Deep underground, in a cave that has been visited by just 30 people, Europe's first species of cave fish has been discovered. The pink translucent fish is a loach from the genus Barbatula and was found by an amateur diver in the deepest parts of the Danube-Aach cave system, in southern Germany.
Cave fish – the generic term for any freshwater fish that has adapted to live in caves – are found across the globe, with around 170 species described. The only continents they had never been found in is Antarctica and Europe – but the latest discovery by a hobby cave diver has changed this.
Joachim Kreiselmaier first spotted the "strange looking" fish in August 2015. He had been in the deepest part of the Danube-Aach system, which is only accessible during dry spells in summer and autumn. The system formed 400,000 to 450,000 years ago and the entrance is described as being a labyrinth, to such a degree that very few people have been to the depths Kreiselmaier reached.
Kreiselmaier took pictures of the fish, which were then shown to Dr Jasminca Behrmann-Godel, assistant professor at the University of Konstanz. She contacted fish taxonomist Jörg Freyhof, who confirmed that the fish appeared to have adapted for cave living. Specifically, it had small eyes, very pale colour and large nostrils.
"It took someone with the 'right eye' to realise that this might be something special and I believe that, on top of the right conditions and the difficult trip, this discovery depended on an exceptional diver like Joachim to realise in the first place that the fish might be special," Behrmann-Godel said.
Kreiselmaier returned to the same spot a few months later and managed to capture a live specimen. "Due to the usually bad visibility, strong current, cold temperature, and a labyrinth at the entrance, most divers do not come back again for diving," he said.
Analysis of the fish showed it was a new species, with morphological and genetic differences indicating an isolated population – and the first cave fish ever to be found in Europe. Their description of it has now been published in the Cell Press journal Current Biology.
The discovery has also shed light on when this new species emerged. "The cave fish was found surprisingly far in the north in southern Germany," Behrmann-Godel said. "This is spectacular as it was believed before that the Pleistocene glaciations had prevented fish from colonising subterranean habitats so far north."
Because the cave would have been frozen until fairly recently, scientists say the new species must have emerged at some point within the last 20,000 years – an extremely short period of time in evolutionary terms.
"The newly discovered European cave loaches do not represent individuals displaced from surface populations, but they follow a unique evolutionary trajectory towards cave life," they wrote. "The available data support that the cave fish are genetically isolated from both surface populations of stone loaches and agree with a recent origin of the cave fish population.