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.' A life underground 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.