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

The use of molecular markers to evaluate species delineation in the genus Sargasssum 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 Sargassum and the monospecific genus Cladophyllum 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, rbc LS, cox 3 and mtsp) on new and existing collections from the region. Sequences obtained from the genus Cladophyllum revealed its nesting inside Sargassum subgenus Sargassum as a close relative of S. pteropleuron , 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, Sargassum sect. Cladophyllum (Bula-Meyer) Camacho, Mattio & Diaz-Pulido, sect. nov., that includes the two sister species mentioned above. Based on these results, we also merged Cladophyllum within Sargassum and renamed its unique species S. schnetteri (Bula-Meyer) Camacho, Mattio & Diaz-Pulido, comb. nov. The remaining Sargassum spp. included in the analyses clustered in a polytomy within the worldwide-distributed S . section Sargassum , 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, S. giganteum 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.