Four frilly corticated nonspiny species of Ceramium are currently recognized in the British Isles, all formerly confused under the illegitimate name Ceramium rubrum (Hudson) C. Agardh. One of these species was known briefly as C. nodulosum (Lightfoot) Ducluzeau but, in 1996, RC. Silva and others [Univ. Calif. Publ. Bot. 79] showed that this is a later homonym Of C. nodulosum de Candolle and advocated conserving C. rubrum with a neotype. As an alternative to this, we investigated whether the name C. virgatum Roth 1797 might be available for this species. The aim of the present study was to typify C virgatum, in the context of better characterizing this and the other fully corticated species of Ceramium that occur in the British Isles, by analysis of plastid DNA (cpDNA) restriction fragment length polymorphism (RFLP) or rbcL sequences (or both) of each species from various sites, including their type localities. The lectotype of C virgatum is Roth's illustration of a Ceramium specimen from Eckwarden, North Sea, which could represent any member of the 'C rubrum' group. Material resembling Roth's from the North Sea island of Helgoland is here designated as an epitype of C virgatum. Its rbcL sequence was aligned with sequences of all C rubrum-like species in the British Isles, viz. C secundatum, C botryocarpum, C pallidum and 'C nodulosum'. The Helgoland material was clearly conspecific with the alga previously identified as C nodulosum from Ireland (0.08% rbcL divergence, compared to a minimum between-species divergence of 1.8%). We therefore suggest that the name C. virgatum Roth should be employed for this species. Each of the four fully corticated species can be identified by morphological features, in particular the number of axial segments between branches. Phylogenetic analyses of rbcL sequences and cpDNA RFLP data showed C pallidum to be basal among the North Atlantic C. rubrum-like species and indicated a close relationship between C secundatum and C. botryocarpum.
