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

Aim

Glaciers cover considerable portion of land and host diverse life forms from single-celled organisms to invertebrates. However, the determinants of diversity and community composition of these organisms remain underexplored. This study addresses the biogeography, population connectivity and dispersal of these organisms, especially critical in understanding during the rapid recession of glaciers and increased extinction risk for isolated populations. By reconstructing the Quaternary biogeographic history of Fontourion glacialis, a widespread in Northern Hemisphere glacier obligate species of Tardigrada, we aim to understand how populations of glacier-dwelling metazoans receive immigrants, respond to disappearing glaciers and to 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 specimens collected across the distribution range. Phylogeographic and coalescent-based approaches were used to detect population differentiation patterns, investigate most likely models of gene flow and test the influences of geographical and climatic factors on the distribution of F. glacialis genetic variants.

Results

Our findings indicate that the distribution of F. glacialis genetic variants is primarily influenced by geographical rather than climatic factors. Populations exhibit a dispersal-limited distribution pattern, influenced by geographical distance and local barriers, even between neighbouring glaciers. Significantly, the genetic structure within Scandinavia suggests the existence of “southern” glacial or low-temperature refugia, where F. glacialis may have survived a period of extensive deglaciation during the Holocene climatic optimum (8–5 kyr ago).

Main Conclusion

The study uncovers complex metapopulation structures in F. glacialis, with impacts of local barriers, population bottlenecks as well as historical ice sheet fluctuations. It suggests that such populations can endure extended periods of deglaciation, highlighting the resilience of glacial refugia. The study highlights the necessity of understanding the diversity and population structure of ice-dwelling fauna in both spatial and temporal contexts.