Whilst climbing through a cave system on the western slopes of Tena, Ecuador, the last thing you might expect to find when looking up is a catfish. But that’s exactly what happened when an international research team was making a flora and fauna inventory of a cave located in the Napo District.

The group of cavers found the fish climbing up a near vertical flowstone waterfall, with some individuals reaching heights of up to three meters (10 feet). Whilst it’s not too unusual for some catfish to make forays out of the water and up rocky rapids, this is the first time it has been documented in this species, and the first time it’s been seen happening in a cave for the armored catfish family to which it belongs.

The researchers were able to identify the fish as Chaetostoma microps, a species that usually lives in the upper reaches of the Amazon and is endemic to Ecuador and Peru. The species is normally known to eat algae, so the reasons for why it was found living in the caves has led to some speculation. The authors of the report, published in the journal Subterranean Biology, suggest that they might simply be passing through the cave system, or could possibly be grazing on microbial films found in the cave streams.

The fish ordinarily uses its sucker-shaped mouth to attach to things such as rocks and trees in fast-flowing sections of rivers, though relatives of C. microps are known to climb over rapids, especially when spawning. Studies have been done on other members of the armored catfish family, Loricariidae, which have demonstrated that the fish are able to simultaneously breath and scrape off algae whilst clinging to surfaces with their mouths, which look like they're made of sandpaper.

The ability to climb, therefore, was not too much of a surprise to the cavers. To them, the fact that the species had previously only been reported from streams above ground, and that instead they found the creature deep within a cave system, was the most interesting. Whatever the reason for why the fish has decided to set up shop in a cave, one of the researchers who found the fish, Geoff Hoese, told the BBC that he thinks there might be some physical differences between the specimens seen in the cave and the ones that normally live above ground.

"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.