1584 TAXON 60 (6) • December 2011: 1584–1595 Wolf & al. • Mediterranean and Atlantic taxa of Ceramium INTRODUCTION The cosmopolitan genus Ceramium Roth (Ceramiaceae) is one of the most speciose genera in the Rhodophyta. It typi- cally occurs in eulittoral or shallow subtidal habitats and has a worldwide distribution (Maggs & al., 2002). The genus is characterized by cylindrical or slightly compressed thalli, axial cells bearing regular rings of periaxial cells and becoming in- completely to completely covered by cortical cells, alternate to pseudo-dichotomous branching, and straight to inrolled apices (Dixon, 1960; Maggs & Hommersand, 1993). Traditionally, three morphological groups have been recognized (Skage & al., 2005): (1) species that bear cortical spines; (2) species able to form cortication that completely covers the internodes; and (3) species with ecorticate internodes. However, these groups are not resolved as clades in molecular studies (Skage & al., 2005; Barros-Barreto & al., 2006). Currently, the systematics of this genus is still rather chaotic despite several recent studies (Maggs & al., 2002; Cho & al., 2003, 2008; Skage & al., 2005; Barros-Barreto & al., 2006). Taxonomic problems are linked to the high degree of variation in morphological characters classically used in species recog- nition: presence of cortical spines, numbers of periaxial cells, developmental patterns of the corticating filaments, branching pattern, and tetrasporangial features (Maggs & Hommersand, 1993). In addition, culture studies, suggesting a strong influence of the environment on morphology, and the results of molecular analyses have questioned the validity of morphological features for discriminating Ceramium species (Maggs & al., 2002). The identification of new Ceramium species is often complicated by their small size and epiphytic habit (South & Skelton, 2000). Molecular markers are therefore of increasing importance for screening and assessing Ceramium biodiversity (Maggs & al., 2002; Cho & al., 2003, 2008; Gabrielsen & al., 2003; Skage & al., 2005; Barros-Barreto & al., 2006). Difficulties in delimiting species result in and are affected by nomenclatural problems. Since the establishment of the ge- nus Ceramium by Roth (1797), taxon circumscriptions have undergone considerable changes. Currently, 893 species (and infraspecific) names are reported in the international database AlgaeBase (Guiry & Guiry, 2011), of which only 177 have been flagged as currently accepted taxonomically. A particular Ceramium Roth (Ceramiales, Rhodophyta) from Venice lagoon (Adriatic Sea, Italy): Comparative studies of Mediterranean and Atlantic taxa Marion A. Wolf, 1 * Katia Sciuto, 1 * Christine A. Maggs, 2 Maria Beatriz B. de Barros-Barreto, 3 Carlo Andreoli 1 & Isabella Moro 1 1 Department of Biology, University of Padova, via U. Bassi 58/B, 35131 Padova, Italy 2 School of Biological Sciences, Queen’s University of Belfast, 97 Lisburn Road, Belfast BT9 7BL, Northern Ireland, U.K. 3 Departamento de Botânica, Instituto de Biologia, Universidade Federal do Rio de Janeiro, Laboratório Integrado de Ficologia, Bloco A, sala 99, sub-solo 2562-6331, Brazil * These authors contributed equally to this work. Author for correspondence: Isabella Moro, isabella.moro@unipd.it Abstract The cosmopolitan genus Ceramium (Ceramiaceae, Rhodophyta) is a large and systematically complex group. The taxonomy of this genus remains in a chaotic state due to the high degree of morphological variation. Culture studies, suggest- ing a strong influence of environment on phenotype, and the use of molecular tools have recently questioned the validity of morphological features used in species recognition. Here we compare three Ceramium taxa from Venice lagoon with samples from northwest Europe using the plastid ribulose-1,5-bisphosphate carboxylase/oxygenase gene (rbcL) and the rbcL-rbcS intergenic spacer combined with morphological observations. A strongly banded species, previously identified as member of a poorly understood and misnamed group, the Ceramium diaphanum complex sensu Feldmann-Mazoyer, is probably conspecific with British samples of Ceramium diaphanum sensu Harvey, for which no valid name has been identified up to now. We show that Ceramium polyceras (Kützing) Zanardini is a valid name for this species. A fully corticated Ceramium species morphologically resembling C. secundatum differs at the species level from Atlantic C. secundatum; a valid name for this entity is Ceramium derbesii Solier ex Kützing, described from Mediterranean France. A third species characterized by cortical spines, previously listed as Ceramium ciliatum var. robustum (J. Agardh) Mazoyer, is shown to be Ceramium nudiusculum (Kützing) Rabenhorst, originally described from Venice. Keywords Ceramium; Mediterranean seaweeds; phylogeny; rbcL; rbcL-rbcS spacer; species delimitation; Venice lagoon Supplementary Material The alignment files are available in the Supplementary Data section of the online version of this article (http://ingentaconnect.com/content/iapt/tax).