A turtle feared to be locally extinct was found to be abundant enough to shed 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 rear-breathing favorite.
The lower Burdekin River is wide and muddy, and its turtles spend most of their time in the depths, so it was only in 1990 that TV presenter Steve Irwin and his father caught one. When it was confirmed that it was a new species, it was named Irwin’s tortoise (elseya irwini). Although it was later found to inhabit tributaries of the Burdekin, it has not been seen on the lower Burdekin for over 25 years, raising fears that it has lost its largest habitat.
However, an article in the journal BMC Ecology and Evolution has revealed evidence that the turtle is still present in many places in the river. The evidence comes in the form of DNA found in water samples collected along the river.
Queensland’s freshwater turtles, including the Irwin’s, have developed a distinctive method of breathing to avoid having to surface too often, thereby reducing the risk of becoming someone else’s lunch. They absorb water through their cloaca (the hole that reptiles and birds use to breed and defecate). The gill-like structures in their digestive tracts capture dissolved oxygen in the water.
No known turtle species can survive solely on its anal-sourced oxygen; everyone has to come to the surface from time to time to get air in the usual way. However, breathing with your butt delays the urge to do so. Under the right conditions, some turtles can obtain 80 percent of their oxygen needs this way, although the figure is lower for other species.
However, bum respiration requires oxygen-rich water, which requires fast-moving currents. It was feared that a dam on the Mary River could drive species in that watershed to extinction by reducing the supply of oxygen. The campaign to stop the dam, in which the rare and endangered tortoise took a leading role, was the first time its ability had been made known outside of herpetological circles.
Consequently, it was feared that the Burdekin Falls dam, completed in 1987, might have started a fatal decline in Irwin’s tortoise numbers in that river. However, Dr Cecilia Villacorta-Rath, Professor Damien Burrows and co-authors sampled 37 sites along the Burdekin River and its tributaries, and turtle DNA was found throughout the watershed.
That does not mean that the turtle is not affected by the prey. Samples taken from the Broken and Bowen tributary rivers were more frequently positive for turtle DNA, indicating a higher abundance there than below the dam. However, turtle DNA was found at four sites below the dam, but above the confluence with the Bowen River.
“Until this rediscovery, we had no formal record to show that Irwin’s tortoise still lived in the lower Burdekin River, and that river has changed a great deal since the construction of the Burdekin Falls Dam,” Burrows said in a statement. . “It’s reassuring to know that they still live there.”
Irwin’s Tortoises share the lower Burdekin with crocodiles, discouraging scientists from diving into the waters. The turbidity of the river below the falls makes underwater cameras useless, and all that nonsense limits the chances of detecting them on the surface. The ability to collect environmental DNA (eDNA) and magnify it to the point where its source can be determined has proven to be 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 its DNA,” Burrows said.
“We don’t know anything about the demographics of this population,” Vilacorata-Rath said, but the fact that they are still alive shows that further work is not without fruit.