Thèse de doctorat
Résumé : How is life distributed on the Southern Ocean sea floor? How can we explain species distribution patterns, their origin and the underpinning drivers? These questions are central to macroecological studies, especially in regions facing fast environmental changes. The main objective of this thesis was to describe and characterise distribution patterns of sea stars species (Echinodermata: Asteroidea) with regards to reproductive strategy (i.e. brooding versus broadcasting), a key life history trait for species dispersal. I tested whether contrasting dispersal abilities between direct developers (brooders) and species with pelagic larvae in their development (broadcasters) result in distinct diversity and distribution patterns. At the scale of the Southern Ocean, I used both biogeographic (14,000 occurrence records) and phylogeographic approaches (five genera – Diplasterias, Notasterias, Lysasterias, Bathybiaster, Psilaster) to show that 1) biogeographic patterns are highly linked to reproductive strategy but these patterns also vary according to taxonomic levels, revealing the importance of historical factors. 2) Main biogeographic patterns are congruent with results obtained in previous works for other benthic taxa highlighting the prevalence of common biogeographic patterns like: the isolation of New Zealand fauna from the Antarctic, the high richness in the Scotia Arc region (particularly in brooders), the differentiation between East and West Antarctic species, and faunal affinities between southern South America and sub-Antarctic Islands. Overall, asteroids show low endemism levels compared to previously reported values, with 29% of species only occurring in the Antarctic Zone. 3) Phylogeographic patterns indicate that reproductive strategy is not directly related to the levels of genetic diversity nor to species richness but rather to the spatial structure of species distribution. Phylogeographic patterns analysed in broadcasters can be explained by different scenarios including deep‐sea colonisation routes, bipolarity or cosmopolitanism, and sub‐Antarctic emergence for the genus Bathybiaster; faunal exchanges between the Antarctic and New Zealand across the Polar Front for the genus Psilaster. Phylogeography of brooders support the hypothesis of a past trans‐Antarctic seaway established between the Ross and the Weddell seas during the Plio‐Pleistocene. These results also show, for the first time, that the Weddell Sea is composed of a mixed asteroid fauna originating from both the East and West Antarctic. Finally, I use an exploratory approach to investigate the phylogeny and phylogeography of the entire class Asteroidea and try to reveal their origin and evolution in the Southern Ocean. I demonstrate that species richness in asteroids has been overlooked and misunderstood in the Southern Ocean and possibly also all around the world. I then argue that modern Southern Ocean sea star fauna result, for the most part, from recent diversifications. These results, altogether, raise questions about commonly accepted principles on biogeographic patterns in the Southern Ocean. This stresses the need for more global and integrative approaches for such studies.