Amateur cave diver Joachim Kreiselmaier, a member of Freunde der Aachhöhle (Friends of the Aach cave), spotted the cave loaches in August 2015 in the Danube-Aach system in southern Germany.
Following the sighting, several fish of various sizes were studied and five live specimens were caught by Kreiselmaier for observation by researchers at the University of Konstanz.
“It took someone with the ‘right eye’ to realise that this might be something special and I believe that this discovery depended on an exceptional diver like Joachim,” researcher Jasminca Behrmann-Godel told Science Daily.
Kreiselmaier was exploring caves deep in the cave system, 600m from the entrance in the Aach Spring, which eventually empties into the River Rhine.
It is believed no more than 30 divers have ever reached the location where the fish were found.
“To reach the cave takes an hour and involves periods of decompression that make the round trip as long as three hours,” Kreiselmaier told New Scientist.
Genetic analysis of the cave fish and geological history of the region indicate that they separated from surface-dwelling fishes when glaciers from the last Ice Age receded 16,000–20,000 years ago.
Experts previously believed if any cave fish were ever to be discovered in Europe it would be likely that they would be located in the fauna-rich caves of the Western Balkans, where a number of other cave-dwelling creatures live.
The strange pink fish have typical adaptations for cave life such as eyes about 10 per cent smaller than those of surface loaches and long sensory whisker-like organs used for navigation in the dark murky waters.
Read the full paper in Current Biology.
Read more wildlife news stories in BBC Wildlife Magazine