A turtle feared to be locally extinct has been found to be plentiful enough to be shedding its DNA all over Australia's highest discharge river. The rediscovery of any species is good news for ecology, but the chelidae in question is a particular favorite for breathing through its rear end.
The lower Burdekin river is wide and muddy, and its turtles spend most of their time at depth, so it was only in 1990 that TV presenter Steve Irwin and his father captured one. On confirmation this was a new species it was named Irwin's turtle (Elseya irwini). Although subsequently found to inhabit the Burdekin's tributaries, it hasn't been seen in the lower Burdekin for more than 25 years, leading to fears it had lost its largest habitat
However, a paper in the journal BMC Ecology and Evolution has revealed proof the turtle remains present at many locations on the river. The evidence comes in the form of DNA found in water samples collected along the river.
Queensland freshwater turtles, including Irwin's, have developed a distinctive method of respiration to avoid having to come to the surface too often, thus cutting the risk of becoming someone else's lunch. They absorb water through their cloaca (the hole reptiles and birds use for reproduction and defecation). Gill-like structures in their digestive tracts capture oxygen dissolved in the water.
No known turtle species can survive purely on its anally-sourced oxygen; they all have to come to the surface now and then to take in air in the usual way. However, bum-breathing delays the need to do so. Under the right conditions, some turtles can get 80 percent of their oxygen needs this way – although the figure is lower for other species.
Bum-breathing requires oxygen-rich water, however, which requires fast-moving streams. It was feared a dam on the Mary River might drive that catchment's species to extinction by lowering the oxygen supply. The campaign to stop the dam, in which the unusual and endangered turtle took a starring role, was the first time their capacity had become widely known outside herpetological circles.
Consequently, there were fears the Burdekin Falls Dam, completed in 1987, might have started a fatal decline in Irwin's turtle numbers on that river. However, Dr Cecilia Villacorta-Rath, Professor Damien Burrows, and co-authors sampled 37 sites along the Burdekin River and its tributaries, and the presence of turtle DNA was found throughout the catchment.
That doesn't mean the turtle is unaffected by the dam. Samples taken in the Broken and Bowen tributary rivers were more frequently positive for turtle DNA, indicating greater abundance there than below the dam. Nevertheless, turtle DNA was found at four sites below the dam, but above the confluence with the Bowen River.
"Until this rediscovery, we didn't have any formal records to prove that the Irwin's turtle was still living in the lower Burdekin River, and that river has changed a lot since the construction of the Burdekin Falls Dam," Burrows said in a statement. "It's reassuring to know they are still living there."
Irwin's turtles share the lower Burdekin with crocodiles, discouraging scientists from diving into the waters. The murkiness of the river below the falls makes underwater cameras useless, and all that bum-breathing limits opportunities to spot them at the surface. The capacity to collect environmental DNA (eDNA) and magnify it to the point its source can be determined has proven a game-changer for finding the turtles, just as it has been for extinct human species. “All we had to do was take a water sample and analyze for their DNA," Burrows said.
"We don't know anything about the demographics of this population,” Vilacorata-Rath said, but the fact they are still alive proves further work is not fruitless.