Fish is also world's most northerly cave fish
Few species are as well hidden as those that live underground. In Europe, there are many animals in the underwater caves and in the groundwater that we do not know about. To date, such animals did not include any fishes in Europe, even though cave fishes have been found in many other continents.
Now, a team of cave divers and researchers from the Universities of Konstanz and Oldenburg/Max-Planck Institute for Evolutionary Biology Plön and the Leibniz-Institute of Freshwater Ecology and Inland Fisheries Berlin have discovered the first cave fish in Europe.
The newly discovered fish is also the world's most northerly cave fish. It was found not in the Balkans where most European cave-dwelling species live, but in one of the least expected places – Germany. “We assume that there is a large population of cave fishes in the 250 square kilometre underground karst water system where percolating water from the Danube flows to the Aach spring north of Lake Constance,” said Dr Jasminca Behrmann-Godel, from the University of Konstanz.
Finding cave fish so far north was unexpected. It was assumed that they are only found in places were ice-age glaciers had not buried everything in the vicinity. The research suggests that the cave fish had vanished into the dark and thus became a troglodyte (cave dweller). Since there are no predators in their environment, life for the loach is very safe. It is likely that they feed on small cave crustaceans and cave snails in the underwater passages.
“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 maximum of 20,000 years ago and seemingly from the Danube. Our genetic analyses are very clear on this,” said Professor Arne Nolte from the University of Oldenburg/Max-Planck Institute for Evolutionary Biology Plön.
The dives were difficult; it took an hour to reach the location where the fishes were found
Dr. Jörg Freyhof, from the Leibniz Institute for Freshwater Ecology and Inland Fisheries (IGB), Berlin, explained, “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.”
The dives were difficult, as the cave divers had to swim against the current from the entrance into the underwater system in the Aach spring. They took an hour to reach the location where the fishes were found, about 600 metres away from Aach spring. “Diving in this area is for real professionals. For instance, on the way, there is a 'siphon shaft' that descends vertically for 40 metres. The divers use special air mixtures here to shorten the decompression time on the return journey. Poor visibility due to stirred-up mud from the current also makes things a lot more tricky,” said Joachim Kreiselmaier. He had discovered the first cave loaches during one of the three-hour dives in August 2015.
Think of the underwater system of the Danube drainage area between Immendingen and Möhringen up to the Aach spring as a flooded tunnel system. Its linear distance is just 12.5 kilometres, before the water drains into a sloping underwater area. “We don't know exactly what the system looks like but there must be farther underground rivers and lakes,” said Roland Berka, who has spent many decades studying the geological formation of the region.
The recent evolutionary history of these fishes promises to be an interesting topic for future research. Indeed, this spectacular discovery shows that even in Germany, one of the world's most widely researched countries, new things can still be found.