Gitchak nakana, the unique groundwater dwelling fish from North East India.
R. Britz, W. K. Marak, K. Velentina, Y. Lokeshwor, R. Raghavan, A. K. Pinion & L. Rüber via Wikimedia Commons.
When we think of groundwater, we usually think of wells, drinking water, irrigation, and water scarcity. Rarely do we think of life. Yet deep beneath the earth's surface, in dark aquifers where sunlight never reaches, entire ecosystems exist.
Hidden within these underground waters are organisms that have evolved over millions of years to survive in complete darkness. Their existence reminds us that groundwater is more than a resource waiting to be extracted. It is a living ecosystem that supports biodiversity while sustaining human societies.
A remarkable new discovery shared by Ralf Britz, Wimarithy K. Marak, Kangjam Velentina, Yumnam Lokeshwor, Rajeev Raghavan, Amanda K. Pinion & Lukas Rüber in their open access paper titled 'A miniature, subterranean, blind cobitid loach, Gitchak nakana, new genus and species, is the first groundwater-dwelling fish from Northeast India' published in Nature Scientific Reports from Assam has brought this hidden world into focus. In the study, these scientists have identified North-east India's first known groundwater-dwelling fish, a tiny blind species called 'Gitchak nakana'.
The finding expands our understanding of subterranean biodiversity in India and highlights the urgent need to protect groundwater ecosystems that are increasingly threatened by over-extraction, pollution, and urban expansion.
Groundwater is one of India's most important sources of drinking and irrigation. Yet aquifers across the country are being depleted at alarming rates due to excessive extraction. What is less widely known is that groundwater also supports biodiversity. Aquifers form one of the world's largest freshwater habitats and harbour organisms specially adapted to life underground.
Groundwater accounts for more than 95 percent of the Earth's freshwater and represents the planet's largest freshwater habitat. Although species living in subterranean aquifers make up only a small proportion of freshwater biodiversity, they provide important insights into the health and ecological integrity of groundwater systems. Because these environments are difficult to access, much of their biodiversity remains poorly understood.
Fish are one of the important residents of the subterranean environment and are found mostly at <30 m depths in caves, aquifers, connected wells - even at depths of 600 m as well as pools, streams, karsts and interstitial waters.
Till November 2023, scientists have documented:
305 species of cave and groundwater fishes
53 species of fishes living between sediment grains
55 species with reduced eyes/pigmentation and enhanced sensory organs, but not living in subterranean environments
15 species of cave-living amphibians
Thus, a total of 428 subterranean fish are found worldwide, which shows how vast and diverse life beneath the surface really is—and how much of it remains hidden. Subterranean fishes have some unique characteristics, which differentiate them from their surface-living counterparts. These include:
Lack of eyesight: Many species are blind or have reduced eyes since vision is useless in darkness.
Depigmentation: They often lack colouration, appearing pale or translucent.
Enhanced senses: They rely on touch, smell, and lateral line systems to detect vibrations and navigate.
Slow metabolism: Adapted to nutrient-poor environments, they conserve energy.
However, lack of these capabilities is compensated by enlarging or enhancing other sensory structures such as the following:
Increased sense of smell: Their sense of smell is heightened, allowing them to detect chemical cues in water that signal food, mates, or predators.
Sensitivity to vibrations: The fish are very sensitive to vibrations and water movement, which helps them “see” with touch and helps them to understand their environment through subtle currents.
The fish have elongated fins or whisker-like barbels: These act like tactile probes, helping them feel their way through tight subterranean spaces.
Subterranean fish species are rare and located only in specific locations
Subterranean fish are among the rarest vertebrates on Earth. Of the approximately 37,500 fish species currently known, only about one percent have adapted to life underground. More than 330 subterranean fish species have been recorded globally. Nearly 70 percent occur in just five countries, with China and Brazil together accounting for almost half of all known species.
India ranks fourth globally in subterranean fish diversity and is home to 21 known species. Scientists speculate that many subterranean fishes of the Indian subcontinent may have ancient evolutionary origins linked to the Gondwanan era, similar to groundwater-dwelling shrimps and burrowing amphibians found in the region.
Majority of the known subterranean fish are found in caves and very few in aquifers
Of the more than 300 known subterranean fishes, fewer than 10% have been found in aquifers. A vast majority of the fish from subterranean habitats are found in caves, while fish species living in groundwater aquifers are very rare and represented by only a few specimens in museum collections and found by accident.
Kerala and North East are hotspots for subterranean groundwater fish in India
Twenty-seven species of fish have been discovered as of now from the groundwater and subterranean systems of the Indian subcontinent, of which fourteen have been found in the peninsular Indian state of Kerala and five in the north-eastern Indian state of Meghalaya.
The nature of subterranean habitats occupied by fish in both these regions is also very different. For example, those in Kerala are dominated by lateritic aquifers, while those in Meghalaya are dominated by limestone caves.
This study found a subterranean fish from a dugout open well in Assam, and it was collected on three occasions from the same well in a small village at the foothills of the Shillong Plateau close to the Brahmaputra valley in the west of Assam. Caves on the Shillong Plateau are home to several completely blind and pigmentless subterranean fishes. However, this new fish is the first found living in groundwater aquifers in North-east India.
The fish lacks body colouration, and its translucent body appears blood-red. It does not have eyes like many subterranean fishes; it has a tiny black spot in the orbital area of the head. The fish has been named Gitchak nakana. The genus name is derived from the Garo word gitchak, meaning red, referring to the striking red colour of this freshwater bottom-dwelling fish, or loach.
The specific name 'nakana' is derived from the Garo words 'na·tok', fish, and 'kana', blind, referring to the absence of eyes in this species. With a maximum length of just around 20 mm SL, it is one of the smallest in size and qualifies as a miniature fish. It is also the only true loach in which the skull roof is completely absent and the brain is covered dorsally only by skin, which is rare in such fish, as it offers protection.
Gitchak lays only a few, but comparatively large, eggs as compared to other fish who lay hundreds to thousands of eggs. These few large eggs hatch to produce large offspring, a clever adaptation to ensure that they can feed on similar food items as the adults, given the sparse availability of food in subterranean habitats.
This fish has currently been found only in a single well in Assam, although efforts were made to look for it in other nearby wells. This area forms the alluvial deposits of the bed of a tributary of the Brahmaputra, which is a geologically dynamic area, and this type of habitat is not expected to be stable for longer periods of geological time.
The discovery of Gitchak in North East India provides the first evidence that this landscape supports unique subterranean fish of the kind that previously has only been known from aquifers in the lateritic lowlands of the Western Ghats.
The study is of great scientific importance as it has found evidence of the first groundwater-dwelling fish reported from the region, expanding knowledge on the current distribution of subterranean biodiversity in India.
Also important is the finding that unlike most cavefish, Gitchak nakana lives in aquifers rather than caves, showing that subterranean ecosystems are more diverse than previously thought.
The adaptation patterns of the fish to subterranean environments like blindness, loss of pigmentation, enhanced sensory systems and size reduction and specialisation, provide a living model to study regressive and progressive evolution patterns.
The findings also identify the urgent need to conserve groundwater as fragile ecosystems that are under immense pressure from extraction, pollution, and urban expansion. The presence of such fish underscores the need to treat groundwater as a living ecosystem, not just a resource.
Subterranean fish often have extremely restricted ranges, making them highly susceptible to extinction if their habitats are disturbed. The species can serve as valuable bioindicators, signalling connectivity and ecological integrity of groundwater systems.
In the longer term, protecting aquifers that host species like Gitchak nakana can also help in strengthening water security for cities. The discovery also offers a powerful narrative to engage the public and policymakers in groundwater conservation. Gitchak nakana is not just a new species; it’s a symbol of hidden resilience and a reminder that aquifers are living ecosystems that need to be conserved.
The discovery of Gitchak nakana offers a rare glimpse into a hidden world beneath our feet. It reminds us that aquifers are not empty underground reservoirs but dynamic ecosystems supporting life adapted to conditions that seem almost impossible.
As India grapples with groundwater depletion, pollution, and growing water demand, this tiny blind fish carries an important message. The future of water security depends not only on how much groundwater we extract, but also on how well we protect the ecosystems that make groundwater systems healthy and resilient.
Gitchak nakana is more than a new species. It is a symbol of the life hidden beneath our wells, farms, and cities and a reminder that conserving groundwater means conserving an entire world that we are only beginning to understand.