| Summary: | Despite its inhospitable conditions, the Southern Ocean is characterized by a surprisingly high biodiversity. Its long-term geographic and thermal isolation produced a diverse and partly endemic benthic fauna. The deep sea also represents an extreme environment where benthic fauna appeared to be much more diverse than suspected. Evolutionary links exist between the Antarctic fauna and that of the world’s deep seas, implying colonisation events between both environments. Unique environmental characteristics generated some assumptions on faunal distributions: circumpolar, eurybathic Antarctic species and cosmopolitan, eurybathic species in the deep sea. Using lysianassoid amphipods as a model group, the aims of this thesis were to investigate, by means of molecular tools, (i) the origin of the biodiversity in the Southern Ocean, (ii) the evolutionary relationships between the faunas of the Southern Ocean and the deep sea and (iii) species richness and distributions in both environments. Phylogenetic analyses revealed incongruence between molecular phylogenies and morphology-based classification and gave an indication of colonisations between the Southern Ocean and the Atlantic deep sea. With DNA barcoding and phylogeographic methods, formerly accepted paradigms of circumpolarity, eurybathy and cosmopolitism were tested. In Antarctic lysianassoids, cryptic species as well as truly circum-Antarctic and eurybathic species were revealed. In the deep sea, we found a cosmopolitan, eurybathic species to be composed of several vertically stratified species-level lineages, of which some were geographically widespread whilst others were only recovered from a single ocean basin. Antarctic and deep-sea species richness appeared to be clearly underestimated for this group. Considering anthropogenic impacts on these pristine environments, these findings are important for determining conservation protocols. L’océan Austral est, malgré ses conditions extrêmes, caractérisé par une faune benthique particulièrement diversifiée et ...
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