On the fish living underground
Blind loaches, blood-red swamp eels and cave-dwelling mahseer are forcing scientists to rethink where freshwater life exists. The recent discoveries across Northeast India suggest that an entire hidden aquatic world may lie beneath ordinary landscapes
This year, my attention has repeatedly been caught by a succession of reports about subterranean fish. One discovery followed another, each revealing a fish living beneath the surface, in places where we hardly imagine fish could exist. My curiosity could hardly remain untouched.
Earlier came Gitchak nakana, a tiny blind loach from Assam. Then, within the first two weeks of August, came two more: Rakthamichthys anchi, a new subterranean swamp eel from Assam, and Gangaichthys indonepalicus, another subterranean fish described from Bihar. Three discoveries in such a short span are a lot to take in.
Gitchak nakana was particularly significant. It was the first groundwater-dwelling, or phreatobitic, fish known from Northeast India, described in February. It was discovered in a dug-out village well in Assam, a relative of our familiar gutum mach. It is a peculiar little fish: pigmentless, blind and barely two centimetres long, with an extraordinary anatomical feature — the roof of its skull is absent, leaving its brain covered only by skin.
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In early August came Rakthamichthys anchi, a new species of subterranean swamp eel and a cousin of our baim mach, also from Assam. It was found in wells in the same village where Gitchak nakana was discovered. It is blood-red, essentially blind and adapted to a life in perpetual darkness. Its discovery is particularly intriguing because it is only the second species of its genus known from Northeast India, following Rakthamichthys rongsaw, a burrowing swamp eel described from the Khasi Hills of Meghalaya in 2018.
And then, almost immediately, came Gangaichthys indonepalicus from the Gangetic tributaries in Bihar, a different type of baim mach. The paper appeared only yesterday. Another subterranean fish. Another hidden lineage emerging from beneath the surface. At this point, the succession of discoveries is becoming difficult to ignore.
A freshwater world beneath the freshwater world
When we think of freshwater ecosystems, we usually imagine what we can see: a river flowing over stones, a forest stream, a wetland filled with aquatic vegetation, or a pond shimmering under the sun. But water does not stop at the surface.
Rainwater seeps through soil and sediments. Rivers exchange water with the ground beneath them. In porous sediments, water fills interconnected spaces between grains, creating enormous underground networks known as aquifers. The fully saturated underground zone is called the phreatic zone.
And in some places, fish live there. These animals are known as subterranean fishes. A smaller group that lives specifically in groundwater aquifers is called phreatobitic fishes. They are among the strangest members of the freshwater fauna. Some have lost their eyes. Some have lost pigmentation. Some have reduced skeletons. Many have elongated bodies, enhanced sensory structures and other anatomical modifications associated with perpetual darkness.
The darkness itself is only one part of the challenge. Underground habitats are often poor in food and oxygen, physically fragmented and extremely difficult to explore. Unlike rivers, an aquifer has no obvious boundary to survey. The habitat is hidden inside rock, sand, gravel and soil.
The discoveries offer a glimpse of that second story. Two very different types of subterranean fish emerged from wells in the same village in Assam. One was unearthed from forest soil. One lives deep inside a cave. And the lineage represented by Aenigmachanna reminds us that subterranean adaptation has evolved in groups we might otherwise associate with entirely different habitats.
This is one reason why subterranean fishes remain so poorly known. A recent scientific account of Gitchak nakana noted that more than 300 subterranean fish species are known globally, but very few have been recovered from aquifers. Such fishes are rarely encountered, often appearing only through chance discoveries.
A village well, therefore, can become a window into an ecosystem that nobody realised was there.
Northeast India's underground fish story
The discoveries from Assam do not stand alone.
Meghalaya has already produced one of the most spectacular subterranean fish discoveries in the world: Neolissochilus pnar, a cave-dwelling mahseer formally described in 2023. It is important to distinguish its habitat from that of Gitchak nakana. N. pnar is a subterranean cave fish, living in a cave environment rather than a groundwater- or aquifer-dwelling fish. At more than 30 centimetres, it was described as the world's largest known subterranean fish.
It is almost difficult to reconcile the image of a mahseer — a vibrant gold-coloured fish generally associated with flowing, azure rivers — with a life deep inside limestone caves. Yet there it is, a pale, blind, slow swimmer in pitch-dark waters.
The other remarkable story is Rakthamichthys rongsaw. It was discovered not while fishing, but while researchers were digging through soil in search of fossorial caecilians. The single known specimen was found roughly 40 centimetres underground in moist, rocky soil in a secondary forest of Meghalaya, close to a stream. Imagine surveying amphibians and having a fish emerge from the soggy underground. This also changes how we should imagine a forest stream.
A stream is not merely the water visible between its banks. It can be connected to an unseen subterranean environment beneath the surrounding forest. Water moves through soil and sediments; organisms can occupy spaces that conventional biodiversity surveys rarely reach.
The subcontinental subterranean fish story extends farther west. Aenigmachanna — the mysterious subterranean snakehead, related to our shol and taki mach — represents another remarkable lineage of subterranean fish, although its known distribution lies farther from Bangladesh. Its discovery showed that even a group of fishes as familiar as snakeheads could conceal a highly unusual subterranean evolutionary history.
The recent discoveries of Gitchak nakana, Rakthamichthys anchi and Gangaichthys indonepalicus make this point even more strongly. Some were found through wells or other human access points into the underground environment. The wells themselves are not necessarily the true habitat. They are windows into a hidden world.
The fish are living in the darkness beneath us.
And what about Bangladesh?
This is where the story becomes particularly interesting.
Bangladesh is a country built around water. We have rivers, floodplains, haors, beels, ponds, canals and an enormous groundwater resource. Yet our understanding of freshwater biodiversity has traditionally been overwhelmingly surface-oriented and, in itself, extremely limited.
But how often do we ask what lives in underground waters? How often do we treat a village well not merely as a source of drinking or irrigation water, but as a possible biological sampling point?
The north-eastern edge of Bangladesh shares the same broad landscape system as the hills and plateaus of Meghalaya and Assam. The Surma–Kushiyara basin, the Sylhet wetlands and surrounding hill forests, and the region hosting Teesta tributaries each belong to distinct but hydrologically connected landscapes. It would be fiction to assume that the subterranean fishes recently discovered across the border must occur in Bangladesh. But their discovery should make us ask the question.
Our knowledge of native fish, therefore, may contain two stories. One is written in the water we can see. The other may be written beneath our feet.
The discoveries offer a glimpse of that second story. Two very different types of subterranean fish emerged from wells in the same village in Assam. One was unearthed from forest soil. One lives deep inside a cave. And the lineage represented by Aenigmachanna reminds us that subterranean adaptation has evolved in groups we might otherwise associate with entirely different habitats.
None of these animals was waiting in a spectacular wilderness reserve to be discovered. They were beneath ordinary landscapes.
Perhaps that is the most fascinating lesson. Biodiversity does not always announce itself with a forest canopy, a bird call or a river teeming with fish. Sometimes it is hidden in plain sight, in the most unexpected of places.
