The Biogeographic Importance of Buoyancy in Macroalgae: A Case Study of the Southern Bull‐Kelp Genus Durvillaea (Phaeophyceae), Including Descriptions of Two New Species¹

Abstract

Long-distance dispersal plays a key role in evolution, facilitating allopatric divergence, range expansions, and species movement in response to environmental change. Even species that seem poorly suited to dispersal can sometimes travel long distances, for example via hitchhiking with other, more intrinsically dispersive species. In marine macroalgae, buoyancy can enable adults-and diverse hitchhikers-to drift long distances, but the evolution and role of this trait are poorly understood. The southern bull-kelp genus Durvillaea includes several non-buoyant and buoyant species, including some that have only recently been recognized. In revising the genus, we not only provide updated identification tools and describe two new species (D. incurvata comb. nov. from Chile and D. fenestrata sp. nov. from the Antipodes Islands), but also carry out biogeographic analyses to determine the evolutionary history of buoyancy in the genus. Although the ancestral state was resolved as non-buoyant, the distribution of species suggests that this trait has been both gained and lost, possibly more than once. We conclude that although buoyancy is a trait that can be useful for dispersal (creating evolutionary pressure for its gain), there is also evolutionary pressure for its loss as it restricts species to narrow environmental ranges (i.e., shallow depths).

Phylogeny and Evolution of the Brown Algae

“…Brown algae vary widely in traits that influence their potential for speciation and diversification, such as life history strategies, dispersal mechanisms and potential, as well as apparent scope for ecological diversification. For instance, the potential for dispersal in brown algae can vary from a few centimeters (Hays, 2007;Barner et al, 2011) to hundreds of kilometers (Smith, 2002;Fraser et al, 2020). In the sister species Postelsia palmaeformis and Nereocystis luetkeana, the difference in potential for long distance dispersal is dramatic, in that P. palmaeformis is characterized by drooping, deeply grooved blades, promoting highly localized dispersal and selfing (Barner et al, 2011), while N. luetkeana produces dehiscent sori on blades near the surface-up to tens of meters from the substratum, presumably promoting greater dispersal distances (Dayton, 1985).…”

“…However, this species does not occur anywhere on Antarctica and likely colonized the subantarctic region recently (Macaya and Zuccarello, 2010). The population structure of Durvillaea antarctica from Sub-Antarctic islands suggest sea ice was extensive during the LGM, pushing polar populations into refugia along the coastlines of New Zealand (Fraser, Hay, et al, 2009;Fraser, Hay, Nikula, et al, 2009Fraser et al, 2020).…”

Pathogen inferred to have dispersed thousands of kilometres at sea, infecting multiple keystone kelp species

“…Additionally, M. ectocarpii has been found to infect gametophytes of kelp species Macrocystis and Desmarestia, which could disrupt the life cycle of these keystone kelp species, particularly in a commercial context (Maier et al 2000). Southern bull kelp (Durvillaea) are large and ecologically important keystone species occupying intertidal and shallow subtidal zones (Fraser et al 2020). To withstand the wave forces in these dynamic environments, Durvillaea species are highly flexible and strong (Kelly and Brown 2000).…”

“…The Durvillaea genus includes three buoyant species, whose thalli contain a gas-filled honeycomb structure (Fraser et al 2020). This trait has promoted long-distance dispersal of Durvillaea species such as D. antarctica, which has been found washed up on coasts thousands-and even tens of thousands-of kilometres away from known source populations (Moore and Cribb 1952;Fraser et al 2011Fraser et al , 2018Waters et al 2018).…”

“…New Zealand is a centre of diversity for southern bull kelp species, with several buoyant and non-buoyant species found in the region (Fraser et al 2020). To date, however, Maullinia infections have not been recorded from New Zealand bull kelp populations.…”