The genera <i>Melanothamnus</i> Bornet & Falkenberg and <i>Vertebrata</i> S.F. Gray constitute well-defined clades of the red algal tribe Polysiphonieae (Rhodomelaceae, Ceramiales)

<p><i>Polysiphonia</i> is the largest genus of red algae, and several schemes subdividing it into smaller taxa have been proposed since its original description. Most of these proposals were not generally accepted, and currently the tribe Polysiphonieae consists of the large genus <i>Polysiphonia</i> (190 species), the segregate genus <i>Neosiphonia</i> (43 species) and 13 smaller genera (< 10 species each). In this paper, phylogenetic relationships of the tribe Polysiphonieae are analysed, with particular emphasis on the genera <i>Carradoriella, Fernandosiphonia, Melanothamnus, Neosiphonia, Polysiphonia sensu stricto</i>, <i>Streblocladia</i> and <i>Vertebrata</i>. We evaluated the consistency of 14 selected morphological characters in the identified clades. Based on molecular phylogenetic (<i>rbc</i>L and 18S genes) and morphological evidence, two speciose genera are recognized: <i>Vertebrata</i> (including the type species of the genera <i>Ctenosiphonia, Enelittosiphonia, Boergeseniella</i> and <i>Brongniartella</i>) and <i>Melanothamnus</i> (including the type species of the genera <i>Fernandosiphonia</i> and <i>Neosiphonia</i>). Both genera are distinguished from other members of the Polysiphonieae by synapomorphic characters, the emergence of which could have provided evolutionarily selective advantages for these two lineages. In <i>Vertebrata</i> trichoblast cells are multinucleate, possibly associated with the development of extraordinarily long photoprotective trichoblasts. <i>Melanothamnus</i> has 3-celled carpogonial branches and plastids lying exclusively on radial walls of the pericentral cells, which similarly may improve resistance to damage caused by excessive light. Other relevant characters that are constant in each genus are also shared with other clades. The evolutionary origin of the genera <i>Melanothamnus</i> and <i>Vertebrata</i> is estimated as 75.7–95.78 and 90.7–138.66 Ma, respectively. Despite arising in the Cretaceous, before the closure of the Tethys Seaway, <i>Melanothamnus</i> is a predominantly Indo-Pacific genus and its near-absence from the north-eastern Atlantic is enigmatic. The nomenclatural implications of this work are that 46 species are here transferred to <i>Melanothamnus</i>, six species are transferred to <i>Vertebrata</i>, and 13 names are resurrected for <i>Vertebrata</i>.</p>