Morphological and Molecular Characters Differentiate Common Morphotypes of Atlantic Holopelagic Sargassum

Amy Siuda , Aurélie Blanfuné (1) , Skye Dibner (2) , Marc Verlaque (1) , Charles-François Boudouresque (1) , Solène Connan (3) , Deborah Goodwin (4) , Valérie Stiger-Pouvreau (3) , Frédérique Viard (5) , Florence Rousseau (6) , Valérie Michotey (1, 7, 8) , Jeffrey Schell , Thomas Changeaux , Didier Aurelle (1) , Thierry Thibaut (1)

1 MIO - Institut méditerranéen d'océanologie
2 Eckerd College
3 LEMAR - Laboratoire des Sciences de l'Environnement Marin (LEMAR)
4 Sea Education Association
5 UMR ISEM - Institut des Sciences de l'Evolution de Montpellier
6 ISYEB - Institut de Systématique, Evolution, Biodiversité
7 MEB
8 MEB

Since 2011, massive new strandings of holopelagic Sargassum have been reported on the coasts of the Caribbean, northern Brazil, Guiana, and West Africa, causing severe economic and ecological damage. Three common morphotypes (S. fluitans III, S. natans I, and S. natans VIII) were identified as responsible for these catastrophic events, with dominance shifts between them over time. However, the taxonomic status of these holopelagic Sargassum morphotypes remains unclear. Using an integrative taxonomy framework, combining a morphological study and molecular analyses, this study aimed to clarify their taxonomic status. Morphological analyses of 54 characters revealed no intermediate form between the three morphotypes, with the overall shape, nature of the axis, and size and shape of blades and vesicles being the most discriminating. An analysis of mitochondrial (IGS, cox2, cox3, mt16S rRNA, and nad6) and plastid (rbcL) markers confirmed the genetic divergence among the three morphotypes, with a lower level of divergence between the two S. natans morphotypes. Without additional molecular characterization, these morphotypes cannot be classified as three distinct species. However, due to their distinct morphological characteristics and sympatry within drifting aggregations, a revision of holopelagic species names is proposed, with Sargassum fluitans var. fluitans (for S. fluitans III), Sargassum natans var. natans (for S. natans I), and S. natans var. wingei (for S. natans VIII). This revision provides necessary clarity on the species involved in inundations of the tropical Atlantic.

Domaines: Botanique

Amy Siuda, Aurélie Blanfuné, Skye Dibner, Marc Verlaque, Charles-François Boudouresque, et al.. Morphological and Molecular Characters Differentiate Common Morphotypes of Atlantic Holopelagic Sargassum. Journal of Phycology, 2024, 4 (2), pp.256-275. ⟨10.3390/phycology4020014⟩. ⟨mnhn-04572926⟩