The coralligenous is a typical Mediterranean marine habitat and hosts the highest concentration of biodiversity of this sea. Paradoxically, unlike Posidonia meadows, it is poorly studied. Like the famous tropical coral reefs, it is a complex biogenic habitat, based on encrusting red algae (numerous metazoan phyla also participate to bio-construction)(Ballesteros 2006). Concerning societal interest coralligenous habitats are known to be very attractive diving spots and but the importance of this ecosystems for fisheries The societal interest relies on the setup of innovative, cost-effective and accurate methods for biodiversity characterization and monitoring, based on genetic tools rigorously inter-calibrated with traditional taxonomy and photo-quadrate approaches. The aim of this project is to improve our understanding of its functioning and resilience capacity, and establish biodiversity and connectivity patterns among localities and ecological profiles, providing information for rationalizing the design of Marine Protected Area networks and monitoring methods. To reach this goal, synergy of interdisciplinarity will be used, combining population genetics, community ecology, microbial ecology and physical oceanography. This requests four tasks, partly using the same data, but different analysis methods. 1- Establishment of the taxonomic composition of the engineer corallinale algae. Several cryptic species in red algae were revealed by genetic markers (cf. B-4). Molecular tools (IMBE) and classical taxonomy (Dr Marc Verlaque, MIO) will be used 2- Establishment of the species composition for numerous small quadrates of coralligenous using meta-barcoding in different ecological profiles 3- Population structure and phylogeography of two selected taxa: a red alga Lithophyllum spp. and a bryozoan Myriapora truncata, on the CIGESMED sampling network, contextualized by ecological factors (cf above). 4- Synthesis and Comparisons: Propose new GES (good environmental status) indicators for the coralligenous and biodiversity management rules.