While in Europe there is a rich fauna in caves, there were no fish known that lived in them — but all that has changed with the discovery of a cave-dwelling loach. This little fish is not only the first cave fish found in Europe but is also the most northerly cave fish in the world.
The loach, from the genus Barbatula, was found in Germany, in a 250 square kilometre underground karst water system where percolating water from the Danube flows to the Aach spring north of Lake Constance.
To find cave fish at all in such a northern region of the earth is completely unexpected. It was assumed that cave fish are only found in areas where the ice-age glaciers hadn't buried every living thing under their path. The results presented by the research team suggest that the newly discovered cave fish, a loach, did in fact first broach the darkness after the end of the ice age and thereafter became a troglodyte (cave dweller). “It was only when the glaciers retreated that the system first became a suitable habitat for fishes. They must have moved there at some point following the end of the Würm glacial period, a max. of 20,000 years ago and seemingly from the Danube. Our genetic analyses are very clear on this,” mentions Professor Arne Nolte from the University of Oldenburg/Max-Planck Institute for Evolutionary Biology Plön.
“In this short period of time from an evolutionary perspective the fishes have developed into real cave fish. Their eyes are much smaller than in surface fish, almost as if they were curved inwards and their colouring has almost disappeared. The fish have elongated projections on their heads, so-called barbels, and their nostrils are larger than those of their cousins who live closer to the surface,” explains Dr Jörg Freyhof from the Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB). There are no predators within the cave fish’s environment, so that life far below the surface is very safe for the loach. Small cave crustaceans and cave snails have also been detected in the underwater passages. They most likely provide the fishes with their staple diet.
The underwater system of the Danube drainage area between Immendingen and Möhringen up to the Aach spring resembles a flooded, labyrinth-type tunnel system. It’s only 12.5 kilometres linear distance, which the water runs off from across a sloping horizontal underground area. Researchers say that while they don't know exactly what the system looks like, there must be further underground rivers and lakes.
For more information, see the study published in Current Biology: Jasminca Behrmann-Godel, Arne W. Nolte, Joachim Kreiselmaier, Roland Berka & Jörg Freyhof: The first European cave fish, Current Biology, 3 April 2017