Researchers working in Borneo have discovered the only known frog with no lungs, the team reported Monday.
National University of Singapore biologist David Bickford, the lead researcher, said in a release that the frog, Barbourula kalimantanensis, has an "amazing ability to breathe entirely through its skin."
The tiny frog, measuring less than 40 mm and weighing 6.5 grams, was found in western Kalimantan, Indonesia, in August by researchers employing new search methods.
The findings are published in Tuesday's edition of the journal Current Biology.
The frog lives in cold, rushing water and is so rare that only two specimens had been found previously. At that time, scientists did not want to sacrifice those frogs for dissection, so Bickford's team, with its larger find, were the first to discover the species was lungless.
Bickford said the research team was "absolutely thrilled" to find the frog, after searching using non-traditional methods such as stone turning and wading in streams. Bickford also said the team was unsure about the size of the population they located.
"Nobody knew about the lunglessness before we accidentally discovered it doing routine dissections to extract tissue samples to be used for DNA work," Bickford told CBC News in an e-mail Tuesday.
"When we did [find the frog] and I was doing the initial dissections — right there in the field — I have to say that I was very skeptical at first," Bickford said in a release. "It just did not seem possible. We were all shocked when it turned out to be true."
The researchers said they tried to be as non-invasive as possible, dissecting only four specimens completely and four partially to confirm the lack of lungs.
The frog receives all necessary oxygen through its skin, the researchers said. Among four-legged creatures, only amphibians are known to breathe without lungs — previously only in two families of salamanders and a species of caecillian, a limbless amphibian.
Frogs may have lost lungs for survival
The researchers hypothesized that the frog may have evolved without lungs, or lost its lungs, in adaptation to the high-oxygen environment of its habitat and the species' preference to sink, rather than float, which would have been hindered by lungs full of oxygen.
Bickford explained that cold water can hold more dissolved oxygen than warm water, meaning that in a fast-flowing river, many molecules of oxygen come into contact with the frog's skin.
"Those factors, along with the fact that having lungs makes you more likely to be swept away in a fast flowing stream (because you would float), make this a very strong context for the evolution of loss of lungs," he said, adding that because cold-blooded animals have a low metabolic rate they do not need as much oxygen to live.
"That might be one of the crucial prerequisites for lunglessness since it lowers the total amount of oxygen you are able to utilize. If you don't need as much oxygen anyhow, it might be easier to change, to lose lungs as the primary respiration organ."
Bickford said the finding could help researchers learn more about lung development and the genes that regulate lung growth and tumour suppression.
"If the frog has no lungs, and it still has the genetic building blocks to make those organs, we could find useful applications of the genes since they might be under neutral selection (i.e. they could have novel and unique mutations). At this stage, it is all guesswork, but there is a real possibility," he said, adding "the real implications of the discovery is whether or not we can stop this evolutionary enigma from going extinct in the wild."
Frog's future is 'grim'
He emphasized that deforestation and illegal gold mining in the area is threatening both the species and further research into its reproduction, feeding and life.
"This frog has a grim future and it is entirely our fault that it is, and our responsibility to try and remedy the situation," Bickford said. "If we want to learn anything about these frogs, we have to protect the remaining habitats where it could exist."
He said the researchers hope to go back and conduct "a larger-scale project to determine the frog's extent of geographic range — where exactly are they found — and measure some habitat characteristics so we know more about where they might be found."
"We know shockingly little about these animals."