Taxonomic Complexity and Conservation Implications of Mediterranean Mesophotic Rhodolith Beds

ABSTRACT Rhodolith beds are bioconstructions formed by coralline algae (Rhodophyta: Corallinophycidae) and are ecologically important worldwide because they provide habitats that support diverse biocenoses. However, conservation efforts are limited by gaps in our knowledge of their spatial distribution and species composition. Ongoing scientific research at mesophotic depths (60‐ to 80‐m depth) in the Menorca Channel, western Mediterranean, shows that rhodolith beds host a wide variety of morphologies and multiple coexisting rhodolith‐forming species. However, knowledge of their taxonomy is deficient. By integrating traditional taxonomy with molecular analysis and using the markers psb A and rbc L, our study revealed that at least 15 different species of coralline red algae belonging to three orders (Hapalidiales, Corallinales and Sporolithales) contribute as rhodolith‐forming species in this area. These findings confirm the existence of the cryptic species diversity of coralline algae in mesophotic habitats and imply that conservation efforts based on the European Union Habitats Directive, which considers only the two species predominant in NE Atlantic rhodolith beds ( Phymatolithon calcareum and Boreolithothamnion corallioides ), may be insufficient. Effective conservation in the Mediterranean must recognize the multispecies and morphologically complex nature of rhodolith beds—an insight made possible by the critical use of molecular data.