‘Demon’ cavefish that could be world’s rarest fish species found during routine survey

A ‘demon’ cavefish that could be one of the rarest fish species in the world has been found during a routine survey.

Scientists were shocked to discover the pale, eyeless creature.

It represents an entirely new genus and species and is currently known from just one cave.

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Researchers have never seen more than around 20 of the fish at once and believe it may spend much of its life hidden deep inside an aquifer.

The fish has several unusual features, including a pronounced hump behind its head, unbranched fin rays and a longer, flatter snout.

Genetic testing also revealed that part of its rhodopsin gene, which is linked to light sensitivity, is missing and no longer works, as reported by What’s The Jam.

“Honestly, my first thought was ‘Don’t get excited yet, but I think it’s a new species’,” researcher Matthew Niemiller said.

“Over the years, I’ve seen how variable cavefish can be, and I’ve been fooled before by animals that looked different but were genetically very similar.

“But this one was different in ways that kept holding up.

“It has unbranched fin rays, the pronounced hump behind the head, and a longer and flatter snout.

“It wasn’t just one odd feature.”

The fish was found in Bobcat Cave on Redstone Arsenal in Huntsville, Alabama, US.

Matthew sent photographs of the fish to Jonathan Armbruster and Pamela Hart before returning to the cave to collect specimens for further analysis.

Jonathan, curator of fishes at the Auburn University Museum of Natural History, initially remained cautious because seemingly unusual cavefish had previously turned out to be known species.

He said: “Matt noticed it right off, but I was skeptical as I had been spurned by things that I thought looked different but turned out to not be.

“He sent the specimens down to me and I immediately saw that they were something different then.

Researchers used comparative anatomy, genetic analysis and CT imaging to confirm the fish was a completely new lineage.

It was named Demogorgonichthys arcanus, commonly known as the Demon Cavefish.

The first part of its scientific name refers to the Demogorgon, an ancient underworld demon that later appeared as a monster in Dungeons & Dragons and Stranger Things.

Arcanus means “hidden” or “secret” – a reference to the fish remaining undetected for so long.

Matthew said: “I’ll happily give the naming credit where it’s due, and that was really Jon and Pam.

“My role was mostly to say ‘yes, absolutely’.

“I loved it instantly, because it works on so many levels.

“It’s the ancient underworld demon of mythology, which later became the monster in Dungeons & Dragons and Stranger Things, and because a blind, pale creature from the permanent dark practically names itself.

“Arcanus, ‘hidden,’ was the perfect finishing touch for a fish that stayed unrecognized for so long.

“I am a sci-fi/fantasy nerd at heart who grew up in a small town in Indiana, so the Stranger Things connection resonated with me.”

Scientists have monitored the cave for roughly 40 years because it is also home to the endangered Alabama Cave Shrimp.

The mysterious creature may even prey on other cavefish, with an X-ray appearing to show one inside a specimen’s stomach.

Jonathan said: “We are CT scanning one of the Demogorgonichthys specimens to see what is in the stomach.

“It is clear that there is something in there.

“I X-rayed it, and it looks to be another cavefish, but the images were not clear.

“They probably do not eat often and will eat any animal that fits in their mouths.”

Despite the long-running surveys, the mysterious fish was not first spotted until February 2025.

The findings were published in a journal on 6 July.

Researchers believe it may have previously remained deeper within the aquifer or could simply have gone unnoticed.

They plan to use environmental DNA and continued monitoring to discover whether the species is restricted to the single cave system and to track its population.

Matthew added: “This fish is a reminder of how much life is hidden right beneath our feet, in the same groundwater we all depend on.

“We’re standing on one of the richest cave landscapes in the country in northern Alabama, and we’re still discovering the animals that live in it.”

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