Islands of ice: Glacier-dwelling metazoans form regionally distinct populations despite extensive periods of deglaciation

Abstract

Aim: Glaciers cover considerable portion of land and host diverse life forms fromsingle-celled organisms to invertebrates. However, the determinants of diversityand community composition of these organisms remain underexplored. This studyaddresses the biogeography, population connectivity and dispersal of these organ-isms, especially critical in understanding during the rapid recession of glaciers andincreased extinction risk for isolated populations. By reconstructing the Quaternarybiogeographic history of Fontourion glacialis, a widespread in Northern Hemisphereglacier obligate species of Tardigrada, we aim to understand how populations ofglacier-dwelling metazoans receive immigrants, respond to disappearing glaciers andto what extent remaining glaciers can serve as refugia.Location: Glaciers across Svalbard, Scandinavia, Greenland and Iceland. Methods: We analysed mtDNA (COI gene) variability of 263 F. glacialis specimenscollected across the distribution range. Phylogeographic and coalescent-based ap-proaches were used to detect population differentiation patterns, investigate mostlikely models of gene flow and test the influences of geographical and climatic factorson the distribution of F. glacialis genetic variants.Results: Our findings indicate that the distribution of F. glacialis genetic variants isprimarily influenced by geographical rather than climatic factors. Populations exhibita dispersal-limited distribution pattern, influenced by geographical distance and localbarriers, even between neighbouring glaciers. Significantly, the genetic structurewithin Scandinavia suggests the existence of “southern” glacial or low-temperaturerefugia, where F. glacialis may have survived a period of extensive deglaciation duringthe Holocene climatic optimum (8–5 kyr ago).Main Conclusion: The study uncovers complex metapopulation structures in F. glacia-lis, with impacts of local barriers, population bottlenecks as well as historical ice sheetfluctuations. It suggests that such populations can endure extended periods of degla-ciation, highlighting the resilience of glacial refugia. The study highlights the necessityof understanding the diversity and population structure of ice-dwelling fauna in bothspatial and temporal contexts. cryoconite ecosystems, glacial ecology and biogeography, ice-fauna survival, impact ofdeglaciation, phylogeography, quaternary history, refugia identification, Tardigrada