A hobby diver has made the 'surprise' discovery of Europe's first cave fish in a cold and labyrinth-like cave in southern Germany.

Noticing that the fish were 'strange looking' to him, diver Joachim Kreiselmaier took some photos and showed them to friends, only to find that he had stumbled upon a rare loach.

Despite only living in the cave for 20,000 years, loaches have quickly adapted to the dark conditions, growing elongated whisker-like barbels and larger nostrils than related fish living closer to the surface.

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Hobby diver Joachim Kreiselmaier first sighted the loaches in August 2015 while exploring the deepest parts of the Danube-Aach system. Genetic studies of the fish showed it arose within the last 20,000 years. Pictured is a male which is about three inches (8.5 cm) long

DISCOVERING THE CAVE LOACH 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 autumn. No more than 30 divers have ever reached the place where the fish were found. In November 2015, on another dive, Mr Kreiselmaier succeeded in catching a live specimen, which allowed the researchers to study its features in greater detail. The following year, he caught four more fish, enabling further study of the loach's form and genetics. Researchers found that the cave loaches were an isolated population and the first known European cave fish. Advertisement

Researchers at the University of Konstanz in Germany carried out genetic studies of the loach which showed that the species had only been living in the cave for 20,000 years.

Comparing the results to other fish caught upstream and downstream of the cave, the researchers discovered that the cave loaches were an isolated population and the first known European cave fish.

Until now, researchers thought that the Pleistocene glaciations had prevented fish from colonising subterranean habitats so far north.

Arne Nolte, co-author of the study, said: 'It was only when the glaciers retreated that the system first became a suitable habitat for fish.'

Despite that relatively short period of evolutionary time, the fish already show adaptations characteristic of 'real' cave fish.

The loach's eyes are much smaller, appearing almost as if curved inwards, and their colour has all but disappeared.

'From the entrance of the cave to where I found the fish it is completely dark. This is a distance of 550m [1800ft] ', Mr Kreiselmaier told MailOnline.

'The only light is the light you bring with you', he added.

Mr 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 autumn.

'No more than 30 divers have ever reached the place where the fish have been found,' Mr Kreiselmaier said.

'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.'

The cave where the fish lived was relatively inaccessible due to usually bad visibility, strong current, cold temperature, and a labyrinth at the entrance (stock image)

In November 2015, on another dive, Mr Kreiselmaier succeeded in catching a live specimen, which allowed the researchers to study its features in greater detail.

The following year, he caught four more fish, enabling further study of the loach's form and genetics.

The findings show that adaptation to subterranean habitats can be fast - requiring only a few thousand years.

Mr 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 autumn

They also come as reminder that 'wonders of nature can turn up anywhere, even in your own backyard,' said co-author Jörg Freyhof.

The researchers say they'll continue to study this new cave fish in more detail, including its genetic, genomic, and behavioural characteristics.

Ultimately, they'd like to explore, through this newly discovered loach, the first steps toward life in caves.