Diversity of <i>Cystoseira sensu lato</i> (Fucales, Phaeophyceae) in the eastern Atlantic and Mediterranean based on morphological and DNA evidence, including <i>Carpodesmia</i> gen. emend. and <i>Treptacantha</i> gen. emend.

The diversity and phylogenetic relationships of specimens in the brown algal genus <i>Cystoseira sensu lato</i> from the eastern Atlantic and Mediterranean were examined using DNA sequences for four genetic markers (<i>psb</i>A, mt23S, <i>cox</i>1 and <i>nad</i>1). Our results provide evidence to resolve most of the species documented in the eastern Atlantic and Mediterranean as <i>Cystoseira</i> in three clades that are interpreted here as representing three genera. Morphological studies confirmed the distinctness of the genera and identified characters that distinguish species. One clade corresponds to <i>Cystoseira sensu stricto</i>, including the type species <i>C. foeniculacea</i>, together with <i>C. compressa, C. humilis</i> and the resurrected species <i>C. aurantia</i>. Species in <i>Cystoseira</i> are characterized by a caespitose habit and receptacles with grouped conceptacles lacking spiny appendages. The second clade corresponds to <i>Treptacantha</i> gen. emend., with <i>T. abies-marina</i> as the type species. A further 11 Atlantic and/or Mediterranean species were transferred to this genus. The concept of <i>Treptacantha</i> was better redefined, using the following morphological characters: smooth primary branches at least in the basal region, cortical cells with thickened walls and square meristoderm cells in cross section. The third clade corresponds to <i>Carpodesmia</i> gen. emend., with <i>C. zosteroides</i> as the type species. Another six Mediterranean species and the Atlantic/Mediterranean <i>C. tamariscifolia</i> were transferred to this genus. The concept of <i>Carpodesmia</i> was better defined to include new morphological features: all branches having spines, cortical cells with thin walls and rectangular meristoderm cells in cross section.