Different paths of success in the sea – what we can learn from seaweeds

Closely related clades of brown marine seaweeds display different patterns of geographical distributions and diversifications.

Above: Meadow of the brown seaweed Lobophora in the Canary Islands. Photograph by Jan Ranson

In studying the factors and processes structuring marine biodiversity and distribution patterns, an unconscious (perhaps inevitable) bias towards animals vs. seaweeds is often reflected in the literature. The emphasis is essentially made on what usually are considered emblematic taxa such as corals, sponges and fishes. This trend somewhat echoes what we observe in society, where seaweeds go mostly unnoticed by the general public even though they play a crucial and irreplaceable role in the functioning of coastal marine ecosystems as primary producers.

Editors’ choice: (Free to read online for a year.)
Vieira, C, Steen, F, D’hondt, S, et al. Global biogeography and diversification of a group of brown seaweeds (Phaeophyceae) driven by clade‐specific evolutionary processes. J Biogeogr. 2021; 48: 1– 13. https://doi.org/10.1111/jbi.14047 

Seaweeds include a heterogeneous set of organisms, which have diverged into evolutionary independent lineages (brown, green and red algae). Despite their ecological importance, our knowledge on seaweeds’ biodiversity, biogeography and evolution is still very fragmentary. Not even that long ago it was not particularly uncommon for seaweed experts to claim that many seaweeds have biogeographic ranges spanning all major ocean basins and that they were present in temperate as well as tropical regions. However, truly cosmopolitan species were thought to be rare, for these needed to be present also in Arctic and Antarctic regions. Needless to say that the use of DNA-assisted species identification radically altered our view on seaweed diversity. Two decades later many so-called widespread species turned out to be species complexes sometimes comprising over a hundred individual species (e.g. Lobophora, Portiera). Alongside the development of a more accurate view of species diversity, a much improved view on distribution ranges and diversity patterns emerged with some clades conforming to a textbook latitudinal diversity gradient, while other clades displayed inverse latitudinal patterns.

Marine brown seaweeds covering the seafloor — in this case a vertical wall — in the Baleares Islands, Spain. Photograph by Christophe Vieira.

Studies focusing on large-scale biogeographical patterns in seaweed are particularly uncommon. Moreover, the factors underlying these patterns have been only rarely addressed. Generally, because species diversity was insufficiently characterized for many groups of seaweed or datasets were too small to allow rigorous statistical testing of alternative diversification hypothesis. This forced phycologists to address macroecological or evolutionary questions at genus-level, as a proxy for species level diversity. However, species- and genus-level diversity patterns and diversification dynamics do not necessarily align well, leaving many questions unanswered with respect to clade-specific drivers of diversification.

The brown marine seaweed Dictyota in the Keys, Florida, USA. Photograph by Ana Tronholm.

In this sense, the driving force behind writing this paper was the long-brewing idea of overcoming the aforementioned impediments and addressing patterns of diversification in seaweeds. To this end, we put together a consortium of phycologists with access to local seaweed diversity and joined efforts for data acquisition (collecting and barcoding). This study illustrates how patterns in distributions and diversification diverge among closely related clades as a result of contrasting evolutionary mechanisms, using the brown seaweed order Dictyotales as an example of successful diversification in the marine realm. This work marks a milestone in the study of global biogeography and diversification of marine seaweeds.

Sampling of seaweeds in Tenerife, Canary Islands. Photograph by Ana Tronholm.

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Written by:
Olivier De Clerck (1), Christophe Vieira (2), Ana Tronholm (3)

1) Professor, Phycology Researh Group, Biology Department,  Ghent University. (2) Postdoctoral Fellow, Kobe University Research Center for Inland Seas, Japan. (3) Researcher, Department of Biological & Environmental Sciences, University of Gothenburg.

Additional information:
https://phycology.ugent.be/;
https://www.facebook.com/groups/Phycology.UGent/