WASHINGTON, April 9 (Reuters) - A rare and primitive frog living in a remote Borneo stream has no lungs and apparently absorbs oxygen through its skin, researchers reported on Wednesday.

The aquatic frog has evolved backwards, re-acquiring a primordial trait, David Bickford of the National University of Singapore and colleagues reported.

Studying the frog could help shed light on how lungs evolved in the first place, they wrote in the journal Current Biology, adding that illegal gold mining in the area may threaten the unique species.

"The evolution of lunglessness in tetrapods (amphibians, reptiles, birds, and mammals) is exceedingly rare, previously known only from amphibians -- two families of salamanders and a single species of caecilian (blindworm)," they wrote.

"Here we report the first case of complete lunglessness in a frog, Barbourula kalimantanensis, from the Indonesian portion of Borneo."

The frog may be endangered because of mining activity, the researchers said.

"In August 2007, we visited ... near NangaPinoh, Western Kalimantan but found that illegal gold mining had destroyed all suitable habitats in the vicinity," they wrote. They snorkeled, waded and turned over boulders to find their quarry.

"The originally cool, clear, fast-flowing rivers are now warm and turbid. Water quality around the ... locality is no longer suitable for the species, but we were able to re-discover two new populations upstream," they added.

"We knew that we would have to be very lucky just to find the frog," Bickford said in a statement.

Animals evolved lungs when they moved from the sea to land millions of years ago. Animals have only lost this important adaptation a few times, Bickford's team said.

"The discovery of lunglessness in a secretive Bornean frog, supports the idea that lungs are a malleable trait in the Amphibia, the sister group of all living tetrapods. Amphibians maybe more prone to lunglessness since they readily utilize other methods for gas exchange," they wrote. "This is an endangered frog that we know practically nothing about, with an amazing ability to breathe entirely through its skin, whose future is being destroyed by illegal gold mining by people who are marginalized and have no other means of supporting themselves," Bickford said.

Only animals with small body sizes, slow metabolisms and living in fast-flowing cold water where oxygen and carbon dioxide are exchanged quickly may be able to survive without lungs, the researchers said.

"We strongly encourage conservation of remaining habitats of this species," they recommended.

(Reporting by Maggie Fox; Editing by Cynthia Osterman)

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