Aim Biogeographical processes underlying Indo‐Pacific biodiversity patterns have been relatively well studied in marine shallow water invertebrates and fishes, but have been explored much less extensively in seaweeds, despite these organisms often displaying markedly different patterns. Using the marine red alga Portieria as a model, we aim to gain understanding of the evolutionary processes generating seaweed biogeographical patterns. Our results will be evaluated and compared with known patterns and processes in animals. Location Indo‐Pacific marine region. Methods Species diversity estimates were inferred using DNA‐based species delimitation methods. Historical biogeographical patterns were inferred based on a six‐gene time‐calibrated phylogeny, distribution data of 802 specimens, and probabilistic modelling of geographical range evolution. The importance of geographical isolation for speciation was further evaluated by population genetic analyses at the intraspecific level. Results We delimited 92 candidate species, most with restricted distributions, suggesting low dispersal capacity. Highest species diversity was found in the Indo‐Malay Archipelago (IMA). Our phylogeny indicates that Portieria originated during the late Cretaceous in the area that is now the Central Indo‐Pacific. The biogeographical history of Portieria includes repeated dispersal events to peripheral regions, followed by long‐term persistence and diversification of lineages within those regions, and limited dispersal back to the IMA. Main conclusions Our results suggest that the long geological history of the IMA played an important role in shaping Portieria diversity. High species richness in the IMA resulted from a combination of speciation at small spatial scales, possibly as a result of increased regional habitat diversity from the Eocene onwards, and species accumulation via dispersal and/or island integration through tectonic movement. Our results are consistent with the biodiversity feedback model, in which biodiversity hotspots act as both “centres of origin” and “centres of accumulation,” and corroborate previous findings for invertebrates and fish that there is no single unifying model explaining the biological diversity within the IMA.

Defining species in the brown algal genus Colpomenia is a challenging endeavour because of their morphological similarity, overlapping phenotypic variation, the absence of conspicuous diagnostic characters, and often lack of reproductive structures crucial for their identification. Thus, the use of molecular tools has become widely used to study Colpomenia taxonomy and evolution. The following four Colpomenia species are described along the Australian coast: C. claytoniae M.Boo, K.M.Lee, G.Y.Cho & W.Nelson, C. ecuticulata M.J.Parsons, C. peregrina Sauvageau, and C. sinuosa (Mertens ex Roth) Derbès & Solier. The objective of this study was to assess the diversity of Colpomenia species in southern and south-eastern Australia by using DNA barcoding techniques and single-marker species delimitation methods. We generated 44 new COI-5P DNA sequences from nine different populations across three Australian states (South Australia, Victoria and Tasmania), and applied 13 variations of four species delimitation methods (ABDG, SPN, PTP, GMYC). Our results recognised three Colpomenia species in the region, namely, C. sinuosa, C. claytoniae, and C. peregrina. Colpomenia sinuosa is the most widely distributed species in Australia. Colpomenia peregrina and C. claytoniae presented high levels of intraspecific genetic divergence. We did not find C. ecuticulata, although it has been previously reported from nearby our sampling area.

Several filamentous endophytic genera are assigned to the Phaeophyceae, in particular to the family Chordariaceae (Cormaci et al. 2012). Brown endophytic filamentous algae are known to cause infections in host marine algae (Schoenrock et al. 2013; Ogandaga et al. 2016, 2017; Gao et al. 2019). Acting as pathogens, they may cause morphological, physiological and ecological changes in the host alga such as production of galls and wart-like spots, changes in metabolism and growth rates, and changes in survivorship and reproduction (Schoenrock et al. 2013; Ogandaga et al. 2016, 2017; Gao et al. 2019). Among brown algal filamentous endophytes, the genus Mikrosyphar Kuckuck is relatively understudied. To date, only Mikrosyphar zosterae Kuckuck (1895: 177) was studied thoroughly (Ogandaga et al. 2016, 2017).