‘Enigmatic’ catfish species that dwells in complete darkness found in India

Newly discovered fish species survives in environment with less oxygen concentration

An “enigmatic” new catfish species that dwells deep underground in a completely dark environment has been found in India.

The blind fish have a “blood-red body”, are just 3cm in length and live in underground aquifers that have a low concentration of oxygen and are low on sources of nutrients.

The fish dwell in local groundwater without any light and surface only during the digging and cleaning of homestead wells, said scientists, including those from the Kerala University of Fisheries and Ocean Studies (KUFOS) in India’s Kerala state.

The catfish are of the genus Horaglanis and have been christened Horaglanis populi by the scientists, who recently published their study in the journal Vertebrate Zoology.

The newly discovered fish species are blind, lack pigments and have a “high level of genetic diversity” that has evolved over millions of years.

“There are very few documented occurrences of these species – as a rule, these elusive little fish only come to the surface when a domestic well is being dug or cleaned,” said Ralf Britz, a study co-author from the Senckenberg Natural History Collections in Germany.

The discovery was the result of a six-year study of water-bearing laterite rock layers in Kerala.

The scientists teamed up with locals and applied targeted collection efforts in wells and above-ground storage tanks, using scoop nets in shallow wetlands, water channels, home gardens and plantations.

They also used baited traps in excavated wells on farmsteads and in ponds and caves to look out for the rare aquifer-dwelling fish.

“This allowed us to generate data sets with a total of 47 new site detections and 65 new genetic sequences,” Dr Britz said.

“These show, among other things, that Horaglanis are endemic to the part of Kerala state south of the Palghat Gap – the mountain pass apparently represents a biogeographical barrier for the subterranean world as well,” he explained.

The study would not have been possible without help from local communities, he explained. The naming of the fish was influenced by the efforts of the communities involved.

“Local people are often the only ones who get to see such well-hidden species. Therefore, they can play an important role in improving our scientific knowledge of this unusual fauna,” Dr Britz said.

“The specific name populi, the genitive of the Latin noun for ‘people’, honours the invaluable contributions of the interested public in Kerala who helped document the biodiversity of these subterranean fishes – including the discovery of the new species,” he pointed out.

“Our Horaglanis project is an excellent example of how public involvement can greatly increase our knowledge of rarely collected organisms that live in relatively inaccessible habitats. Local people expand the researchers’ ‘eyes and ears’ by several orders of magnitude,” Dr Britz said.

The scientists conducted a series of workshops over a six-year period along with focus group discussions and informal conversations with local communities at several sites.

“We informed local villagers about the importance of subterranean fish species and their conservation needs and asked them to share information, photos, or videos with us when they encountered and/or collected these species,” he added.

The rarity of the discovery is underscored not just because the fish live exclusively in underground aquifers, but also because such species are considered to be at a high risk of extinction.

Fish populations are threatened by groundwater extraction and mining of laterite rock layers.

“Currently, 289 fish species are known from subterranean aquatic habitats worldwide – less than 10 per cent of them live in aquifers,” Dr Britz explained.

The fish also get little to no protection under local or regional laws, with their habitats embedded in densely populated landscapes, they explain.

“Utilising new information derived from these samples, we significantly advance knowledge of, and reduce key biodiversity shortfalls for, these enigmatic catfishes,” said the scientists.