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Stygichthys typhlops reemerges from the dark
An incredibly rare blind fish that lives underground in Brazil has been rediscovered by scientists.
A number of individual fish have been found almost 50 years after the only known specimen was collected and then described by American experts.
Biologists cannot be sure, but they suspect the fish may be a living relict that has survived deep under the ground while its relatives above went extinct.
A team of Brazil-based researchers led by ichthyologist Dr Cristiano Moreira of the University of Sao Paulo undertook an expedition to rediscover the fish, named Stygichthys typhlops.
The first and only known specimen was caught in 1962 from a communal water well used by inhabitants of the city of Jaiba, in Minas Gerais, Brazil.
The fish was taken to American ecologist Dr Joseph Tosi Jr who was in the region at the time, and then described by specialists back in the US.
"This was the most enigmatic species of the order Characiformes, a group of freshwater fishes that includes piranhas and tetras," says Dr Moreira.
Not only did the fish come from underground, but it was blind, whereas most characiforms live above ground and have good eyesight.
"Because it was never collected since, it was assumed that is was an endangered species. However, no one knew. There was no information of its distribution, abundance, anything."
To find the fish, the expedition team interviewed local people around Jaiba, who reported they had seen the fish swimming in open wells.
But because the region is so dry, the people there rely heavily on underground water.
That has caused water levels in most aquifiers to drop considerbly since the 1980s.
"This was one of the problems we encountered to find this fish, since most of the open wells we could access to collect or put traps were dry," Dr Moreira told the BBC.
Eventually, the team found two wet wells. In both, they could see fish swimming, which turned out to be the elusive S. typhlops.
Overall, the team collected 34 specimens, which have allowed them to find out more about this strange species.
As well as being blind, the fish lacks pigmentation, a common trait in organisms that live underground where there is little or no light.
However, "morphologically, Stygichthys is very different from any species in the group, so much so that we are still not able to say to which species it is more related," says Dr Moreira.
One another characiform fish lives underground, a blind tetra called Astyanax fasciatus.
But only some of the species have reduced eyes and pigmentation and live underground in caves.
Other populations of the same species continue to live above ground, without these adaptations.
But that does not appear to be the case for S. typhlops.
"It could represent the last remaining fish of an extinct group in the Characiformes, what we would call a relict species," says Dr Moreira.
"The surface species of this group could have gone through extinction, while Stygichthys because of its habitat was spared. But this is only speculation."
What is certain is that the fish's survival is threatened.
"This species seems to be the most threatened underground fish species in Brazil," says Dr Moreira.
It appears to be restricted to a specific 25km-long aquifier that runs underground in the region.
"The fact that most of the wells in the region are drying is very worrying.
"The excess withdrawal of water from that aquifier will certainly lead to the extinction of the species."
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