It is not a beauty, pale and rather inconspicuous - but in terms of evolutionary biology it is a sensation: the first known cave fish in Europe. Oldenburg genomicist Arne Nolte was also involved in the spectacular discovery north of Lake Constance.
Few animals live as hidden as those that are at home underground. But there is a rich fauna in caves, soil and groundwater that hardly anyone knows about. However, fish living in lightless, subterranean caves were previously unknown on the European continent. Now a team of cave divers and researchers from the Universities of Oldenburg and Constance and the Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB) Berlin have described Europe's first cave fish in the journal "Current Biology".
The newly discovered species is a loach, a few centimetres long, quite colourless and presumably blind. Amateur diver and co-author Joachim Kreiselmaier first noticed it in August 2015 during a dive in the Aachtopf (Baden-Württemberg). He took photos; a few months later he was able to bring a live specimen to the surface.
Experts from Constance, Berlin and Oldenburg analysed the surprising find. According to Prof Dr Arne Nolte from the Institute of Biology and Environmental Sciences (IBU) at the University of Oldenburg, scientists had previously only suspected cavefish further south on the globe, where the glaciers of the Ice Age had not buried all life. Apparently, the cave loach only ventured into the darkness after the ice age and became a cave dweller.
"With the retreat of the glaciers, the system only became colonisable for fish. At some point after the end of the Würm Ice Age, a maximum of 20,000 years ago, they must have migrated there from the Danube. We can clearly see this from our genetic analyses," says Nolte, who had already conducted research on this topic at the Max Planck Institute for Evolutionary Biology in Plön. First author Dr Jasminca Behrmann-Godel from the Limnological Institute at the University of Konstanz suspects that a large population of cavefish could be living in hiding in the relevant seepage area of the Danube.
Regardless of the short period of time since its "descent" from an evolutionary point of view, the loach has obviously already developed into a true cave fish. "The eyes are greatly reduced, almost as if they were turned inwards," says co-author Dr Jörg Freyhof from IGB Berlin. Instead, the fish have elongated tactile appendages on the head, known as barbels, and the nostrils are larger than those of the cave loach's above-ground relatives.
The subterranean life of the animals is quite safe, as they have no predators in their habitat. Divers were also able to identify small cave crabs, isopods and snails, which presumably serve as food for the cave loaches, in the underwater passages. To reach the site where the fish were found, around 600 metres from the source of the Aach, cave divers have to swim against the current and take around an hour. Co-author Roland Berka, who has been studying the geological formations for several decades, suspects a labyrinthine system with other underground rivers and lakes.
The relatively recent evolutionary history of the fish in particular makes them interesting for future research. They could help to better understand evolutionary adaptation processes.