«The new species Gitchak nakana, of which only a few specimens are known, was brought to light during the cleaning of an excavated well in a small village in the Indian state of Assam», says ichthyologist Lukas Rüber, curator at the Natural History Museum in Bern. The tiny, eyeless loach is so unique morphologically and genetically that it represents not only a new species, but even a new genus. Another special feature of the tiny blind loach is the absence of a bony skull; the upper part of the brain is protected only by skin, an anatomical feature that has so far only been found in a very few tiny fish species.
Of the more than 37,000 known fish species worldwide, around 300 species live underground, mostly in caves. These species often exhibit troglomorphies – special morphological adaptations to life in permanent darkness, such as reduction or loss of eyes, lack of pigmentation and an enhanced sense of touch or smell. Only about 10% of all fish species living underground occur in aquifers, i.e. porous rock layers that carry groundwater. «Fish that live in this unique habitat are found only very rarely and purely by chance», says Rüber.
Significance for biodiversity and evolutionary research
The discovery provides important insights into the evolution of subterranean species. As the first aquifer-dwelling fish from northeast India, Gitchak nakana draws attention to this sensitive and hitherto unexplored ecosystem and underscores the role of aquifers as habitats for hidden biodiversity. Molecular dating of the loach family tree suggests that Gitchak split from its closest relatives 21–45 million years ago – significantly older than the age of the porous aquiferous rock layers in which the loach was found.
This can be explained either by the long-term geological stability of similar aquifers in the region or by the possibility that Gitchak's ancestors originally lived above ground and only later migrated into the underground aquifer system. «The discovery of the groundwater-dwelling species Gitchak nakana shows how little we still know about the evolution of fish living in aquifers and their diversity – research in this field remains exciting», says curator Lukas Rüber.
