Fish not only live in lakes or rivers, a few species also inhabit underground water resources. Researchers have now discovered one of them in a well in northern India. The small, blind loach “Gitchak nakana” is the first groundwater loach in this region. Their discovery suggests that other unknown species could also occur in the underground waters.
Underground bodies of water such as aquifers are among the least explored habitats. But there are also fish in these dark, underground water deposits. However, only about one percent of the more than 37,000 known fish species have adapted to live underground, and fewer than ten percent of these species live in aquifers, which, unlike caves, are difficult to reach. “These so-called ‘phreatobiont’ fish are discovered extremely rarely – when they do, it usually happens by chance,” explains Ralf Britz from the Senckenberg Natural History Collections in Dresden. The locations are usually wells that are being emptied or cleaned.
A unique find
This is what happened to Britz’s colleague Wimarithy Marak in northeast India. In a well she dug herself in a small town in the state of Assam, the researcher came across an unknown fish that was only around 20 millimeters long, apparently a loach. “I was immediately sure that this was a very special find,” says Britz. To check this, the research team examined several specimens in more detail. They analyzed the body shape and skeletal structure using, among other things, high-resolution CT scans and compared the characteristics with other loach species. In addition, they determined genetic sequences to clarify the evolutionary position of the fish in the loach family tree.
The investigations revealed that the small fountain fish belongs to a previously unknown species and even to a new genus. The fish named “Gitchak nakana” is fully adapted to life underground. This is revealed by some typical characteristics of animals that live underground. These include, but are not limited to, severely reduced or completely absent eyes, lack of pigmentation with a pale, transparent appearance, heightened non-optical senses, and elongated appendages.
A fish without a skull cap
“But one anatomical detail is particularly spectacular: the animals do not have a bony skull roof. The upper part of the brain is only protected by skin – a unique feature among the known loaches,” says Britz. The small size of these fish is also striking. At a maximum length of around two centimeters, Gitchak nakana is one of the smallest loaches of all, as the biologists report. The females of this fish species, which lives in groundwater, only lay a few, but relatively large, eggs. Researchers suspect that this is an adaptation to the nutrient-poor underground habitat.
Gitchak nakana is the first phreatobitic loach from northeast India – and at the same time the first known aquifer-dwelling fish from this region. The new species was only found in a single well, although researchers also examined other wells in the area. “The discovery of Gitchak nakana provides evidence that this region also has a highly specialized underground fauna in aquifers that now needs to be researched,” says Britz.
Source: Senckenberg Society for Natural Research; Specialist articles: Scientific Reports, doi: 10.1038/s41598-026-40425-6