Tiny African Tadpole a Big Sucker to Its Prey

John Roach
for National Geographic News
November 7, 2002
Scientists have discovered that the tiny tadpoles of a species of African frog have an unusual feeding method: They suck in their prey, just as fish do.

Suction feeding, which biologists from the University of California at Berkeley observed during high-speed video filming of African dwarf clawed frog (Hymenochirus boettgeri) tadpoles, is extremely rare among frogs.

Most tadpoles are suspension feeders, filtering out tiny particles while continuously pumping water. As adults, most aquatic frogs use their hands to scoop up their prey or capture it in their large mouths.

Tadpoles of the African dwarf clawed frog (Hymenochirus boettgeri), however, visually track their prey, chase it, and then suck it down through an extendable tube-like mouth, said Stephen Deban, co-author of a report on the discovery published in the November 7 issue of the journal Nature. Deban is now a postdoctoral researcher the University of Utah in Salt Lake City.

He likens the Africa frog's behavior to a human sucking down soda with a straw.

"The tadpole basically lowers the floor of its mouth very quickly, while raising the head and extending the mouth tube," he said. "These movements increase the space inside the mouth and water flows inward as a result."

Once the prey is inside the tadpole's mouth, the water is slowly squeezed out through gill slits at the rear of the mouth cavity.

Wendy Olson, a co-author of the study who is currently a postdoctoral researcher at Dalhousie University in Canada, said that when she and Deban show the tadpole-feeding video to other scientists, "most of them assume right away that they are looking at a fish."

The African dwarf clawed frog is native to rain forests of central and western Africa and is a common pet in household aquariums.

Suction Advantage

Tadpoles of the African dwarf clawed frog start feeding when their bodies are less than 0.04 inches (one millimeter) long, making them one of the smallest free-swimming vertebrates in the world when they start eating.

The tadpoles grow continuously and their bodies reach about 0.4 inches (one centimeter) before they transform into frogs.

Water for animals of such a small size, including larval fish, brine shrimp, and water fleas, is essentially like syrup. They cannot achieve the speeds necessary to overcome the viscosity of water, explained Deban.

The filter feeding mechanism of most tadpoles may not be efficient for animals of this size. It is like trying to put syrup through a coffee filter, he said.

"The strategy that larval fish and Hymenochirus take is not to filter the water, but to bite off or suck up a piece of the water with the food in it and then squeeze out the water forcefully," he said.

Scientists estimate the relative viscosity, or syrupiness, of fluids for any given aquatic animal with an index of fluid dynamics known as the Reynolds number. The lower the Reynolds number, the more syrupy the water and thus the harder it is to move around.

Larger animals can get more water as they move faster, so their momentum results in a greater force than the syrupiness of the water, resulting in a higher Reynolds number.

Tadpoles of the African dwarf clawed frog overcome the syrupiness of the water with brute force. They are able to move quickly and therefore operate in a slightly less syrupy world, said Deban.

Olson and Deban estimated that the African frog tagpoles have a Reynolds number of 300 as they capture prey, compared with 5 to 70 for comparably sized larval fish.

Fast Eater

The higher Reynolds number for the tadpole indicates that it is "faster and better at overcoming the viscous drag that typically confronts small aquatic organisms," the biologists report in Nature.

"It might have larger feeding muscles or a proportionately larger mouth opening, either of which might help," said Deban.

The tadpole can suck down its prey in just seven milliseconds. A comparably sized larval fish takes up to 12 milliseconds to engulf its prey, the researchers note in their paper.

For comparison, a blink of the human eye takes more than 100 milliseconds. A bee flaps it wing once every 10 milliseconds. Deban cautioned, however, that the tadpole's movements occur over a very short distance, and therefore are not all that impressive from a human perspective.

"The velocity of the prey entering the mouth is 0.6 meters per second, which is about 1.35 miles per hour," he said. "Nothing to write home about unless you're a few millimeters long."

Olson and Deban came to this research out of an interest in how animals move and how their biomechanics evolve. They say it is interesting that the suction-feeding mechanism evolved independently in the frogs and larval bony fish.

It makes a strong statement about the importance of the environment for the evolution of an organism, said Olson.

"Animals do not live in a vacuum, and they do not evolve in a vacuum," she said. "There is so much focus on genes these days, but the morphology is what makes it exciting."

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