Old and new problems in the circumscription of Mediterranean species of Lithophyllum (Corallinales, Rhodophyta)

Coralline algae are among the most common seaweeds in benthic habitats worldwide, as well as one of the most species-diverse groups. Unfortunately, they are also among the most problematic in terms of species circumscription and characterization. Their taxonomic history has been plagued by problems of cryptic diversity, phenotypic plasticity and taxonomic misinterpretations related to the importance of some morphological characters. Key tasks in current coralline taxonomy are to clarify the extent of genetic variation of individual species and link type specimens with molecular phylogenetic clades. We examined three Mediterranean species of Lithophyllum (L. byssoides, L. incrustans, L. stictaeforme) using a combination of molecular markers (cox2,3 spacer and psbA, rbcL and 18S rDNA genes). The results unraveled different scenarios for the three species.Mediterranean L. byssoides represents a well-defined entity that should be separated at species level from Atlantic populations reported under the same name. Cox2,3 and psbA data showed an unexpectedly high variation, suggesting limited dispersal in populations of this species. Lithophyllum incrustans is the type species of the genus Lithophyllum and its characterization is therefore critically important. A partial rbcL sequence obtained from the type material allowed to bridge the gap between molecular sequences and name-bearing type. In light of this result we concluded that L. incrustans is common in subtidal habitats and may easily form rhodoliths. Intertidal crusts reported as L. incrustans both in the Mediterranean and in Atlantic Europe probably do not belong to this species. Lithophyllum stictaeforme, as currently circumscribed on morphological basis, represents a complex of cryptic species for which a taxonomic and nomenclatural reassessment is necessary. Taken together, our results indicate that the genetic diversity of Mediterranean coralline algae is probably much higher than currently believed and sequences from type specimens will be essential to unambiguously identify many species.