The long pink blind swamp eel

This is the only specimen of the blind swamp eel to have ever been found © Rachunliu G Kamei

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New species of blind eel that burrows through the soil discovered

Considered by many to be the least fish-like of fishes, swamp eels are a real oddity and rarely documented. Now Museum scientists have described an entirely new species. 

The fish was discovered not in water but in damp soil. Museum researcher Dr Rachunliu G Kamei uncovered it while searching the rainforest for an entirely different group of animal, the legless amphibians called caecilians.     

'We were digging all day every day for caecilians in the Khasi Hills of Meghalaya, in the north-east of India, when I chanced upon this fish,' explains Rachunliu. 'This is the only specimen of the species, as we couldn't find any more.'

Compared to the two other species of swamp eel found in the region, the specimen was so distinct that even from this single individual the researchers could be certain it was a new species. It is described in the journal of Ichthyological Exploration of Freshwaters.

A burrowing fish

Even among the 34,000 or so species of bony fishes, swamp eels are rather weird.   

Many swamp eels are found close to freshwater streams, making a living in the mud and between roots. But some have evolved far more specialised, subterranean lifestyles.

Some of these eels live in caves while others are found in aquifers deep underground, but a few actually burrow through the soil. This new species, for example, was found burrowing some 50 metres from the nearest stream.

Unsurprisingly this means that very little is known about these ichthyological oddities, as those who study fishes tend to be looking in the water.

Dr Ralf Britz, fish researcher at the Museum and co-author of the paper with Rachunliu, says, 'More swamp eels have been collected by my herpetologist friends than I have ever collected with my fishing net.' Herpetology is the study of amphibians and reptiles.

CT scan of blind swamp eel's skull

CT scans of the fish's head reveal its formidable looking skull © Britz et al. 2018

Living in the dark

Living an entirely subterranean lifestyle means that the fish has a suite of adaptations - including being blind. Ralf explains, 'We suspect that the eyes just never really develop. We can't say that for certain for this particular species as we don't have early life stages, but that is something I would expect.'

The vivid pink colouration is also a result of this underground existence. Living in perpetual darkness means that the fish have lost all pigmentation. This is combined with an increase in the number of blood vessels to their skin to allow a certain amount of oxygen exchange to occur. The result is their hot pink appearance.

This leads to another of their major adaptations, shared with other species of swamp eel. The damp soil helps to keep the eels moist, but not so much with breathing.    

'All swamp eels rely on breathing air,' says Ralf. 'Their gill filaments - which most fishes use for oxygen exchange - are almost gone entirely. Instead the covering of the mouth cavity is highly vascularised with lots of blood vessels. These serve as accessory gills.'

This allows the fish to breathe air through the lining of their mouth. Yet this is not the case for the eels' entire lives: while adult swamp eels have no fins, the young eels do.

'The early larval stages have very large pectoral fins,' explains Ralf, 'and they use these as accessory gills instead of their mouths.'

During their development these fins then disappear. Whether they simply drop off, or are partly reabsorbed by the fish, is still not fully understood. 

A stream running through a lush rainforest

The region in which the eel was found is covered in rich rainforest © Rachunliu G Kamei

An overlooked diversity

There are only 25 species of swamp eel described by science, but they have an interesting distribution.

'There are several species of swamp eel from South and Central America, then three species in West Africa, and the rest are found in India and further east,' explains Ralf. 'This is what we call a Gondwanan distribution.'

This refers to the pattern of distribution of many species of animals and plants that evolved on the supercontinent Gondwana before it split to form South America, Africa, Antarctica, Australia and the Indian subcontinent. As this event began around 170 million years ago, these lineages tend to be very old.

The latest eel was found in a very poorly known, remote part of India. Traditionally it has been regarded as a transitional zone between the Himalayas and the Indo-Burma hotspots, with few unique species of its own to speak of.

'But this has turned out not to be true,' Rachunliu explains. 'Rather than being a transitional zone, it seems to have its own set of species with deep evolutionary roots. It's a really fascinating area, but greatly under sampled.'

It is now recognised by the International Union for the Conservation of Nature as the Eastern Himalaya Biodiversity Hotspot. It is hoped that further studies in this poorly documented region will reveal a host of hidden diversity to keep the new species of blind swamp eel company.