Bridging the Scotia Arc: Climate-Driven Shifts in Connectivity of the Freshwater Crustacean Branchinecta gaini in Sub-Antarctic and Antarctic Ecosystems

Aim: To integrate the historical and contemporary connectivity of Branchinecta gaini (B. gaini), in order to better predict future distribution changes within its fragmented, dynamic and isolated habitat range. Location: The study covers 20 locations of freshwater ecosystems across B. gaini distribution within the maritime Antarctic, sub-Antarctic South Georgia, Falkland/Malvinas Islands and southern South America (SSA). Methods: We used two mitochondrial DNA loci and 7446 SNP markers to assess genetic diversity, population structure and connectivity of B. gaini. Additionally, we applied an ensemble ecological niche modelling (ENM) approach to project current and future species distributions under various climate scenarios. Results: High genetic diversity was found in most sampled locations, with SSA exhibiting the greatest variation in terms of haplotype and nucleotide diversities. Antarctica exhibits short topologies with a limited number of shared haplotypes among its different regions. Overall, there is significant genetic and phylogeographic differentiation among biogeographic regions. Historical demographic analyses indicated population expansion in Antarctic regions but stability in SSA. Contemporary population structure analyses revealed six genetic clusters with limited gene flow and a clear pattern of isolation by distance. Ecological modelling suggested future habitat loss in the sub-Antarctic and potential expansion in Antarctic regions. Main Conclusions: Our integration of historical and contemporary connectivity potentially provides a solid foundation for the development of conservation strategies, especially in fragile areas with dramatic changes projected. The strong genetic structuring and limited connectivity of B. gaini across its range highlight the need for region-specific conservation strategies. These findings emphasise the importance of integrating genetic and ecological approaches to predict species' responses to environmental change and guide conservation strategies for vulnerable Antarctic freshwater ecosystems.

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