The Mediterranean Cystoseira Complex (Fucales, Ochrophyta): An Updated Illustrated Guide and Identification Key

Macroalgal forests dominated by Cystoseira sensu lato (s.l.) (genera Cystoseira sensu stricto, Ericaria, and Gongolaria) are among the most ecologically significant habitats in the Mediterranean Sea, providing structural complexity, biodiversity support, and key ecosystem services. Despite their ecological importance and protected status under multiple international directives, they are experiencing a severe decline and local extinction driven by anthropogenic pressures and climate change, making accurate monitoring and species identification a conservation priority. However, identification at the species level remains challenging, even for expert researchers, due to the high variability of diacritical characters and the lack of comprehensive photographic documentation in existing identification tools. To address this gap, we developed a photographic atlas key covering all Cystoseira s.l. taxa with confirmed presence in the Mediterranean Sea. The atlas consists of an identification key, a glossary, and species-specific descriptive cards. The cards, arranged alphabetically within each genus, include photos of complete thalli alongside macro-scale diacritical characters, captured in the field and in the laboratory, taxonomic synonymies, ecological and distributional information, and notes on potentially confounding species. This resource provides a user-friendly, stepwise identification tool accessible to both expert and non-expert users, with the aim of improving knowledge of the species-level distribution and ecological status of Cystoseira s.l. across Mediterranean coastal waters.