On Earth, about 250 species of underground fish are known that live in a world of constant darkness. They are usually small, several centimeters in length. But in an underground chamber in northeast India, researchers found cave fish, which is much larger: it grows to almost 0.5 m in length and weighs about 10 times more than any known species.
Local residents reported that they sometimes saw fish in the Chimp cave, where waterfalls cascade into the underground pool.
When biologist Daniel Harris first saw fish during the 2019 expedition, he was astounded. And puzzled.
“My first reaction when I saw the fish myself was this: I need a bigger net, ” he said.
"Fish recently described in Cave and Karst Science may still be in the process of evolution to become a separate new species, " says Harris, co-author of the study, "and can provide scientists with a unique opportunity to understand this evolutionary process."
The find raises many questions
The find raises many questions, for example, about how fish maintain their body size, what they eat, and how they adapted to live in these caves, which are extremely vast and deep, and many of them have not yet been explored. This creature is mostly blind and eyeless, although it seems to have some ability to sense light.
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Harris met with fish on an expedition led by Thomas Arbens, a professional cave explorer in the hilly Indian state of Meghalaya. There are many caves in the region due to the abundance of karst and limestone that can be carved by rainwater. And there are a lot of them - the state is one of the rainy places on Earth.
The team saw a photograph of a fish taken by one of the researchers, and suspected that it was a new species. But they still could not believe what they found in a small underground cave Um Lado, more than 90 meters below the surface.
There, the team discovered dozens of large creatures swimming in the pool. “I had such a small net that they usually use to catch tropical fish in their aquarium, and I stood there looking down, ” says Harris. Realizing that he needed a different way to catch the fish, he eventually put the cookies in an underwater bag. The trick was successful.
The cave can be visited only in the winter dry season; during the monsoon, the entire territory is flooded and access there is impossible.
"How fish get so big and what they eat is a mystery, " says Harris. The team has not yet weighed the fish, because it would be difficult to take the scales to the cave, but he believes that the weight is more than 1 kg.
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Of course, there is something rather strange in that in such an environment there are so many large fish.
Patricia Ornelas, a researcher from the National Autonomous University of Mexico, who did not participate in this discovery, agrees. “This is very interesting … This cave could contain not only fish with significantly larger body sizes, but also a relatively large population.”
What fish are known before
Prior to this discovery, several species of cave fish were known. The largest of them:
- blind swamp eel (Ophisternon infernale, which means “serpent from hell”), originally from the Mexican Yucatan,
- blind cave eel (Ophisternon candidum) from Western Australia.
These endangered fish are much thinner than the recently discovered creature. "It is much more bulky, with a body weight probably exceeding the mass of the next largest cave fish at least an order of magnitude, " the researchers write.
Their biological identity also remains a mystery. The team works with scientists from India, Nilesh Dahanukar and Rajiv Raghavan, to track his genetics and determine if this is a new species. During a return visit to the cave in January 2020 with photographer Robbie Sean, Harris and colleagues, the team collected several live fish, as well as tiny pieces of their fins for laboratory analysis.
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The new fish is undoubtedly closely related to the fish that lives on the surface, known as the golden maxir (Tor putitora).
The only noticeable difference between these two animals is that cave fish lack pigmentation - they appear white, almost transparent, and their eyes are poorly developed or even absent.
Cave creatures are also smaller than golden maxhirs.
Although they are very similar in body shape and structure, scientists believe that cave fish can be quite different from the golden maxirs living on the surface to qualify as a unique species.
At first glance, a similar situation is now observed with the Mexican tetra (Astyanax mexicanus), cave fish, which is very similar to the tetra living on the surface, although it is devoid of eyes and pigment. The theory goes that some surface fish were isolated underground a long time ago, and then began to develop traits to better match their new, lightless home.
