The genus Lithophyllum (Corallinales, Rhodophyta) in the Mediterranean: current knowledge of its taxonomy, phylogeny and biogeography.

With 142 species currently accepted, Lithophyllum is the most species-rich genus in the coralline algae, a cosmopolitan group of calcareous seaweeds belonging to the phylum Rhodophyta (red algae). The genus has a particularly high floristic and ecological relevance in the Mediterranean Sea, where its presence since the mid Miocene is documented by fossil records. Our understanding of the taxonomy, distribution and biogeography of Lithophyllum in the Mediterranean has progressed in recent years thanks to the use of molecular data (DNA sequence data and transcriptomic data). At present, 17 species of Lithophyllum are known from the Mediterranean; some are recent records, for which the presence has been revealed by DNA sequence data. The actual diversity of the genus, however, is certainly higher, as the existence of a complex of cryptic species (the Lithophyllum stictiforme complex) was recently demonstrated. Molecular studies also revealed contrasting biogeographic patterns: whereas some species are widespread, others appear to have a circumscribed distribution and are known only from a few locations or regions. These patterns are considered a consequence of speciation/population fragmentation events related to hydrogeological changes of the Mediterranean in the last 5 mya. A full taxonomic and biogeographic reassessment will require further studies based on extended geographic sampling, re-examination of herbarium collections and sequencing of selected type specimens.