About 250 species of subterranean fish are known on Earth, eking out a living in a world of permanent dark and scant food. They are usually small, generally a few inches long, since there’s usually little food or prey to eat.
But in an underground chamber in northeastern India, researchers have discovered a cave fish that is much bigger—growing to nearly a foot and half in length and weighing about 10 times more than any known species.
When biologist Daniel Harries first saw the fish during a 2019 expedition, he was amazed. And perplexed.
“My first reaction upon seeing the fish myself was, I’m going to need a bigger net.”
The fish, described recently in the journal Cave and Karst Science, may still be in the process of evolving to be a separate new species, says Harries, a study co-author—and could present scientists with a unique opportunity to understand this evolutionary process.
The finding raises many questions, such as how the fish maintain their body size, what they feed on, and how they’ve adapted to live in these caves, which are extremely extensive and deep, many of which haven’t yet been explored. Like most other troglobites, the creature is basically blind and eyeless, though it apparently has some ability to sense light.
Into the caves
Harries encountered the fish on an expedition led by Thomas Arbenz, a professional cave explorer, in India’s hilly Meghalaya state. There are many caves in the region due to the abundant karst and limestone, which can be carved by rainwater. And there’s a lot of it—the state is one of the rainiest places on Earth.
The team had seen a photograph of the fish from a fellow explorer, and suspected it was a new species. But they still couldn’t believe what they found in a small underground cavity, called the Um Ladaw Cave, over 300 feet below the surface.
There, the team found dozens and dozens of the large creatures, swimming in a pool. “I had this little net, the sort that you use to catch tropical fish in your tank, and I was standing [there], looking down,” Harries says. Realizing he needed another method to catch them, he eventually put biscuits in an underwater bag, a ploy that proved successful.
The fish likely feed on vegetation washed underground by rain, but none could be seen at the time of the visit, says Harries, a hobbyist cave explorer and marine biologist at Heriot-Watt University in Edinburgh, Scotland.
The cave can only be visited in the winter dry season; during the monsoons, the whole area is flooded and impossible to access.
How they get so large, and what they feed on remains a mystery, Harries says. The team didn’t weigh them yet, because it would’ve been difficult to take a scale down into the cave, but he estimates they are a little over two pounds.
“There’s certainly something rather odd going on to have quite so many large fish in that kind of environment.”
Patricia Ornelas, a researcher with National Autonomous University of Mexico, who wasn’t involved in this discovery, concurs. “It’s very interesting that... this cave could support not only a fish with a considerably large body size, but also a relatively large population.”
Before this discovery, the two longest known subterranean fish species, both narrow and ribbon-like, are the blind swamp eel (Ophisternon infernale—meaning “chest-serpent from hell”), native to Mexico’s Yucátan, and the blind cave eel (Ophisternon candidum), from western Australia. These endangered fish are much thinner than the newfound creature, which “is considerably more bulky, with a body mass likely to exceed that of the next largest cave fish by at least an order of magnitude,” the researchers write.
Their biological identity also remains enigmatic, for now. The team is working with collaborating scientists in India, Neelesh Dahanukar and Rajeev Raghavan, to sequence its genetics and determine if it is indeed a new species. On a return visit to the cave in January 2020 with photographer Robbie Shone, Harries and colleagues the team collected a few live fish, as well as tiny bits of their fin, for the lab analysis.
“I’ve photographed wildlife in caves over the last 20 years but never seen anything so big,” Shone says. “I was amazed how big they were.”
Evolution in action?
The newfound fish is undoubtedly closely related to a surface-dwelling fish known as the golden mahseer (Tor putitora), Harries says.
The only observable differences between the two animals, he explains, are that the cave fish lack pigmentation—appearing a white, almost translucent color—and their eyes are poorly developed or even non-existent.
The cave creatures are also smaller than golden mahseers, an endangered fish that can grow to many feet in length. (Learn more: How this rare, good-luck fish is thriving in Bhutan.)
Though they look very alike in body shape and structure, the scientists think that the cave fish may be different enough from surface-dwelling golden mahseer to qualify as a unique species.
A seemingly analogous situation is occurring right now with the Mexican tetra (Astyanax mexicanus), a cave fish that’s very similar to tetras that live at the surface, albeit lacking eyes and pigment. The theory goes that some surface fish were isolated underground long ago, and then began to develop traits to better suit their new, lightless home.
There are multiple studies aimed at understanding the genetic process by which the Mexican tetra loses its color and eyesight, and similar research on this Indian species could provide “opportunities to explore the genetic basis of these adaptations,” Ornelas says. A thorough understanding of pigmentation and vision could have wide and perhaps unexpected applications.
The Mexican tetra, and perhaps the newfound Indian fish, could be an example of “speciation and evolution in progress,” Harries says. (Read on: How this cave-dwelling fish lost its eyes to evolution.)
Many people imagine evolution to be a very slow and irreversible process, Harries says. But that’s not so. “Studies of these systems seem to indicate that very different animal forms might be able to evolve relatively rapidly,” he adds.
Furthermore, this discovery shows that caves harbor unique animals, and need to be protected, Harries says. Caves, usually formed in karst and limestone, are under threat worldwide from cement production, coal mining, and water pollution, which could “cause extinction of cave species before they have even been documented.”
So much to find
Despite living in total darkness, the fish move and swim around quickly, clearly able to sense the confines of their watery home, Shone says. They were also quite curious, at least at first—and hungry.
“If you place a boot or a finger in the water, they’ll come and chew it,” Harries says.
At first, it didn’t seem like the fish could sense the light. However, after a few encounters, the creatures would flee when the team turned on artificial lights.
Light was necessary, of course, for getting around—and taking pictures. Cave photography is indeed tricky, Shone adds, in part because you must provide all your own lighting.
“It’s something that took me years and years to get a base level of competence,” he says, “and today I’m still learning.”
But new findings like this propel him onward, exploring that which is hidden from most.
“There’s just so much to discover,” he says.