Tadpole Science Gets Its Legs . . .
. . . and reveals complex lives for the swimming squiggles
The wonder of tadpoles morphing into frogs or toads has always provided preschoolers with an enticing entrée into the world of science. But the little critters aren’t just for kids. An ever-widening pool of adult scientists is turning to tadpoles.
Biologists interested in development have focused on the tadpole-frog transition. Yet a profitable focus of research, little attention went to the physiology, behavior, ecology, and natural history of the tadpole until the past few decades.
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Tadpoles just didn’t do enough–many scientists held–to be interesting, recalls ecologist Rick Relyea. Now, however, he expresses enthusiasm about the emerging view that as remarkably sensitive organisms, tadpoles do plenty.
At the University of Pittsburgh, Relyea watches tadpoles grow into different body shapes depending on what predator lurks nearby. He sees them react differently if the predator has been eating insects or, horrors, other tadpoles. He sees them distinguish between predators and competitors. “They’re amazingly complex animals,” he insists.
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Tadpoles are also handy, abundant, and easy to tend. The more favorable view of them arose in the early 1970s, when, researchers began to construct miniature aquatic ecosystems in tubs. These biologists used tadpoles as an appropriate minuscule inhabitant for studies that teased apart the workings of multifaceted ecological communities. Those observations led to a greater appreciation of the intricacies of tadpole biology. Some biologists deadpan that if their field keeps up its current growth rate for another 15 years, every new scientific paper in the world will be about the once-uninspiring tadpoles.
In about three-quarters of the world’s 4,400 frog and toad species, the young spend weeks in a fishy form that bears little resemblance to their adult shape. That disparity raises a practical question for field biologists. “I go to a stream, and I catch five tadpoles and five frogs–how do I tell which goes with which?” says Joe Mendelson of Utah State University in Logan.
Researchers have matched all of North America’s frog larvae with adults, he says, but the amphibian abundance of the rest of the world poses plenty of challenges. Where he works, in the New World tropics, no tadpole’s been described for hundreds of species. African amphibians raise even more questions. Worldwide, scientists can identify tadpoles for only about a third of the species that almost certainly have them, according to the tome Tadpoles (1999, Roy McDiarmid and Ronald Altig, University of Chicago Press).
Tadpoles come in spectacular variety, contends Tadpoles coeditor Altig, who’s at Mississippi State University in the town of Mississippi State. He’s seen abundant examples: stubby, sleek, fluttering through the water, flicking along on land, or wriggling in tree branches.
What Altig speaks most passionately about, however, is the variety of tadpole mouthparts. Taxonomists have long used the eating equipment of tadpoles for classifying the creatures, but Altig maintains that scientists still don’t know much about how the tadpoles use it.
A tadpole grows its teeth outside its mouth, where they can scrape nutritious particles from surfaces or break up a chunk of tasty detritus. Tadpoles consume a wide variety of snacks, including bacteria, fungi, algae, and leaf fragments. The teeth sprout above and below the mouth in curved rows, sometimes just a few rows, sometimes more than 20. “It’s as if you had hundreds of teeth on your upper lip and chin,” Altig says.
The jaws work in an unusual style, too. The lower one has an extra hinge, so the tadpole can open its mouth to a superwide gape.
For a typical slurper of whatever’s in the tadpole’s pond, the floor of the mouth cavity moves up and down, pumping an appetizing slurry over an amphibian version of a lint remover. The trap–often a pair of continuously secreted, gummy cords–picks up particles before the water whooshes over the gills and out through an opening on the side. The tadpole swallows the cords along with whatever nourishing gunk they’ve picked up. Champion particle trappers can strip, in one pass, more than three-quarters of the bacteria they encounter, says Altig.
The young animals’ prowess in using their myriad forms of mouthparts to glean particles from water presents serious challenges to the scientists who observe them. Altig hopes that the next generation of tadpole researchers will link the abundant variety of mouth structures to particular feeding styles and ecological niches.
Although tadpoles have long been viewed as simple creatures, they may have complex relationships with each other. Altig says. “It’s so much more subtle than a bird sitting up in a tree screaming his head off.”
Tadpoles certainly gather in clusters, some so dense that they make a crackling sound as individuals pop through the water’s surface. One species associates in a dense mass that looks like a football rolling through the water. Are the tadpoles seeking each other’s company, or do they just happen to show up in the same place at the same time?
A series of experimenters in the 1970s and 1980s approached this question by penning tadpoles in compartments where they could see or chemically sense another of their own kind on the other side of a divider. Response varied by species, but for some, such as the western toad and the African clawed frog, individuals spent more time near the divider, and their neighbor’s compartment, than in the other sectors of their domain.
Some species of tadpoles can discriminate between kin and non-kin. Perhaps the most dramatic example comes from studies of cannibalistic tadpoles by David Pfennig at the University of North Carolina in Chapel Hill. He and his colleagues have studied species of Spea spadefoot toads in the United States.
Some eggs of this species grow into medium-size, round-headed larvae that simply suck food particles out of water, as many other tadpoles do. Other larvae grow explosively, however, and develop big heads with gaping jaws. The oversize tadpoles attack and eat the mild-mannered ones. Pfennig’s research team found, however, that cannibals generally don’t eat siblings if alternative victims of different lineages are available.
Tadpoles of some dart-poison frogs eat primarily eggs that might otherwise have been siblings. Providing those eggs within easy reach, in fact, is their mother’s way of caring. Females of one Central American dart-poison frog, Dendrobates pumilio, lay their eggs on dry, flat leaves. When they hatch, the mother frog carries the tadpoles one by one on her back to drop each into a water-filled crevice at the base of a plant leaf. She returns to the crevice at intervals of 1 to 9 days, and her approach sends the resident tadpole into a vibrating frenzy. Instead of rippling around its tiny pool as it normally does, the tadpole stiffens and shakes its tale, which roils the water. Mom backs into the pool, and the tadpole bumps against her as she releases an egg. The tadpole gulps it down.
Among these frogs, the feed eggs aren’t fertilized, but tadpoles of another Central American dart-poison frog, Osteocephaus oophagus, do eat fertilized eggs. These tadpoles grow up in water that pools in the straplike leaves of tree-clinging bromeliads. Every 5 days or so, a female carrying a male piggyback returns to a particular bromeliad. She lays an egg, which the male fertilizes. If one of her tadpoles is still in residence at the plant, the lucky youngster gets egg for lunch. If the previous tenant has perished, the egg can develop into a tadpole that will feed on eggs that the mom delivers later.
For tadpole cannibals, it’s not always older ones eating the little brothers and sisters. Tadpoles lose some of their speed as they near their transformation into adults. At least in laboratory aquaria, hungry tropical Osteopilus septentrionalis youngsters sometimes nail older tadpoles that are in the throes of metamorphosis.
New studies of tadpoles are uncovering tales of family life that go beyond eating the sibs. Tadpoles interact with their dads, as well as their moms. For example, some male dart-poison frogs transport their tadpoles to bromeliad-leaf pools.
Among giant bullfrogs, males rescue their tadpoles from puddles that are drying out. In the March 2001 Journal of Herpetology, Willem Ferguson of the University of Pretoria in South Africa describes male bullfrogs plowing channels through the mud. The conduits connect substantial puddles to failing ones containing tadpoles. When water flows into the shallow puddles, the temperature can fall, making it more healthful for the tadpoles. Sometimes, the frog sloshes from the drying puddle back to the deeper water and the tadpoles follow in his wake.
Responding to danger
One of the most exciting findings in tadpole science of recent years has come from the discovery of the way that tadpoles respond to danger. Tadpoles are pretty much the universal snack food of aquatic life, and they tune in to the menaces in the neighborhood.
To test the range of the animals’ reactions, Relyea raised six species of tadpoles in tubs equipped with a mesh cage holding a predator, such as a dragonfly larva or giant water bug. The tadpoles knew when to worry. Other experiments had shown that the presence of a predator, especially if it had been eating other tadpoles, can practically immobilize them or alter their body shape.
Relyea and his colleagues reported in the February 2001 Ecology that tadpoles adjusted their behavior and body shapes appropriately given the deadliness of the predator. For example, tadpoles of the wood frog Rana sylvatica grew extra-deep tail fins when sharing a tub with a threatening mud minnow or a dragonfly larva, a somewhat deep tail fin when raised with a less dangerous newt, but nothing special when exposed to a more innocuous water beetle.
To do these studies, Relyea’s team had to determine how much of a threat each kind of predator posed for the tadpoles. In so doing, the group upset an old truism of ecology. The conventional wisdom predicts that any defense mechanism will get stronger as predators get more dangerous. “That makes sense until you think about it carefully,” Relyea says.
The real world proves more complicated than that. Each animal balances several defenses in the wild, he notes. For instance, in a dangerous environment, a tadpole has several options: It can avoid attention by staying still most of the time, or it can grow a deeper tail to make quick escapes. One defense, therefore, may not show a straightforward rise with increased risk. In his experiments, Relyea finds that different species make different trade-offs.
Predators aren’t the only problem that a tadpole faces; a crowd of other tadpoles often competes for the same food. Relyea finds evidence that tadpoles act differently when their main problem is predators or competitors.
In general, a lurking predator makes a tadpole grow high-speed tails and stay comparatively inactive. When tadpoles grow up away from predators but in a crowd, the youngsters get lively and grow big heads with proportionately big mouths–adaptations that allow them to get to and take in more food.
Variation in tadpole life can carry over to adulthood, Relyea has found. He raised tadpoles of wood frogs in water with caged dragonfly larvae, which are tadpole predators. As expected, the tadpoles developed the shape they typically have in predator-infested environments and took longer to mature than tadpoles in safe water did. When the tadpoles did metamorphose, those in water with dragonfly larvae grew hind limbs that were slightly but significantly longer and wider than frogs’ from uninfested water.
In work examining the chemistry of predation cues, Relyea happened onto a dramatic side effect of predator risk. He and his colleagues raised gray tree frog tadpoles Hyla versicolor in water laced with a low concentration of the pesticide carbaryl, which is commercially sold as Sevin. Adding a caged predator, a salamander larva, to the water rendered the pesticide two to four times as lethal as it was alone, Relyea and Nathan Mills of the University of
Missouri in Columbia reported in the Feb. 27, 2001 Proceedings of the National Academy of Sciences.
Since then, Relyea has exposed more frog and toad larvae to the double menace of pesticide and predator. His preliminary results, he says, indicate that some tadpoles respond even more dramatically to the combination than the gray tree frog tadpoles do.
In this era of declining amphibian populations, Relyea and Mills were the first biologists to test for interactions between pesticide pollution and other environmental stresses. In the world outside laboratories, predators abound, and
Relyea argues that their deadly boost to pesticide lethality could be widespread.
Scientists don’t know just what cue a tadpole picks up to determine that a predator’s nearby. Nor have researchers worked out the chemistry of how that stress might increase the tadpole’s vulnerability to a toxic chemical.
Relyea is now starting to work with chemists to search for active compounds. He says, not for the first time, “This is an exciting time to study tadpoles.”
When tadpoles veer to the left
A few years ago, Richard Wassersug of Dalhousie University in Halifax, Nova Scotia, noticed that tadpoles surfacing to gulp air tended to turn left instead of right when they dove back underwater. The split was 60:40.
Perhaps, the researchers speculated, the asymmetric turning of this North American tadpole of the genus Rana relates to the water outlet, or spiracle, on the left side of its body. The idea gained ground when the researchers tested Xenopus tadpoles. They expel water through a pair of spiracles, one on each side of their bodies, and in the experiment, they showed no lefty tendencies.
In 2000, however, Wassersug and his colleagues reported a turning bias in another species that’s physically symmetrical. Microhyla ornata has just one spiracle, but it’s located on the tadpole’s midline. Immediately after hatching, these tadpoles show no preference in turning. During the middle 2 to 3 weeks of their larval period, however, they turn left roughly 75 percent of the time when they’re disrupted. As they mature, that bias disappears.
Now, the researchers speculate that they’re on to something more complicated than a simple quirk of spiracle position. Perhaps the animals’ brain dictates handedness (SN:12/15/01, p. 375: Crows appear to make tools right-handedly) and does so in a manner that varies with age.–S.M.