In a cave system near Tena in the Napo District of Ecuador, a species of catfish has been filmed doing something that scientists have never seen it do before – climbing walls.

An international research team observed several fish moving up a near vertical flowstone waterfall, with some individuals spotted nearly three metres above the cave floor.

This makes climbing a new recorded behaviour for this species of catfish.

It is also the first documented observation of any member of the armoured catfish family climbing in a cave.

The findings were published in the journal Subterranean Biology.

Footage courtesy of Aaron Addison.

The fish, Chaetostoma microps, is a member of the armoured catfish family (Loricariidae) from the upper reaches of the Amazon basin, occurring in relatively limited areas in Ecuador and Peru.

They feed mainly on algae with their sucker-shaped mouths, which they also use for attaching to things like rocks and trees in fast flowing sections of water.

Relatives of C. microps are known to climb open rapids on surface streams but the authors believe this is the first time that climbing behaviour has been documented in this species.

However, it is where this behaviour occurred that is of most interest to the research team, as the catfish had previously only been recorded in waters above ground.

"It's not too surprising to find another catfish that climbs rocks. What is surprising is the environment that they are doing it in," said Geoff Hoese, naturalist and lead author of the study.

"This is a significant observation that merits investigation into why they are there," he added.

There are many environmental factors that result in species moving into cave habitats and, over time, for these species to become cave adapted.

According to Mr Hoese, there were some possible physical differences between the specimens he and the team recorded in the cave and those known from surface streams.

This, together with the newly observed climbing behaviour, throws up some exciting questions.

"There isn't enough data at this point to do more than speculate, but it's nice to think that we may be watching a small but significant evolutionary step as a species moves from one niche to another,” Hoese told BBC Earth.

But Hoese is quick to point out that simple answers are "usually best", and without further studies we won't know if these fish are making an evolutionary step or are simply individuals that got lost while heading upstream.

"It's a beautiful and fascinating region where there is much more to discover, and we hope to return in the near future," he said.

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