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."
Copyright
National Geographic
Reprinted from http://news.nationalgeographic.com/news/2002/11/1107_021107_tadpoles.html
