Lip-O-Suction
With teeth in its lips and its mouth open 180°, a hungry tadpole turns a scrape into a close shave.
Story by Adam Summers Illustration by Shawn Gould

Though the leap from frog to prince gets all the press, I would argue that the metamorphosis from tadpole to frog is just as impressive (even more so when one considers the greater frequency with which it occurs). The transformation from frog to prince is no mean feat--requiring rapid weight gain, change of coloration, and minor rearrangement of facial features. But pollywogs must grow legs, lose a tail, and completely reconfigure their jaws and digestive tract in order to prepare for a life spent eating flies. Biologists have been fascinated with the frog’s protrusile tongue for decades, but until recently the biomechanics of the tadpole mouth was a mystery.

There are more than 4,000 species of frogs, and the diversity of their tadpoles is nothing short of astonishing. Plankton-eating tadpoles spend much of their time hanging motionless in the water. Others inhabit temporary ponds, where they dine on their fellow tadpoles. But by far the most common way of life involves feeding on the algae and microbes that cover rocks and mud at the bottom of a pond. Tadpoles that live this way have a broad tail; a wide, rounded body; and a peculiar mouth totally unlike the familiar smiling gape of a frog.

The mouth of an algae-eating tadpole has a beak that looks a bit like a squooshed version of a parrot's beak and is set in the middle of a floppy oral disk; also sprouting from the disk, above and below the beak, are close-set rows of tiny teeth. Both beak and teeth are made of keratin, the stuff of fingernails and hair. (Tadpoles tend to look an awful lot alike, and the number and arrangement of the tooth rows are important for determining what kind of frog a particular tadpole will become.)

Richard Wassersug, of Dalhousie University in Nova Scotia, has devoted hiscareer
to understanding tadpoles and, with his colleague Masamichi Yamashita, ofthe Institute of Space and Astronautical Science in Japan, recently described the biomechanics of two quite different tasks for whichtadpoles use their mouths: breathing and feeding.

Early in development, a tadpole breathes with gills, but as it starts the transition to froghood, it develops lungs, which means it must now swim frequently to the surface to gulp air. This is a dangerous business, because the average tadpole would be a tasty treat for any number of fish, reptiles, and birds. Such tastiness puts a premium on its being able to rush to the surface, take a speedy sip of air, then dash back to the safety of the pond bottom.

If you have ever watched a water strider gracefully scooting across a pond, or floated a needle on the surface of a glass of water, you have witnessed surface tension: the “desire” of molecules at the surface of a liquid to stick to those below. For the water strider, this cohesion is a good thing, allowing the insect to walk on, rather than fall through, water. For a little tadpole in a big hurry to take a breath, however, surface tension is an obstacle to be overcome, and some high speed video footage by Wassersug and Yamashita shows how the bullfrog tadpole manages this. When the tadpole is not breathing, its oral disk is closed, folded into the shape of a half-moon. But as the tadpole speeds close to the surface of the water, it flips the disk open, throwing water away from its beak in the process. As the disk unfolds, little papillae at its corners stick up, further blocking the water from flooding in.

The unfolded disk is now flush with the water surface, and the beak is projecting into the air just above. The tadpole opens its beak, takes a quick gasp of air, slams the beak shut, and turns for the bottom. All this happens in the blink of an eye (not much more than a hundred milliseconds), hardly time for any but the luckiest predator to take advantage of the situation.

Of course, the mouth is also for feeding. To get enough to eat, algae feeders must forage quickly and thoroughly, because algae generally grow in a thin layer. Most algae eaters, including cichlids, catfish, and surfperch, have fine, spatulate teeth (made of the same stuff as ours-­enamel and dentin) for scraping, as well as soft lips for scooping up and holding the particles of algae loosened by the scraping and for feeling along the surface for more algae. Tadpoles, rather than having separate teeth and lips, combine the two--the floppy disk serving as lips, in which are embedded the rows of tiny keratin scrapers. Again using high-speed video, this time of tadpoles feeding on algae-covered glass slides, Wassersug and Yamashita revealed the importance of the multiple rows.

When feeding, the tadpole’s first task is to place its oral disk flat on the algae-covered surface. To do this, the little frog-to-be opens its mouth to an astonishing 180 degrees--the oral equivalent of a gymnastic split and, by a large margin, the widest gape of any vertebrate. The teeth then anchor the mouth to the surface while the beak nips off long pieces of algae. As the beak closes, the rows of teeth perform a scraping operation on any shorter algae it missed. This operation resembles the action of a multiblade shaving razor: As the first row of teeth starts to scrape, it puts tension on the second, which soon pops free of the surface, cutting a little more algae in the process and taking a pass at the swath cut by the first row. As row after row of teeth breaks loose, the algal surface is shaved closer and closer. By the time the last row has done its scraping, the beak is completely closed, leaving the algae trimmings to be sucked in during the next chomp. The tadpole performs six of these all-in-one chomps per second.

Some stream-dwelling tadpoles have more than twenty rows of teeth. The extra rows may hold the tiny creatures in place during feeding or perhaps ensure a closer shave -- a possibility that might interest razor manufacturers, whose offerings have thus far been limited to triple-bladed models.

Adam Summers is an assistant professor of ecology and evolutionary biology at the University of California, Irvine (asummers@uci.edu).