Discovery Of World’s Largest Cave Fish In Meghalaya: ‘Flooding in caves, rich fauna made fish grow to 40 cm’

Anjali Marar

Discovery of the fish, measuring 40 cm, has left more questions for scientists to answer — on its evolution, presence in the caves of Meghalaya and its large size.

Regular flooding leaving ample trails of vegetation and scores of unknown organisms found inside the caves located in the dense forests of Jaintia Hills in Meghalaya could have been the main feed for the recently discovered world’s largest cave fish.

Discovery of the fish, measuring 40 cm, has left more questions for scientists to answer — on its evolution, presence in the caves of Meghalaya and its large size.

“This fish, Tor putitora, is commonly found in the rivers that flow along the foothills of the Himalayas. But we still do not know how it landed up in caves in Meghalaya,” Neelesh Dahanukar, co-author and researcher from Indian Institute of Science, Education and Research (IISER), Pune told The Indian Express.

A team of international scientists stumbled upon the fish in February last year, when they were carrying out some cave studies in the region as part of Caving In the Abode of Clouds Project, which was launched in 1992. Under the project, scientists have been studying and mapping caves in Meghalaya.

Tor putitora are commonly found in river waters across India, Pakistan, Afghanistam, Bhutan, Nepal and Myanmar. Some varieties of this fish were previously discovered from Garo Hills in Meghalaya. “As the fish is known to enter into rapid-flowing streams and pools, which are present adjacent to Jaintia Hills, experts suggest this to be one of the sources for their reaching and breeding inside the Meghalayan caves,” one of the researchers said.

It is not the first time that cave fish species has been discovered in Meghalaya. However, they have mostly been smaller in size. There are about 250 known varieties of subterranean fish with a majority of them ranging between 20 mm and 130 mm in size. Very few subterranean fish species discovered so far have been found to grow between 130mm and 230mm. Normally, cave fish do not grow into large fish due to sheer lack of sunlight, thereby making the environment limited in nutrients and scare in food resources.

The research team recently visited the caves to carry out further studies. Dahunakar plans to undertake genomic studies of this discovery.

“This will reveal if there are genetic markers and compare with the available data of Tor putitora available in the ranges. We need to check if the fish is the same species or any unique species in the genus,” said Dahanukar.

Yet another striking feature that the biologists noted was the development of eyes among the fish. The smaller fish were slender while the larger ones were found to be bulky in nature. Significantly, in all the fish inside the caves, eyes were found to be regressed. Black-coloured regression marks were more prominently visible among the smaller fish. The eyes in case of the largest fish, the scientists said, were severely regressed and was invisible unless closely examined.

On this feature, Dahanukar said, “This must be a development phase, and it is common for cave fish to lose eyes as they grow. Also, since they live in dark environments, the role of eyes is limited.”

But the researchers noticed that despite having no eyes, the fish were sensitive to light. This was concluded when the fish disappeared when the team members flashed cave torch light onto them. The smaller fish were capable of showing rapid response to light and were difficult to approach, in comparison to the larger ones, the research stated.