Within the intriguing realm of India’s cave-dwelling fish that are blind.

Two years prior, zoologist Khlur Baiaineh Mukhim noticed something fascinating in a stream within a secluded underground cave located in India’s northeastern Meghalaya state. It was a fish he had never encountered prior, featuring long barbels – the whisker-like extensions near a fish’s mouth – of a yellowish-green hue and, crucially, with eyes.

Cave-dwelling fish, which are species that inhabit only caves, typically lack eyes due to their adaptation to dark environments, making the fish Mr. Mukhim observed particularly noticeable to him. Scientists in Meghalaya now claim it is a new fish species, one that has adjusted to existing both above and below ground—a distinctive trait among cave-inhabiting species.

Their results were published earlier this month in the most recent edition of the Journal of Fish Biology, a top peer-reviewed journal focused on fish studies. The researchers designated the fish as Schistura ‘densiclava’ due to the prominent black stripe on its tail.

It is claimed that the species is native to Krem Mawjymbuin cave in the eastern Khasi Hills, and it has been identified in water pools 60m (196ft) deep within the cave, as well as in a stream located above ground nearby.

Dandadhar Sarma, a zoology professor and researcher of the study, mentions that the cave environment is extreme, with temperatures falling to 18C (64.4F) – significantly lower than the ideal temperature needed for tropical fish survival – and very low oxygen levels.

“So it’s remarkable that the fish can adapt to both – harsh subterranean conditions as well as more favourable surface conditions,” Mr Sarma says.

Schistura densiclava is the sixth species of cave-dwelling fish identified in Meghalaya in the last two to three decades, yet the only one known to demonstrate the ability to thrive in two distinctly different types of environments.

The state is recognized for possessing some of the most intricate cave systems globally, yet many of its approximately 1,500 to 1,700 limestone and sandstone caves have yet to be explored due to their placement in hard-to-reach, secluded forest areas. According to Mr. Sarma, these cave systems host many animal species exhibiting intriguing evolutionary traits, yet they largely remain unexplored due to a lack of research.

In the last five years, a group of researchers from the state, supported by federal funding, has been methodically investigating Meghalaya’s extensive cave system to find and record new fish species inhabiting these spaces. In 2019, the research group identified Neolissochilus pnar, the biggest cave-dwelling fish species globally, according to Mr. Sarma.

The fish was discovered within the Krem Umladaw cave in the western Jaintia Hills in a deep pool hundreds of metres underground. Mr. Mukhim, a team member with extensive cave expedition experience, notes that fish inhabiting caves exhibit evolutionary characteristics that are as intriguing as those found in animals residing at the poles or in the depths of the oceans.

“Cave ecosystems are one of the harshest environments to live in,” he says.

“These fish usually live in perpetual darkness, stagnant, shallow water pools with dangerously low oxygen levels and sometimes, go for months with little to no food.”

Nature has aided their survival by eliminating the unnecessary and fortifying what is essential for life. As a result, they’ve sacrificed their vision and capacity to generate vibrant pigments, which would be an unnecessary expenditure of energy in a completely dark cave.

They possess a more acute sense of taste and smell, and sensory receptors on their skin assist them in sensing vibrations for navigating the substrate and evading predators. Their food sources consist solely of what can be found within the cave, such as leaf litter and aquatic creatures brought in by seasonal floods, along with bat droppings. In this extremely tough environment, these cave-dwelling fish species spend their lives, with some living for nearly ten years and even reproducing.

“Our headlights are the only source of light,” Mr Mukhim says.

Catching fish involves squatting near pools for hours, and swiftly sweeping up the skittish creatures in a net as they present themselves.

Mr Mukhim, who has been studying fish found in the caves of Meghalaya for over a decade, says that there’s a need to study these species as that is the only way we will be able to conserve them.

“Once a species is wiped out, you can never bring them back,” Mr Mukhim says.

“It’s painful to think that an entire ecosystem in our midst, one of the most fascinating ones, has been studied so little,” he adds.

“It’s time we paid a little more attention to these cave-dwelling marvels of nature.”