Morphological and molecular assessment of <i>Sargassum</i> (Fucales, Phaeophyceae) from Caribbean Colombia, including the proposal of <i>Sargassum giganteum</i> sp. nov., <i>Sargassum schnetteri</i> comb. nov. and <i>Sargassum</i> section <i>Cladophyllum</i> sect. nov.

<div><p>The use of molecular markers to evaluate species delineation in the genus <i>Sargasssum</i> is of critical importance given the large variability of morphological traits among and within taxa. To date, most molecular studies of this group have focused on the central Indo-Pacific region, even though the genus is also well represented in the Atlantic Ocean. In the present study, we re-assessed the diversity and taxonomy of <i>Sargassum</i> and the monospecific genus <i>Cladophyllum</i> from Caribbean Colombia (especially from the Santa Marta-Tayrona National Natural Park) and extended to the western tropical Atlantic towards a comprehensive revision of the genus in this basin based on morphological and molecular analyses (ITS-2, <i>rbc</i>LS, <i>cox</i>3 and <i>mtsp</i>) on new and existing collections from the region. Sequences obtained from the genus <i>Cladophyllum</i> revealed its nesting inside <i>Sargassum</i> subgenus <i>Sargassum</i> as a close relative of <i>S. pteropleuron</i>, a broadly distributed species in the Caribbean. Analysis of molecular divergence in this newly identified clade demonstrated a similar range of variation as that present in existing sections leading to the description of a new section, <i>Sargassum</i> sect. <i>Cladophyllum</i> (Bula-Meyer) Camacho, Mattio & Diaz-Pulido, sect. nov., that includes the two sister species mentioned above. Based on these results, we also merged <i>Cladophyllum</i> within <i>Sargassum</i> and renamed its unique species <i>S. schnetteri</i> (Bula-Meyer) Camacho, Mattio & Diaz-Pulido, comb. nov. The remaining <i>Sargassum</i> spp. included in the analyses clustered in a polytomy within the worldwide-distributed <i>S</i>. section <i>Sargassum</i>, thus providing no support for the other eight morphological species currently recognized in the Caribbean Colombia and other western tropical Atlantic areas. Based on the morphological analysis we further described a new species, <i>S. giganteum</i> Camacho & Diaz-Pulido sp. nov., endemic from TNNP. Detailed morphological descriptions and illustrations, as well as an identification key, are provided for all studied taxa.</p>