Deep beneath the Amazon rain forest lives a cave-climbing fish—and scientists have captured it on film for the first time.

A team exploring limestone caves near Tena, Ecuador (map), found the gravity-defying fish scaling near-vertical rock walls some 10 feet (3 meters) above an underground stream.

They later identified the species as Chaetostoma microps, a member of the suckermouth armoured catfish family (Loricariidae), according a study published April 16 in the journal Subterranean Biology.

Although that identification isn't 100 percent certain—for one, the expedition team wasn't authorized collect animal specimens in the caves—C. microps has previously been recorded only in aboveground rivers, where it clings to rocks. (Also see "How Fish Evolved to Climb Waterfalls With Their Mouths.")

While climbing fish are nothing new, the only other cave climber known to study leader Geoff Hoese is a loach from Thailand dubbed the waterfall-climbing cave fish (Cryptotora thamicola).

This fish is able to climb rapids and waterfalls, but Hoese questioned whether it can negotiate "cave walls outside of the main stream flow, such as we observed here."

Hoese and his team observed C. microps shimmy up cave walls where water streamed down from tiny underground tributaries. (Also see "New Climbing Catfish Identified.")

View Images The catfish (seen climbing a 75-degree wall) likely uses water pressure and structures in their mouths and skin to stay attached. Photograph by Geoff Hoese

This impressive climbing ability is due to "a number of modified structural elements of their fins, skin, and mouths," said Hoese, a naturalist and speleologist based in Austin, Texas.

"The thin film of water flowing over them as they climb likely provides pressure to help hold them to the wall, and these various structures help keep them from sliding down," he added. "They can then wriggle their way up, or also down, as you can see in the video."

Traveling Fish?

The puzzling thing about the newfound cave-climbing fish is that they don't appear to be adapted for underground life. These fish weren't colorless or blind, for instance, unlike other fish adapted to live in caves.

So what was C. microps doing down there in the darkness?

"This species is known to primarily eat algae, and as there's no sunlight in the caves, there's not much algae, so it seems unlikely that they are there to feed," Hoese said.

"The simplest explanation is that they simply occupy the fullest extent of the range they can reach," he added. (Also see pictures: "'Walking' Fish a Model of Evolution in Action.")

View Images Scientists suspect the cave-climbing fish is C. microps, a known species of suckermouth armoured catfish. Photograph by Geoff Hoese

Hoese noted the fish live in an Amazonian region where rivers are born in springs that issue from caves in the foothills of the Andes Mountains.

Maybe the catfish "just continue up the streams and into the springs, and on into the caves," he said.

Going to the Dark Side

Many fish have a natural instinct to head upstream, in order to breed, for example.

Another possibility is that the climbing catfish mark the beginnings of an evolving species of permanent cave-dwellers. (See National Geographic's cave pictures.)

"Evolution is a process that's constantly at work," Hoese said. "It's interesting to look at examples where there may be changes, such as here, where we have a surface fish exhibiting specific behaviors and occupying an unusual habitat."

A model for comparison is another suckermouth catfish that the expedition team recorded, Astroblepus pholeter. A colorless species with miniscule eyes, it's already made the transition to cave life. (See "Pictures: New Suckermouth Armored Catfish Discovered.")