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

Mediterranean Lithophyllum stictiforme (Corallinales, Rhodophyta) is a genetically diverse species complex: implications for species circumscription, biogeography and conservation of coralligenous habitats

Lithophyllum species in the Mediterranean Sea function as algal bioconstructors, contributing to the formation of biogenic habitats such as coralligenous concretions. In such habitats, thalli of Lithophyllum, consisting of crusts or lamellae with entire or lobed margins, have been variously referred to as either one species, L. stictiforme, or two species, L. stictiforme and L. cabiochiae, in the recent literature. We investigated species diversity and phylogenetic relationships in these algae by sequencing three markers (psbA and rbcL genes, cox2,3 spacer), in conjunction with methods for algorithmic delimitation of species (ABGD and GMYC). Mediterranean subtidal Lithophyllum belong to a well-supported lineage, hereby called the L. stictiforme complex, which also includes two species described from the Atlantic, L. lobatum and L. searlesii. Our results indicate that the L. stictiforme complex consists of at least 13 species. Among the Mediterranean species, some are widely distributed and span most of the western and central Mediterranean, whereas others appear to be restricted to specific localities. These patterns are interpreted as possibly resulting from allopatric speciation events that took place during the Messinian Salinity Crisis and subsequent glacial periods. A partial rbcL sequence from the lectotype of L. stictiforme unambiguously indicates that this name applies to the most common subtidal Lithophyllum in the central Mediterranean. We agree with recent treatments that considered L. cabiochiae and L. stictiforme conspecific. The diversity of Lithophyllum in Mediterranean coralligenous habitats has been substantially underestimated, and future work on these and other Mediterranean corallines should use identifications based on DNA sequences