Communities of Mediterranean mediolittoral caves and overhangs

Mediolittoral caves and overhangs are a special habitat whose main distinctive trait is the low irradiance. The habitat can be found on most rocky shores regardless of wave exposure. The shaded nature of caves and overhangs diminishes the amount of desiccation suffered by biota during periods of low tides which allows certain species to proliferate.

Algal growth is restricted to a very few species that can withstand low light levels, low to high hydrodynamism, and extended periods of desiccation. Canopy-forming macroalgae are absent whilst encrusting rhodophytes are dominant. Some variation in the species composition of the individual caves must be expected depending on local conditions although in general the communities are very poor in species. At the entrance to mediolittoral caves where there is sufficient light red algae may be present, particularly an association of non‐calcified encrusting red algae Hildenbrandia rubra and Phymatholithon lenormandii, under the red alga Lithophyllum byssoides.

In general, the biomass and diversity of algal species found in upper and mid‐shore littoral caves decreases with increasing depth into the cave as the light levels diminish. Fauna usually only occur on the lower and mid-level walls of the caves and generally comprise barnacles, anemones and tube‐forming polychaetes.

Indicators of quality:                                                                                                                      

Both biotic and abiotic indicators have been used to describe marine habitat quality. These include: the presence of characteristic species as well as those which are sensitive to the pressures the habitat may face; water quality parameters; levels of exposure to particular pressure, and more integrated indices which describe habitat structure and function, such as trophic index, or successional stages of development in habitats that have a natural cycle of change over time.

There are no commonly agreed indicators of quality for this habitat, although particular parameters may have been set in certain situations e.g. protected features within Natura 2000 sites, where reference values have been determined and applied on a location-specific basis.

Synthesis of Red List assessment

Confidence in the assessment

Pressures and threats