Tiny, Blind, and no Skull Roof: New Fish Species Discovered from a Well

An international team led by Senckenberg researcher Dr. Ralf Britz has made an extraordinary discovery in Northeast India when they found an as yet undescribed species of fish in a well. The blind loach with a length of just 20 millimeters, now described as Gitchak nakana in the journal “Scientific Reports,” is characterized by almost non-existent eyes, a transparent body, and a missing skull roof. Gitchak nakana is the first aquifer-dwelling fish species described from Northeast India and provides the first evidence of a previously unknown subterranean fauna in the region.

Ever since the scientific description of the olm (Proteus anguinus) in the 18th century, subterranean animals have been a source of great fascination. Most of these creatures are known from caves, but some species have also adapted to other habitats hidden beneath the earth’s surface, such as aquifers – water-bearing layers of rock deep below ground. “Of the more than 37,000 fish species known worldwide, only around one percent have conquered the subterranean habitat and adapted to life underground. In turn, fewer than ten percent of these species live in aquifers. These so-called ‘phreatobitic’ fishes are discovered extremely rarely and usually only by chance,” explains Dr. Ralf Britz from the Senckenberg Natural History Collections in Dresden.

Unlike caves and their more easily accessible fauna, the “phreatic zone” – i.e., the subterranean groundwater layer – is difficult to access. Fishes that live there are usually discovered accidentally – for example, when wells, which are the only window into this habitat, are emptied and cleaned and their hidden inhabitants are thus literally brought to light. “Taking water from a well is exactly how we discovered the new fish species,” says Britz. “The loach, which is only 20 millimeters long, was found by biology student and doctoral candidate Wimarithy Marak in a self-dug well in a small town in the state of Assam, in Northeast India. When I was presented with this fish while on a research trip to the region in 2024, I immediately knew that this was a very special discovery.”

Together with his colleague Dr. Amanda Pinion from Dresden and the Indian doctoral student Velentina Kangjam, who worked in Dresden last year funded by a Senckenberg Global Fellowship, Britz examined the newly discovered fish and produced various micro-CT images of the skeleton. This allowed the international research team to describe the fish as  a new genus and species, Gitchak nakana. “The tiny, blind loach is unusual enough that it warrants an entirely new genus,” says Britz. The initial specimen, as well as additional individuals discovered in the well in 2025, show typical characteristics of subterranean animals, such as greatly reduced or completely absent eyes, a lack of pigmentation with a pale, transparent appearance, enhanced non-optical senses, and elongated body appendages. “However, one anatomical detail is particularly spectacular: the animals do not possess a bony skull roof. The brain is only protected dorsally by a layer of skin – a unique feature among any of the loach species known to date,” adds the ichthyologist from Dresden.

With 21 known species found from either caves or groundwater systems, India ranks fourth in the world in terms of subterranean fish biodiversity. The main centers of this diversity are Northeast India – in particular the state of Meghalaya with six species – and the southwestern state of Kerala with 13 species. The discovery of Gitchak nakana represents a milestone: It is the first phreatobitic loach from Northeast India and also the first known aquifer-dwelling fish from this region. “The discovery of Gitchak nakana provides evidence that this region also harbors a highly specialized subterranean fauna in aquifers, which now awaits investigation,” adds Britz, looking ahead.