MEDITERRANEAN CORALLIGENOUS ASSEMBLAGES: A SYNTHESIS OF PRESENT KNOWLEDGE

Coralligenous concretions, the unique calcareous formations of biogenic origin in Mediterranean benthic environments, are produced by the accumulation of encrusting algae growing in dim light conditions. This review provides an overview of the results obtained by the main studies dealing with these formations, including the environmental factors which influence the development of coralligenous communities, their distribution, types, assemblages, builders and eroders, the biotic relationships and processes that create and destroy coralligenous assemblages, their dynamics and seasonality, and the functioning of several outstanding and key species. Special attention is devoted to the biodiversity of coralligenous communities and a first estimation of the number of species reported for this habitat is provided. Major disturbances affecting coralligenous communities are discussed, ranging from large-scale events that are probably related to global environmental changes to degradation by waste water or invasive species. Degradation by fishing activities and by divers is also considered. Finally, the main gaps in current scientific knowledge of coralligenous communities are listed and some recommendations are made regarding their protection. Introduction and description Encrusting calcareous algae are important components of benthic marine communities within the euphotic zone (Blanc & Molinier 1955, Adey & McIntyre 1973, Littler 1973a, Lebednik 1977, James et al. 1988, Dethier et al. 1991, Adey 1998) and their historical roles as reef builders have been chronicled thoroughly by Wray (1977). Coralline algae are major contributors to coral reef frameworks (Finckh 1904, Hillis-Colinvaux 1986, Littler 1972) where they usually are the dominant reef-forming organisms (Foslie 1907, Odum & Odum 1955, Lee 1967, Littler 1973b). Although encrusting corallines are adapted to grow at low light conditions (Littler et al. 1986, Vadas & Steneck 1988), coralline algal reef frameworks are usually restricted to littoral or shallow sublittoral environments throughout the marine realm (e.g., Littler 1973b, Adey & Vassar 1975, Laborel et al. 1994) because they easily withstand turbulent water motion and abrasion (Littler & Doty 1975, Adey 1978). The only known exception to this restriction is the coralligenous framework, a coralline algal concretion that thrives exclusively in Mediterranean deep waters (20–120 m depth). There is no real consensus among scientists studying benthic communities in the Mediterranean Sea about what a coralligenous habitat is. In this review a coralligenous habitat is considered to be a hard substratum of biogenic origin that is mainly produced by the accumulation of calcareous encrusting algae growing in dim light conditions. Algae and invertebrates growing in environments with low light levels are called sciaphilic in opposition to photophilic, that is, growing at high light levels. All plants and animals thriving in coralligenous habitats are, thus, sciaphilic. Although more