How many habitats are there in the sea (and where)?

Current policies of habitat conservation, recovery, and management are strongly biased in favour of terrestrial systems, being poorly applicable to marine environments. A sound habitat classification, leading to spatially explicit accounts on the distribution of marine habitats and communities, is a prerequisite to identify conservation priorities, based on appropriate methods for assessing habitat sensitivity to human disturbance, aimed at preventing habitat loss. The ten major European marine habitat classifications, recognizing a total of 1121 marine habitats, have been here revised, and their major differences have been formally tested in terms of multivariate dissimilarity. Mediterranean-based classifications resulted rather uniform, their habitats forming a separate cluster from the rest of European ones; these differences might be due to either distinct ecological features, or to divergences in the way habitats are classified. Either too vague or too detailed classifications, leading to cumbersome appreciations of biodiversity at habitat level, fail to provide proper tools for the conservation and management of marine environments. Different species assemblages can inhabit the same habitat type, representing the well-know natural variability that, at large scale, should not affect the appreciation of habitat distribution. Intra-habitat natural variability, in fact, causes a misleading qualitative interpretation of small-scale biodiversity distribution. Mediterranean classifications have been integrated and simplified by identifying habitats according to explicit criteria: level on the shore, type of primary substrate, presence of bioconstructors, presence of habitat formers, presence of ecosystem engineers. The motivating idea is to limit the current emphasis on spatial dominance as the only criteria for the introduction of species, assemblages, and habitats in the lists, towards a clearer recognition of the structural and functional role of biodiversity. The reduction of previous classifications to a list of 94 Mediterranean marine habitat types represents an initial attempt at providing a simple and flexible tool for the evaluation of biodiversity at habitat level, leading to more feasible conservation measures, potentially extendable at European scale.