light intensity and water flow levels, but species richness was highest in the intermediate community. The most structured community (with the most heterogeneous species distribution pattem) had the largest organisms with the longest life spans. It is suggested that the difference of water flow conditions in these three communities leads to the development of different spatial pattems of individuals and colonies. The size, shape and small-scale distribution of these organisms are probably correlated to their ability of enhancing prey capture under different water flow conditions.