A cave diver first sighted the fish, a loach in the genus Barbatula, living in a hard-to-reach, underground water system in South Germany.
Joachim Kreiselmaier first sighted the loaches in August 2015 while exploring the deepest parts of the Danube-Aach system, which can only be reached under particularly dry conditions in summer and fall. Noticing that the fish were "strange looking" to him, he snapped some photos and showed them to the hobby geologist and co-author Roland Berka, who contacted Jasminca Behrmann-Godel of Germany's University of Konstanz, knowing her from former work. Study co-author and fish taxonomist Freyhof later confirmed that the fish could be showing cave adaptations.
Adaptations evolved real fast
Genetic studies of the fish together with knowledge on the geological history of the region suggest that the cave loach arose recently, within the last 20,000 years. "It was only when the glaciers retreated that the system first became a suitable habitat for fish," says Arne Nolte from the University of Oldenburg/Max Planck Institute for Evolutionary Biology, Plön.
Despite that relatively short period of evolutionary time, the fish already show adaptations characteristic of "real" cave fish.
As Jörg Freyhof from the Leibniz Institute for Freshwater Ecology and Inland Fisheries (IGB) Berlin explains, their eyes are much smaller, appearing almost as if curved inwards, and their color has all but disappeared. The fish also have elongated whisker-like barbels on their heads and larger nostrils than related fish living closer to the surface.