Diversity of Mediterranean coralline algae: recent advancements in the knowledge of a highly threatened group of seaweeds

Coralline algae are globally distributed ecosystem engineers that in the Mediterranean Sea act as key builders of habitats (such as coralligenous concretions, rims of Lithophyllum byssoides, rims of articulated corallines and rhodolith beds) extending on large stretches of coastal rocky bottoms. Due to the semi-enclosed nature and oceanographic features of the basin, Mediterranean corallines are considered particularly vulnerable to future climatic changes. For this reason they have attracted much interest in the last decades, with many investigations focusing primarily on responses to ocean acidification and responses of coralline-dominated communities to anthropogenic stressors. Several aspects of their diversity, however, have also received great attention. Taxonomy and systematics of Mediterranean corallines have been studied for more than two centuries but, until recently, the classification of these seaweeds was based entirely on traditional morphoanatomical grounds. The introduction of molecular data has substantially reshaped our view of their taxonomic diversity. The first DNA sequence data for Mediterranean corallines were published in 2009 and unraveled the presence of Corallina caespitosa, an articulated species now known to be widespread and formerly confused with Ellisolandia elongata. Overall, Lithophyllum is the genus that has been most studied by an integrative approach combining morphological and molecular data. Such studies revealed in the Mediterranean a striking case of cryptic diversity, a phenomenon that is now known to be pervasive in coralline algae: the Lithophyllum stictiforme complex. Two recent studies showed that this alga, previously known as a single species, represents a complex of at least 11 different cryptic species (of which not less than 8 present on the French coasts). Lithophyllum incrustans, the type species of Lithophyllum, has been shown to be primarily an infralittoral species, despite of frequent reports in mesolittoral habitats. Mediterranean populations of Lithophyllum byssoides, known as builders biogenic rims, are likely to represent a separate species from Atlantic populations. Molecular data also allowed the phylogenetic assessment of Pneophyllum cetinanensis, the first coralline alga discovered in freshwater. Additional molecular data led to new records for the Mediterranean and to an improved biogeographic circumscription for several species, especially of the genus Mesophyllum and Phymatolithon. The current number of 59 coralline species recorded for the Mediterranean should not be considered a definitive estimate and new species can be expected to be discovered in the near future (particularly in the genera Lithophyllum and Neogoniolithon).