Hydrodynamism and the percentage of organic matter in the silt is the variable combination that best explains the biotic structure. While all ascidians show a certain tolerance to diverse environmental factors, some species such as Ciona intestinalis, Diplosoma spongiforme, Phallusia mammillata, Microcosmus squamiger, Styela plicata and Synoicum argus could be considered as indicators of areas which have been subjected to intense stress over long periods of time, whereas others such as Aplidium conicum, Aplidium punctum, Clavelina dellavallei, Halocynthia papillosa and Stolonica socialis could be categorized as species very sensitive to stress.