The fate of Cystoseira crinita, a forest-forming Fucale (Phaeophyceae, Stramenopiles), in France (North Western Mediterranean Sea)

In the Mediterranean Sea, Fucales, and in particular the species of the genus Cystoseira C. Agardh, are habitat-forming species dominating several benthic assemblages from the littoral fringe down to 70–80 m depth in the clearest waters. They generate high primary production involved in the maintenance of diversified trophic levels, they provide shelter, food, habitat and nursery areas to many species, and they are long-lived species that constitute a good model for studying human impact on species diversity. Cystoseira crinita Duby is a Mediterranean endemic cespitose species, living in shallow waters (0–5 m depth), in places with little wave action and high solar irradiance. To reconstruct the long-term patterns of change in its distribution along the French Mediterranean coast (French Catalonia, Languedoc, Provence, French Riviera and Corsica), all the historical data (published articles, unpublished reports and herbarium vouchers) collected since the 18th century were searched and analysed. To assess the current status of the species, several field surveys were conducted between 2010 and 2016 by snorkelling and SCUBA diving. Cystoseira crinita is currently extinct in French Catalonia, near-extinct in Languedoc and western Provence, and functionally extinct on the French Riviera coast, while the eastern Provence and Corsican populations have remained stable. The main probable causes of decline are habitat destruction and overgrazing by herbivores.