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A species of subterranean fish described last year has been found to be so different from its closest relatives that it belongs to an entirely new family of fish called the dragon snakeheads.
These fish are so distinct that they may be a relic population that has existed for perhaps over a hundred million years.
Deep below the Indian state of Kerala lies a hidden world that is teeming with life.
Aquifers tens of metres below the surface are home to numerous species of fishes and at least one species of shrimp that are found nowhere else on Earth.
One of these fish is called Aenigmachanna gollum. It is an eel-shaped fish known more widely as the Gollum snakehead that was recently described from these pockets of subterranean water after it was flushed out of the aquifers following the devastating floods that hit Kerala in 2018.
But further study of the Gollum snakehead, named after the Lord of the Rings character, is revealing even more about this underground oddity.
Dr Ralf Britz, a scientific associate at the Museum based at the Senckenberg Natural History Collections, Dresden, first helped describe Aenigmachanna gollum in 2019, and has since been looking at the fish in further detail.
'When I first saw the pictures of this fish I was just completely puzzled, I didn't even know what family it belonged to,' explains Ralf.
It turns out that this was because the fish belonged to a completely new family, one which Ralf has now described with colleagues in the journal Scientific Reports as Aenigmachannidae, giving them the common name of dragon snakeheads.
The group of fish known as the snakeheads are normally found prowling the freshwater rivers and streams of western Africa, central Asia and China. As active predators they will feed on anything they can fit in their mouths. Prey ranges in size from tiny invertebrates to large fish and even small mammals.
But in 2018, Ralf got an email that shed new light on these fish. It contained a picture that had been posted on social media in India that showed a mysterious fish unlike anything he had seen before.
It would not be until a year later that reports of a similar fish had been found, and Ralf could finally get a better look at this unusual find.
'It was then that I knew that this was a really sensational find,' says Ralf, 'and I almost immediately got on a plane and went to India.'
Ralf and his colleagues were able to formally describe it as not only a new species, but the first known subterranean snakehead. A few weeks later, a second species of underground snakehead from southern India has also been described.
Along with local people who alerted Ralf’s colleagues, he and his collaborators were able to collect more specimens of the Gollum snakehead for further studies. The more Ralf and his team looked at A. gollum, the more unusual it seemed when compared to other snakeheads. The most obvious difference was to do with an organ found in their head.
'Snakeheads have what we call a suprabranchial organ,' explains Ralf. 'This is an organ that they use to breathe air. It is situated in a big cavity above the gill arches so that it is well supplied with blood as this is where the gas exchange happens.
'All snakeheads have that, but Aenigmachanna doesn't. It doesn't even have a trace of it, which means that it is not as if these fish reduced this organ, I think they never had it. That was one of the first big differences.'
This suggests that the new family of snakeheads is much more basal than the more common ones. This means that, when looking at a family tree of the fish, they are located on a much lower branch, something which is supported by another significant characteristic.
'Another substantial difference is the swim bladder,' says Ralf. 'In all snakeheads the swim bladder goes all the way to the tail, giving them a very long abdominal cavity supported by ribs.
'But in Aenigmachanna it is much more like a regular fish in that the swim bladder and their supporting ribs stop at about the middle of the body.'
These characteristics which are shared with fish outside of the regular snakeheads are known as primitive characteristics, as it is likely that they are similar to what ancient ancestors of snakehead fishes once had.
It was clear evidence that Aenigmachanna species had been separated from the more regular snakeheads for a long time and were so different they belonged to an entirely new family, Aenigmachannidae. This has been dubbed the dragon snakeheads due to their resemblance to the mythical beasts and the fact they live deep underground.
The tricky question now is figuring out not only how long the two groups of snakeheads have been separated, but why they have evolved such dramatic differences in their bodies.
'When we dated the split in the groups we came up with two ages,' says Ralf. 'The older age gives us a range of 85-130 million years ago and, to be honest, the very old one fits better with what we think has happened.'
Ralf suspects that the dragon snakeheads are a relic of Gondwana. This was the landmass comprising of South America, Africa, Antarctica and India that existed until it began breaking up some 120 million years ago.
It is possible that the two families of snakeheads had diverged before that break-up of the continents, and that the dragon snakeheads were then transported in the huge chunk of land that would eventually hit Asia and become India.
Why one group of the fish evolved the ability to breathe air while the other lives deep beneath the surface is not known.
Due to the lifestyle of the dragon snakeheads, it is impossible to actually study them in the wild and so all Ralf and his colleagues can learn about them has to come from the rare occasions when they are washed out from the rocks and enter into the light.