The Sargassum conundrum: very rare, threatened or locally extinct in the NW Mediterranean and still lacking protection

Species of the genus Sargassum are large canopy-forming marine brown algae (engineer species) found worldwide from tropical to cold-temperate latitudes. Among this very diversified genus (335 taxa accepted taxonomically), only 9 species (including the invasive S. muticum ) have been reported from the Mediterranean Sea. We have analysed the changes over more than two centuries in the Sargassum’s pattern of distribution along ~2,970 km of north-western Mediterranean coasts, using all available historical sources (literature and herbarium vouchers) and 2003–2014 field surveys. Though common in the past, all the long-lived native Mediterranean species (except S. vulgare ) have become extremely rare or locally extinct, while the invasive S. muticum has developed large populations in some coastal lagoons. The increase in water turbidity, trawling and fishing nets is possibly responsible for the regression of the deep populations of S. hornschuchii, S. acinarium , whereas overgrazing by sea urchins and habitat destruction (coastal development) are probably responsible for the decline of shallow populations. In contrast with the terrestrial realm, where thousands of species are protected, even some relatively common species, Mediterranean species of Sargassum that seem to have become extinct in extensive areas, such as S. acinarium and S. hornschuchii , are, surprisingly, still lacking proper protection status and inclusion on the IUCN Red List.