The subarctic ancient Lake El'gygytgyn harbours the world's northernmost 'limnostygon community' and reshuffles crangonyctoid systematics (Crustacea, Amphipoda).

The northward distribution limit of groundwater fauna is generally dictated by the extent of glacial ice sheets during the Pleistocene. However, some taxa can be found far above this limit, sometimes on isolated oceanic islands, implying long-term survival in subglacial subterranean refugia. Here we report a peculiar assemblage comprising two new depigmented and blind (stygomorphic) amphipods from the subarctic ancient lake El'gygytgyn (northern Far East): Palearcticarellus hyperboreus sp. nov. and Pseudocrangonyx elgygytgynicus sp. nov. Molecular phylogenetic analyses based on five markers confirm their affinity to Crangonyctidae and Pseudocrangonyctidae, respectively. Fossil-calibrated molecular dating indicates that the ages of both species predate the onset of Pleistocene glaciations by at least an order of magnitude. Although both species are clearly adapted for subterranean life and are related to groundwater taxa, they are only known from the lake waters (5-170-m depth). Despite being nested within Pseudocrangonyctidae, P. elgygytgynicus has an atypical third uropod that preserves a vestigial inner ramus, a trait characteristic to the monotypic sister family Crymostygidae. Given that this character was the main distinguishing feature between the two families, we propose merging Crymostygidae with Pseudocrangonyctidae. Our findings represent the world's northernmost record of stygomorphic amphipods, emphasising their relictual biogeography and the importance of Lake El'gygytgyn as a long-term, high latitude refugium for ancient pre-glacial fauna. ZooBank: urn:lsid:zoobank.org:pub:3A51D1F8-E65D-4A3A-B663-D5C40272E68B.