Sargassum is a cosmopolitan brown algal genus spanning the three ocean basins of the Atlantic, Pacific and Indian Oceans, inhabiting temperate, subtropical and tropical habitats. Sargassum has been postulated to have originated in the Oligocene epoch approximately 30 mya according to a broad phylogenetic analysis of brown macroalgae, but its diversification to become one of the most widespread and speciose macroalgal genera remains unclear. Here, we present a Bayesian molecular clock study, which analyzed data from the order Fucales of the brown algal crown radiation (BACR) group to reconstruct a time‐calibrated phylogeny of the Sargassum clade. Our phylogeny included a total of 120 taxa with 99 Sargassum species sampled for three molecular markers – ITS‐2, cox3 and rbcLS – calibrated with an unambiguous Sargassaceae fossil from between the lower and middle Miocene. The analysis revealed a much later origin of Sargassum than expected at about 6.7 mya, with the genus diversifying since approximately 4.3 mya. Current geographic distributions of Sargassum species were then analyzed in conjunction with the time‐calibrated phylogeny using the dispersal‐extinction‐cladogenesis (DEC) model to estimate ancestral ranges of clades in the genus. Results strongly support origination of Sargassum in the Central Indo‐Pacific (CIP) region with subsequent independent dispersal events into other marine realms. The longer history of diversification in the ancestral CIP range could explain the much greater diversity there relative to other marine areas today. Analyses of these dynamic processes, when fine‐tuned to a higher spatial resolution, enable the identification of evolutionary hotspots and provide insights into long‐term dispersal patterns.

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Sargassum species play key ecological roles on coral reefs, yet their diversity remains poorly known. Precise identification of Sargassum species, however, is improving with molecular genetic tools, though these have yet to be applied rigorously in Singapore. Historical records list 41 species, but no more than ten were verified based on herbarium vouchers, and even fewer (five species) were confirmed in the field based on a single nuclear gene marker in a previous study. Here, we revised the diversity of Sargassum in Singapore by examining all the morphologically distinct forms collected from the local coral reef environment. A total of six morphotypes, Sargassum aquifolium (Turner) C.Argardh (1820), S. cf. granuliferum C.Argardh (1820), S. ilicifolium (Turner) C.Argardh (1820), S. swartzii C.Argardh (1820), S. polycystum C.Argardh (1824), and an undescribed taxon ‘Sargassum sp.’ (Mattio and Payri 2009), were delineated based on morphological characteristics. The morphotypes were placed in five molecular clades based on phylogenetic analyses of the nuclear ITS-2, chloroplastic partial RuBisCO operon rbcLS, and mitochondrial cox3. Sargassum cf. granuliferum, though morphologically distinct from all other species, is not phylogenetically distinct from S. polycystum. Our results provide a species list for Singapore that will be valuable for future studies on macroalgal biogeography and species-specific ecological relationships with other reef organisms, particularly corals.

The brown macroalgal genus Lobophora (Phaeophyceae: Dictyotaceae) plays an ecologically significant role in many marine ecosystems, but their diversity and taxonomy remain poorly studied. Until 2012, six Lobophora species had been recognised globally based on morphological features. Yet, with more than 100 evolutionary taxonomic units characterised to date, it is now acknowledged that Lobophora comprises many cryptic species and its diversity was vastly underestimated. In light of a growing body of research integrating molecular and morphological data to delimit cryptic species, this study assessed the diversity and phylogeny of Lobophora in Singapore. A combination of molecular data and morphological observations were used to delimit and identify species from 33 specimens collected at eight sites in the southern islands of Singapore. The mitochondrial cox3 and chloroplast psbA genes were amplified and sequenced for phylogenetic analysis. Three formally described species, L. challengeriae, L. lamourouxii, L. pachyventera (comprising two morphotypes), as well as one undescribed putative species, Lobophora sp61, were recovered. These findings replace the record of the Atlantic species L. variegata in Singapore and suggest that there are more species to be discovered in the biodiverse region of Southeast Asia. Precise understanding of Lobophora diversity is critical for ongoing and future work on coral–macroalgal ecological relationships.

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