A blood-red, snake-like burrowing fish from Meghalaya

A blood-red, snake-like creature with an elongated body and sightless black spots for eyes, was spotted burrowing into the moist soil of a rainforest in Meghalaya. Amphibian biologist Rachunliu G. Kamei was digging for legless amphibians called caecilians near a stream in the Khasi Hills, close to the famous living root bridge, when she came across this startling specimen.

“It is a unique adrenalin-rush excitement when you see a special animal like this in the field digging for hours (or days) in sweltering heat,” said Kamei when asked about her first thoughts on seeing the animal. “What struck me most was its bright colour and the slenderness of the specimen.”

The elongate, hot pink creature was not a snake, or an amphibian, but a fish. It was a swamp eel, a group of fish that the authors call the “least fish-like” of bony fish. All swamp eels live in a dark habitat: either in stream beds and marshes, or underground, or in caves. Adaptation to a subterranean habitat means all swamp eels are blind. Their skin, which is never exposed to light, lacks pigment. Swamp eels have an extensive network of blood vessels just under the skin, meant to allow the swamp eels to breathe, giving the blood red colour.

Kamei and her colleagues from the Natural History Museum in London have called the new species Monopterus rongsaw. ‘Rongsaw’ is the Khasi word for red — alluding to the blood red colour of the species in life.

“It [the new species] is special because it is the first swamp eel of this whip-like body type and with this soil-dwelling life-style from northeast India,” said Ralf Britz from the Natural History Museum, a co-author of the study. “The most special feature is its seemingly complete independence from water, which leads us to ask questions about its anatomical and physiological adaptations, its reproductive behaviour and all other life history features.”

Not a very fishy fish

Swamp eels belong to the family Synbrachidae — a family of unique bony fishes that lack eyes, fins and ribs and sport an elongated body. There are about 25 species of swamp eels across the world, found in all continents except Antarctica, said Britz.

Swamp eels have a modified breathing apparatus. Rather than breathing through gills like regular fish, this group breathes through their mouth and skin.

“The epithelium [tissue] in their mouth and pharynx [wind pipe] has a dense net of capillary beds in which the gas exchange takes place and their gills are rather reduced with fewer and much shorter gill filaments,” said Britz in an email to Mongabay-India. “This adaption has made swamp eels less dependent on water for breathing and they can therefore live in habitats that most fishes would be unable to tolerate.”

This curious lifestyle means swamp eels are more the exception than the rule when it comes to fish. “More swamp eels have been collected by my herpetologist friends than I have ever collected with my fishing net,” said Britz in a press release.

M. rongsaw was a serendipitous ‘by-catch’, said Kamei. “Since caecilian species (those in northeast India) need water for reproduction, field work is generally done during the monsoon season (~early May to ~late August),” she said. “I generally choose sites that are close to a water source (permanent/temporary) in a given locality (can be in forest, or agricultural areas), best generally is damp soil. And of course, these are sites you encounter swamp eels as well!”

The new species was found burrowing about 50 metres from the nearest stream, states the press release. Kamei was digging with a spade, following her usual protocol. “One powerful strike into the soil, yank the spade towards you to immediately pull back a chunk of soil, check if there’s anything. I usually dig to 20-40 cm depth.”

There is no data on how these underground dwellers (caecilians, swamp eels and their neighbours) partition their homes. “Burrowing eels such as this new species are an overlooked group of animals (like their soil-mates caecilians) because of their underground lifestyle,” said Kamei.

Sisters separated by thousands of miles

Within the Indian subcontinent, swamp eels have been recorded from Kerala and in the northeast. The enigmatic M. rongsaw is very different from the other swamp eels found in northeast India (NEI). Monopterus cuchia, a common swamp eel found in the rice fields of Assam, is actually cooked into a delicacy. The more widespread Ophisternon bengalense is a “brackish water mud-dwelling fish,” said Britz, found in river tidal zones of south and Southeast Asia, in areas like the Ganges delta. Monopterus ichthyophoides is known from one stream in Mizoram.

“Right now, I would expect the species most closely related to M. rongsaw to be those in the aquifers in Kerala,” said Britz. “No whip-like soil dwelling synbranchid is known from east of India. But let us not forget that the Eastern Ghats are very poorly sampled and I would bet that there is a similar swamp eel to be discovered there.”

The Monopterus genus has five species described from Kerala:  Monopterus fossorius, M. indicus, M. eapeni, M. roseni and M. digressus.

“When I saw a photograph of the new species, my first reaction was one of surprise – I felt I was looking at one of the monopterans I described from Kerala. The same blood red colour because of skin lacking in pigments,” said K. C. Gopi, the deputy director of the Zoological Survey of India and an amphibian biologist.

All the Kerala species have been recorded from deep, man-made wells found in areas with lateritic soil. “Laterite rocks are porous, with canal like structures inside which may provide a subterranean network for the species to move from one place to another,” said Gopi.

Britz is of the same opinion. “The wells are connected to large underground water bodies, called aquifers, in which these animals probably thrive naturally,” he said. “But these aquifer inhabiting organisms cannot be collected from these underground waterbodies and only turn up when they enter the wells.”

The hidden mysteries of the northeast

As of now, there are more swamp eels recorded from south India compared to the northeastern parts of the country.

“I think this is largely a sampling artefact,” said Kamei. “NE India is far less studied than the Western Ghats for herps or fish (or for that matter, biodiversity science in general) for several reasons. Dedicated field research for swamp eels, I am quite certain, will show NEI to have a higher swamp eel diversity than currently know.”

Kamei illustrates this with an example of her study group: burrowing amphibians called caecilians. Until 2009, northeast India was shown to have only four species of caecilians and the Western Ghats more than 20 species.

“This was not because the NEI were less diverse, but because no one went out to the field digging and searching for caecilians specifically. With dedicated research nine new species (plus one new genus and one new family) were described from the region,” said Kamei.

The uniqueness of northeast India and the Eastern Himalayas in general, is becoming apparent with increased research. Because of a fascinating geological past, the area has evolutionary connections to three very different biogeographic realms: peninsular India, Indo-China and Southeast Asia.

Waikhom Vishwanath, a professor at Manipur University who has been working on fish from northeast India, said, “We can divide the fish in the region into southeast Asian, Chinese and Indian fish.” He further explained that the Upper Brahmaputra and the Ayeyarwady river in Myanmar were connected millions of years ago. Constant flux in the river basins over the years resulted in breaking and forging of connections between different water bodies, resulting in an interesting mix of aquatic species in northeast India, he explained.

Study co-author Ralf Britz agrees. “Often there are fish species in the Brahmaputra/Ganges whose closest relatives are in the Ayeyarwady and the closest relatives of both are in the Western Ghats. That is a pattern we have found repeatedly.”