190 Years of Sargassum Taxonomy, Facing the Advent of DNA Phylogenies

Sargassum C. Agardh is one of the morphologically most complex phaeophyceaen genera and represents the most species-rich genus of the brown algal order Fucales Bory de Saint-Vincent (Phaeophyceae). The genus' classification system dates back to the 19th century and is based on observed differences in macro-morphological characters. Those morphological characters may display important variation within individual species, and several authors have linked the taxonomic complexity of the genus to its highly polymorphic nature and phenotypic plasticity. Among the large choice of existing species and subspecies epithets (about 1000), identifying taxa accurately is a difficult task, often relying on authors' interpretation of short Latin diagnoses or descriptions published in local Floras. Recently, the study of individual species' morphological range and DNA phylogenies underlined inconsistencies within low taxonomic levels (sections, subsections, series and species groups). Results highlighted the weak taxonomic value of traditional characters used to classify species, and pointed out significant taxonomic issues. The four Sargassum subgenera (S. subgen. Arthrophycus, Bactrophycus, Sargassum and Phyllotrichia) are now subdivided into a total of 12 sections and further subdivisions were abandoned. Two possible new sections need to be assessed. In the present paper, we raise the hypothesis that S. subgen. Arthrophycus could be merged to S. subgen. Bactrophycus, and that S. subgen. Phyllotrichia could be transferred to the recently reinstated genus Sargassopsis Trevisan. We also propose that two sections of the S. subgen. Bactrophycus: S. sect. Halochloa and Repentia be merged. A summary of the actual classification is given along with an identification key for Sargassum subdivisions.