The story of tadpoles

As a rule, one cannot make many generalizations about most aspects of reproduction in amphibians; their reproductive habits simply are too diverse across the group. Their reproductive diversity perhaps is rivaled only by plants, fishes, and many groups of invertebrates. Some of their strategies are just bizarre, such as the Surinam “toad” (Pipa pipa) of South America in which the male and female conduct a cooperative underwater dance such that their fertilized eggs wind up adhered to her back. Then the flesh on her back grows into a swollen matrix that sequesters each egg in a little fleshy cupola reminiscent of a bee’s honeycomb. It is unsightly and, honestly, it looks like the frog has a terrible inflamed skin disease. The eggs develop and eventually tiny little froglets emerge from her flesh and swim away. Yes—I said froglet, not tadpole. There is no tadpole in this species, as the embryos develop straight into froglets, entirely skipping the familiar tadpole stage. This phenomenon in some amphibians is known as direct development and it is very common.

Amphibians perhaps are best known for their two-stage lifestyle that includes a distinct free-living aquatic larval stage. Most schoolchildren are taught this cycle, typically using a frog as an example, early in their education and they know that tadpoles turn into frogs. Yet, you have to admit that tadpoles bear absolutely no resemblance to adult-form frogs and so the connection between the two is not obvious. It is impossible to determine which species a tadpole represents unless somebody has done a lot of work to associate a distinctive tadpole anatomy with the adult form of a particular frog.  That can be done with molecular-DNA matches, or by raising tadpoles through metamorphosis to see what they turn into! I just finished writing an identification key to all the known tadpoles of Mexico.  Whew, that was a lot of work! It also made me realize that the majority of amphibians in Mexico do not actually have a larval stage.

When we learn about tadpoles in school, we are led to assume that all frogs have that stage. If you went to school in places like Mexico or Ecuador, you might learn that “Some frogs have this weird free-swimming larval stage that in no way resembles a frog, but most just  lay eggs that hatch into miniatures of themselves like everything else.” I have personally been schooled by well-intentioned local persons that tadpoles are, in fact, fish because everybody knows that frogs have well developed legs and hop around on land or vegetation. That is a perfectly reasonable perspective if one did not learn about tadpoles in school and does not have the time to wait by a swamp to see what becomes of all these weird looking little globular, tailed fishes. It was German naturalist August Johann Rösel von Rosenhof in his influential work Historia Naturalis Ranarum Nostratium (1753–1758) who first illustrated and described the non-intuitive life history of frogs for modern science. Nevertheless, a very large number of frogs, salamanders, and caecilians undergo direct development in some form and bypass the characteristic larval phase.

Few people realize that most salamanders in the U.S. do not have a larval stage; they are direct developers. This identifies the often-taught fallacy that amphibians must return to water to breed.  Nope, many amphibians never willingly enter water in their entire life. They just seek humid microhabitats under rocks and logs and such, or live in humid environments like cloudforests. While metamorphosis in frogs requires a drastic reorganization of most aspects of the anatomy and physiology of the individual organism, the metamorphic transition in salamanders is not quite so drastic. Aquatic salamander larvae look like a typical salamander, but with big bushy external gills that disappear when they undergo metamorphosis.

The more I learn about amphibians in my career, the more I realize that general statements about what amphibians do and cannot do are impossible. It seems there’s always some oddball species out there that came up with some weird way to make their babies!  One scholar categorized 39 fundamentally different reproductive strategies in frogs alone. That would be a very long blog-post to write. In fact, it is turning into a very long chapter in the new Herpetology textbook I am writing.

On your next visit to the Zoo, check the exhibit for the Panamanian golden frogs to see if there are tiny gold-flecked tadpoles gripping the rocks with their mouthparts (tadpoles don’t have jaws), or check the backs on any of the poison frogs to see if a parent is carrying around a tiny cluster of tadpoles there, hopping around looking for water to feed in. The parent, male of female, depending on the species, will remember where they were placed and return later after they have had time to feed, signal them that it’s time to move on then they crawl of the water on the back and get moved to the next pasture so to speak. They literally “drop the kids off at the pool”, and return a few hours later to pick them up!  I sure to enjoy studying amphibians.

Thanks for reading!

Joe Mendelson, PhD
Director of Research