A taxonomic revision of the genus Dictyota (Dictyotales, Phaeophyceae) in South Africa

The genus Dictyota is a widely distributed group of brown algae occurring in temperate and tropical environments. Species of Dictyota are ecologically important and are known for their rich diversity of secondary metabolites that have been shown to have potential applications in the blue bioeconomy. This genus, however, is taxonomically challenging due to morphological plasticity, and the presence of morphologically similar species, highlighting the need for integrative approaches in taxonomic revisions. This study aimed to clarify the species diversity and distribution of Dictyota in South Africa, an understudied region that may harbour substantial undiscovered diversity. A total of 83 specimens were collected over 2 000 km of coastline, spanning over four marine provinces, that differ widely in temperature. An integrative taxonomic approach combining molecular and morphological data was employed, targeting three genetic markers: the plastid photosystem II protein D1 (psbA) and ribulose-1,5-biphosphate carboxylase/oxygenase large subunit (rbcL), and the mitochondrial cytochrome c oxidase subunit I (cox1). The results revealed eight previously undescribed species, four of which are formally described in this study (D. virilabris nom. prov. D. caerulea nom. prov., D. variolata nom. prov. and D. natalensis nom. prov.), as well as two new distribution records – D. stolonifera in South Africa and D. dichotoma in Tristan da Cunha. The study did, however, fail to confirm the presence of four species that have previously been recorded in South Africa based on morphology (D. friabilis, D. implexa, D. bartayresiana and D. ceylanica), with contemporary results suggesting these might have been historical misidentifications due to morphological similarities with several new species reported in South Africa or that these species are not present anymore. In addition, the assumption that Dictyota dichotoma is invasive in South Africa is challenged by the presence of multiple haplotypes within the region, suggesting a more complex biogeographic history and species origin. This study makes a substantial contribution to the taxonomy of Dictyota by updating the South African species list from 13 to 18 species and revising the known distribution ranges for all studied taxa. This study also illustrates the importance of sequencing type or topotype material, as the newly described Dictyota variolata nom. prov. could only be distinguished from Dictyota liturata after the designation and sequencing of topotype material during this study. Future research should build on these findings by expanding geographic and seasonal sampling, incorporating deepwater habitats, and including reproductive material to further refine species boundaries and improve taxonomic resolution.

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University of Galway

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University of Galway Theses (Research Masters Theses)