Although they have gills, most tadpoles need to breathe fresh air to survive and develop by breaching the surface of their watery home, especially in oxygen-depleted water.
But new research shows that newborn tadpoles are too small and weak to break the layer where water and air meet that’s caused by surface tension. So how do they breathe? It turns out they suck bubbles, according to a paper published in the journal Proceedings of the Royal Society B: Biological Sciences.
“The initial observation was completely accidental,” says evolutionary biologist Kurt Schwenk, from the University of Connecticut, US. “We had tadpoles in the lab for another project on larval salamander feeding. I noticed a tadpole rapidly surface and do something strange, leaving a little bubble behind.”
Schwenk and his PhD student Jackson Phillips set up a high-speed camera to get a closer look. They recorded days-old tadpoles just three millimetres in size belonging to five frog species swimming to the surface. By slowing down the footage they could see what was going on.
“They use the surface tension to adhere their open mouth to the under-surface of the water. Once the mouth forms a seal with the surface, they drop the floor of the mouth and pharynx, expanding the oral cavity,” says Schwenk. “This creates suction, pulling the surface down into the mouth to form a bubble.”
With a quick snap of the jaws, the tadpoles “pinch-off” the bubble from the surface. It contains fresh air from above and a small amount of deoxygenated air that’s exhaled from the lungs during the process. The floor of the mouth then lifts up to squeeze the bubble, forcing air into the lungs.
“There is so much more fresh air than depleted air in the bubble that the problem [of breathing some deoxygenated air] isn’t as bad as it might be,” says Schwenk.
Since the mouth space is bigger than the capacity of the lungs, tadpoles end up making bubbles that contain more air than they can use. So leftover air is simply forced back out of the mouth, which produces the residual bubble that Schwenk first noticed floating to the surface.
“I assumed that the behaviour had been described before, but when I checked the literature, I was surprised to see that it hadn’t,” he says. “On the other hand, high-speed video with really good resolution is still a relatively recent thing, so we benefitted from the technology now available.”
The researchers also discovered that bubble-sucking isn’t unique to tadpoles. Other footage showed that salamander larvae and lung-bearing aquatic snails do a similar bubble-suck to get around the surface tension barrier.
What’s more, another study by the duo, published in The Journal of Experimental Biology, revealed that tadpoles from two of the frog species they originally investigated continue to bubble-suck even when they grow big enough to breach – but with a twist.
The technique becomes more elaborate with two bubble-sucks in quick succession.
“The advantage of the double bubble-suck is that it prevents the mixing of fresh and depleted air and is presumably a more efficient way of exchanging gas to breathe,” says Schwenk.