Distribution and phenotypic variability of the Mediterranean gorgonian Paramuricea macrospina (Cnidaria: Octocorallia)

The genus Paramuricea is present in the Mediterranean Sea with two species, P. clavata and P. macrospina. These species have similar bathymetric distributions and can live in sympatry. P. macrospina shows morphological plasticity leading sometimes to an erroneous classification as P. clavata. Studying four ambiguous morphotypes of P. macrospina collected in different localities of the Mediterranean Sea, we provide new insights on both the taxonomy and the distribution of this species. Our analyses reveal high morphological plasticity within the sampled colonies, which in two cases were characterised by a peculiar pigmentation and a morphological pattern resembling that of P. clavata. After having confirmed the genetic identity of the samples, we tested the validity of traditional taxonomic characters, and found that the most reliable character to discriminate between P. macrospina and P. clavata is the number of rows of spindles in the collaret of the polyps. All other features are highly variable and therefore do not allow a correct identification. The bathymetric and geographic distribution of P. macrospina is here updated, showing, with the exception of the south-eastern side, broad preference in the whole basin. Moreover, further studies are needed to investigate the gene flow among and within the Mediterranean populations of P. macrospina and to assess whether the morphological plasticity is driven by adaptive genetic processes of populations exposed to different environmental, climatic and bathymetric conditions.