The aquatic glacial relict fauna of Norway – an update of distribution and conservation status

The aquatic “glacial relict” fauna in Norway comprises a group of predominantly cold-water animals, mainly crustaceans, which immigrated during or immediately after the deglaciation when some of the territory was still inundated by water. Their distribution is mainly confined to lakes in the SE corner of the country, east of the Glomma River in the counties of Akershus, Ostfold and Hedmark. We review the history and current status of the knowledge on this assemblage and of two further similarly distributed copepod species, adding new observations from the last decades, and notes on taxonomical changes and conservation status. By now records of original populations of these taxa have been made in 42 Norwegian lakes. Seven different species are known from Lake Store Le/Foxen on the Swedish border, whereas six species inhabit lakes Femsjoen, Oymarksjoen and Rodenessjoen, and five are found in Aspern, Aremarksjoen and in the largest Norwegian lake, Mjosa. From half of the localities only one of the species is known. The most common species are Mysis relicta (s.str.), Pallaseopsis quadrispinosa  and Limnocalanus macrurus . Some populations may have become extirpated recently due to eutrophication, acidification or increased fish predation. Apart from the main SE Norwegian distribution, some lakes of Jaeren, SW Norway, also harbour relict crustaceans, which is puzzling. The region is disjunct from any current fresh- or brackish-water sources, whereas following the early deglaciation it bordered the large, dry landmass of Doggerland, now the submerged bottom of the North Sea. While the Jaeren  Mysis population indeed is found to represent a different, plausibly more salt-tolerant species than that in SE Norway, the recent discovery of the freshwater amphipod Pallaseopsis quadrispinosa from the same lake upholds the zoogeographical enigma.