A male cave loach, measuring 8.5cm in body length. Image: Jasminca Behrmann-Godel
An amateur diver has made a chance discovery in the depths of subterranean caves, with a loach representing a significant first in Europe.
The deepest parts of the Danube-Aach underwater cave system in the south of Germany are only accessible when it is particularly dry, with the summer months offering the best opportunity to explore.
In 2015, Joachim Kreiselmaier was doing just that when he spotted something different: a “strange-looking” fish, far from where it was believed to dwell.
Pleistocene glaciations had prevented fish from colonising subterranean habitats in Europe, and the fact that this creature was seemingly different to other European fish got Kreiselmaier thinking.
With a few flashes of a camera and a chat with some scientists who work in the field, it turned out the diving hobbyist had made a major discovery: Europe’s first cave fish.
“No more than 30 divers have ever reached the place where the fish have been found,” said Kreiselmaier. “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.”
Kreiselmaier was intrigued, though, and he returned to obtain some specimens for further research.
Genetic studies of the fish paired with the geological history of the region suggest that the cave loach emerged within the last 20,000 years.
“It was only when the glaciers retreated that the system first became a suitable habitat for fish,” said Arne Nolte of the Max Planck Institute for Evolutionary Biology.
20,000 years is pretty short in evolutionary terms, though the loach already showed cave fish characteristics, such as small eyes, faded colouring and elongated whiskers.
Based on morphological and genetic comparison to surface fish caught upstream and downstream of the cave, the researchers confirmed that the loaches are indeed an isolated population and the first-known European cave fish.
