Biodiversity of <i>Sargassum</i> (Fucales, Sargassaceae) from Eastern Samar (Philippines) inferred from nuclear ribosomal internal transcribed spacer (ITS) sequence data

Sargassum is the most diverse genus in the brown algal order Fucales, which is an ecologically important primary producer in marine ecosystems. The Philippines has about 73 species of Sargassum identified based solely on morphology, representing 18% of its reported global diversity. The genus is known for its morphological plasticity which makes identification of its species challenging. Taxonomy is particularly problematic in the Philippines where molecular data for Sargassum is lacking. We collected 34 samples of Sargassum from Eastern Samar, Philippines, an area known to have significant species diversity. Using newly designed primers to amplify the nuclear ribosomal internal transcribed spacer (ITS) region, we generated gene sequences and reconstructed the phylogeny of Sargassum to infer species diversity of the Eastern Samar samples. Based on morphological assessments of samples, we found S. corderoi, S. polycystum and five other morphotaxa. Molecular assessments, on the other hand, show the presence of only three morphotaxa. Bayesian inference, maximum parsimony and maximum likelihood generated almost identical trees which showed that S. corderoi and Sargassum sp. aff. siliquosum samples are nested in a clade of S. ilicifolium. This is the first time that S. corderoi has been reported in Eastern Samar. Another species, S. polycystum, formed a clade consisting of well-supported subgroups. This study is a step towards updating the taxonomic identification of Sargassum species found in the Philippines using molecular and morphological data. The biomarker used in our study could improve the identification of Sargassum species in a species-rich subtropical-tropical geographical area such as the Philippines, in phylogenetic studies of Sargassum.