Not many people have the skill or the courage to explore the deepest reaches of flooded underwater caverns. But in August 2015, Joachim Kreiselmaier was doing just that, spelunking in the Danube-Aach cave system deep below south Germany. And it was down in those dark depths that he made an exciting discovery: a blind, pale cave fish – the first ever discovered in Europe.  

This male cave loach is about 8.5cm long. Notice its tiny eyes, pale colour and long face-barbels. Image: Jasminca Behrmann-Godel

A cave is an unusual habitat, and creatures that spend their whole lives within the dark parts of caves tend to evolve unusual features. For a start, they're often blind (or have lost their eyes altogether) – who needs sight in a completely dark environment, right? And in a place where no one can see your colourful exterior, there's not much point to pigment, which makes pale white skin and scales another common adaptation.

These features have popped up in plenty of cave critters (known as troglobites) worldwide, including salamanders, crabs, spiders and lots of fish. Across the globe, you'll find more than 150 species of cave fish living on every continent except Antarctica and – until now – Europe.

The newly discovered fish is a close cousin of the stone loach, which swims in rivers and streams across Europe. But unlike its surface-dwelling relatives, the German cave fish has tiny eyes and is mostly colourless – classic cave adaptations. It even has long whisker-like barbels on its face, and particularly large nostrils, likely to help it navigate its lightless habitat.

The fish were found in a large cave system in south Germany.

"No more than 30 divers have ever reached the place where the fish have been found," Kreselmaier notes in a press release. "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."

But Kreiselmaier did go back. After sharing photos with local scientists, he returned to the caves first in November of 2015, when he captured a live specimen, and later the following year to catch four more. These specimens allowed researchers to get up close and personal with Europe's first cave fish.

"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," says Jasminca Behrmann-Godel of the University of Konstanz, lead author of a new study describing the find.

The mysterious absence of cave fish in Europe has not gone unnoticed by scientists. Geological evidence indicates that the extensive glaciers of the last Ice Age – which covered much of the northern continents – blocked fish from making their way into subterranean water systems.

"It was only when the glaciers retreated that the system first became a suitable habitat for fish," notes Arne Nolte of the University of Oldenburg. This suggests that the cave fish were able to settle here only in fairly recent times, geologically speaking.

With live specimens on hand, researchers were able to compare the cave loaches genetically and morphologically to their river-dwelling cousins. This not only confirmed them as an isolated population, but it also suggested that the cave loaches arose within the last 20,000 years. This is a surprisingly short time considering how well-adapted the fish have already become to the troglobite lifestyle.

There's still a lot we have to learn about the evolution of cave traits. Exactly how and why do these animals lose their eyes? Just how quickly can such changes occur? These remain open questions – and the cave loaches have an important part to play in helping us to answer them. Not only are these animals the first (and possibly only) European cave fish, but they also have the potential to offer key insights into the early stages of troglobite evolution.


Top header image: Jasminca Behrmann-Godel