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A taxanomic review of Sargassum (Fucales, Phaeophyceae), with an emphasis on the Australian species

Dixon, Rainbo (2015) A taxanomic review of Sargassum (Fucales, Phaeophyceae), with an emphasis on the Australian species. PhD thesis, Murdoch University.

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The taxonomy of the common and conspicuous brown algal genus Sargassum C. Agardh, with 76 species in Australia and 336 worldwide, was investigated at a range of scales using genetic and morphological data. New DNA sequences derived from Australian collections were supplemented by co-author contributions from South Africa, Madagascar, New Zealand, New Caledonia, Japan, Netherlands and France. Variation in the loci cox3, rbcL-S spacer and ITS- 2, as well as a morphological assessment of new and herbarium collections, led to a number of substantial changes to the taxonomy of Sargassum and its four currently recognized subgenera. Neither the genus Sargassum, nor its most early-diverged subgenus, Sargassum subgenus Phyllotricha, were resolved as monophyletic taxa. To correct this, species of Sargassum subgenus Phyllotricha were transferred either to the newly resurrected genus Phyllotricha Areschoug or to the genus Sargassopsis Trevisan. These species were well differentiated, with the exception of Sargassum decurrens and Sargassum peronii, and the latter was placed in synonymy with the former. The separation of Sargassum subgenera Arthrophycus and Bactrophycus, previously distinguished primarily by their disjunct distributions, was not supported by the genetic or morphological data. Species of Sargassum subgenus Arthrophycus were genetically and morphologically nested within Sargassum subgenus Bactrophycus section Halochloa (Kützing) Endlicher, and were transferred accordingly. Sargassum section Halochloa species, particularly from the southern hemisphere, could not be easily differentiated, with intraspecific variation equal to or greater than intraspecific variation in all but one case (Sargassum incisifolium).

The final subgenus treated was Sargassum subgenus Sargassum. As the most species rich and widespread subgenus, a geographically broad-scale approach, as was undertaken with the other subgenera, was not feasible. Instead this effort was concentrated in a floristic survey of the previously poorly known north-western Australian members of the brown algal order Fucales, which was composed largely of Sargassum subgenus Sargassum species. Seventeen species of Fucales were recorded for north-western Australia, 10 of which were in Sargassum subgenus Sargassum. The floristic treatment includes full descriptions of each taxonomic level, incorporating current nomenclature, morphology, keys, and numerous figures, and represents the first detailed account of the Fucales of tropical Western Australia. As part of this study, a number of range extensions were recorded, as well as new records for Australia and a new species, Sargassum rastum R.Dixon & Huisman. In addition, the phylogenetic analyses of the north-western Australian species confirmed recent revisions at the sectional level of Sargassum subgenus Sargassum taxonomy.

The present study has reassessed the taxonomy of Sargassum using a combination of detailed morphological examination and molecular analyses, the latter incorporating a triple-loci approach. The investigation confidently differentiated many Australian Sargassum species, yet indicated that to unequivocally resolve species level boundaries in the genus, a greater collection and sequencing effort is required.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Veterinary and Life Sciences
Supervisor(s): Huisman, John
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