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The new species is thought to spend its life in the aquifers deep beneath Kerala, India, and so has been named after Gollum from J R R Tolkien's works.
The new species belongs to the snakeheads, a group of fishes native to Africa and Asia. They are distinguished by their long dorsal fins, large mouths, and ability to breathe air using what is known as a suprabranchial organ in their gill cavity.
Usually, these fishes live in freshwater rivers throughout the forests of western Africa, the wetlands of central Asia and the mountains of China. They are active predators that feed on a range of prey, from animals the size of plankton up to small mammals, depending on the species.
But in the state of Kerala, South India, researchers have found a snakehead fish unlike any seen before. The new species, named Aenigmachanna gollum, has been described in the journal Zootaxa.
Reports of the fish initially surfaced about a year ago, when someone posted a picture on social media of an unidentified species of fish that baffled scientists.
It was not until the devastating floods that hit Kerala in August last year that researchers managed to get a better look at this strange creature.
Mr Ajeer, a local resident of the small village Oorakam in Kerala, uploaded a picture onto the internet of this fish, which he had found in a paddy field. The animals had seemingly been washed out of the aquifers – underground, permeable rocks that carry water.
This time the picture was spotted by Rajeev Raghavan, from the Kerala University of Fisheries and Ocean Studies, who sent his team to the site to take pictures of the live animal.
Dr Ralf Britz, a researcher at the Museum who co-authored this latest paper finally describing the species, says, 'As soon as I saw higher quality images it became pretty clear that it was a snakehead.
'But having a snakehead from a subterranean habitat is a really exciting find. This fish is very unusual.'
Globally, there are around 250 species of fish that are known to live underground.
These tend to have a suite of adaptations to living in the pitch darkness of caves and aquifers, such as a lack of pigment in their skin making them ghostly white, reduced or absent eyes, and the improvement of non-optical senses such as pressure sensing – or lateral-line - systems and taste organs.
The vast majority of these subterranean fishes belong to just a couple of groups.
'If you look at the number of subterranean species of fish in the world, then there at just two groups that contribute three quarters of all species,' says Ralf. 'These are the Siluriformes, or the catfishes, and the Cypriniformes, or carps, loaches and their relatives.
'Some people argue this is because many species in these groups are already pre-adapted to life underground, so if they end up in a subterranean habitat then it is easier for them to cope with finding food and surviving.'
A. gollum, however, doesn’t seem to fit that pattern. Snakeheads don't tend to have any adaptations that might also be suited to an underground lifestyle.
'The surprising part is that - assuming it is subterranean - it doesn't have the typical adaptations you would expect from a subterranean fish,' explains Ralf. 'This could mean one of two things.
'Either the fish only acquired this subterranean mode of life recently, or it is living in some kind of transition zone where it is still moving between the underground and above ground habitats. Again, we simply don’t know and so that is something we'd be very interested in finding out.'
But this new species is even more unusual in that the larger group to which it belongs, the percomorphs, is not known to have many other subterranean species. Percomorpha is a massive group, containing an estimated 41% of all bony fish species, including the perches, tunas, seahorses, cichlids and anglerfishes.
The new fish is highly distinctive when compared to the snakeheads found in both Africa and Asia.
In comparison to these other species, A. gollum has an incredibly long and eel-like body, numerous scales long its body, a very long anal fin running along its belly and tail and has even lost the ability to maintain buoyancy in the water column. It is possible that some of these aspects, such as the slender body and inability to remain buoyant, could be adaptations to their underground existence, but more needs to be done to establish this.
These differences in anatomy are backed up by differences in their genetics. The snakeheads from Asia and Africa form two separate genera, called Channa and Parachanna respectively.
'With the help of Neelesh Dahanukar from the Indian Institue of Science Education and Research in Pune, we conducted some molecular analysis where we sequenced a couple of genes in the new species,' explains Ralf. 'We found a difference in the gene sequence that is equivalent to what we tend to find between genera, so it was pretty clear that this fish belonged to a third genus of snakehead that we had to name.'
They have called the genus Aenigmachanna, as a reflection of just how little we know about this animal. The species name, gollum, is in recognition of The Lord of The Rings character who himself took to a life underground and in turn developed various adaptations to the gloom.
This is now the eighth species of subterranean fish that has come out of the aquifers beneath Kerala, leading Ralf to believe that there must be a hidden ecosystem existing over 40 metres below the surface.
The fact that this environment is by its very nature so difficult to study means that Ralf and his colleagues are reliant on local residents finding and recognising these unique fishes as they get flushed out of the aquifers or end up in the bottom of wells. It also makes any studies looking into their behaviour or life history near impossible.