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.
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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