By Matt Walker

Editor, Earth News



Common musk turtles have special abilities

One type of turtle possesses an extraordinary organ that allows it to breathe underwater and stay submerged for many months.

The common musk turtle has a tiny tongue lined with specialised buds, scientists have discovered.

Rather than use this tongue for eating, the turtles use it to exchange oxygen, solving a mystery of how these reptiles can remain submerged for so long.

Details are published in journal The Anatomical Record.

"I was very surprised, I really didn't expect that," says zoologist Egon Heiss, who is studying for his PhD at the University of Vienna in Austria.

Mr Heiss and colleagues made the discovery while studying the feeding habits of the common musk turtle (Sternotherus odoratus), a freshwater species that inhabits lakes and rivers in southern Canada and the eastern US.

Adults spend most of their lives underwater, but juveniles occasionally come onto land to search for food.

While filming these juveniles trying to feed, the researchers noticed something unusual: when the animals found food, they could only eat it after dragging it back into the water.

Out of land, they struggled to swallow their prey.

A closer examination of the turtle's tongue revealed why.

The common musk turtle has a weak and tiny tongue covered with and surrounded by specialised bud-like cells called papillae.

Further tests revealed that the turtle uses these cells around its tongue to breathe, by drawing in oxygen from water that passes over them.

"We knew that an organ for aquatic respiration must be present somewhere but finally discovered it accidentally," says Mr Heiss.

Hibernating underwater

Some turtles cannot breathe underwater at all.

All marine turtles, for example, must come to the surface at least every few hours to gulp air.

Of freshwater turtles, some cannot breathe underwater, while others do so via their skin.

Scanning electron micrograph showing tongue (T) and the respiratory lobe-like papillae (indicated by arrows)

Other species, such as the side-necked turtles of Australia, cope by using specialised cavities in their rear, known as cloacal bursae, to draw in water and remove the oxygen.

Such turtles often need to spend long periods of time underwater, where they hibernate, remaining asleep and still, not feeding and slowing their metabolic rate down.

"Musk turtles, however, lack cloacal bursae and their skin is relatively thick and lacks a well developed capillary network," Mr Heiss told the BBC.

So how these turtles can spend months underwater without coming to the surface has remained a mystery, as they cannot take in enough oxygen through their skin.

"We found the large papillae in the throat and were immediately fascinated," says Mr Heiss.

He and his colleagues believe the musk turtle's tongue is likely to be an ancient trait.

Turtles are among the longest surviving group of higher land vertebrates, known as amniotes, having persisted for 220 million years.

"I truly believe there's still a lot to discover," he continues.

"This study shows how plastic adaptations to certain environmental circumstances can be in turtles."