Tadpoles: Not to be confused with fish

StoryImage( ‘/Images/Story//Auto-img-113702404613135.jpg’, ‘Photo by Dr. Robert A. Hedeen’, ‘Tadpoles are not small fish, but rather immature forms of frogs and toads.’);

Last spring, I was sitting on the front porch when two boys with their fishing rods and a can of worms passed by. I asked where they were going, and they replied they were going to catch a mess of fish from the small pond in the Gingerwood Section of Loves Park. I told them I didn’t think they were going to have much luck as the pond is shallow and “freezes out” during the winter, and fish cannot survive there. They replied that I was wrong, as they had seen a lot of little fish along the edges of the pond, and where there were little ones, there had to be big ones.

I started to tell them that what they thought were little fish were actually tadpoles, but I did not want to destroy their optimism. There could be a couple of bullhead catfish in the pond they might hook.

Tadpoles, or pollywogs as they are sometimes called, are actually the larval or immature forms of frogs and toads. Frogs and toads are members of the class Amphibia, and are irreversibly tied to water as their eggs are pervious and subject to drying out. Toads lay their eggs embedded in gelatinous strings, and frogs lay them within a jelly-like mass.

The eggs hatch into tadpoles that are quite unlike an adult frog or toad. At the time of hatching, the tadpole has a distinct body and head, and a laterally compressed tail for swimming. The mouth is on the underside, and is equipped with horny jaws used for scraping algae and other vegetation from objects for food. An adhesive disc can be found behind the mouth for clinging to objects while feeding. Swellings are found on each side of the head, and these later become external gills. These are eventually replaced by internal gills. In the neck region, there is an opening called the spiracle through which water flows after it has been taken in by the mouth and passed over the gills. The gills eventually develop into the lungs of the adult. Hind legs appear first, and then the forelimbs that adhere to the body for some time. As the tadpole develops, the tail is gradually resorbed and utilized as food.

When metamorphosis is complete, the eager tadpole moves into shallow water, passes through some additional changes, and then hops out onto dry land as a full-fledged toad or frog. If something prevents it from getting into shallow water, it will not change into a toad or frog. Experiments have shown that if tadpoles are kept in deep aquaria, they will not go through this transformation and remain juveniles.

Migrations of frogs and toads are correlated with their breeding habits. Males usually return to a pond or stream in advance of the females, which they then attract to the mating site by distinctive calls. These mating calls are specific for each species, and prevent mismatings between closely related forms. Though the calls may all sound alike to the human ear, audiospectographs show decided differences in the calls that are detectable by the hearing mechanism of the female frog or toad.

The common grass or leopard frog usually completes the phase of its life as a tadpole in about three months, while the bull frog may require two to three years to develop into an adult. Many of these common amphibians go to college as they are frequently used as laboratory specimens in freshman biology classes to illustrate the anatomy of a simple vertebrate animal.

Tadpoles are mainly vegetarians, but an exception is made when mosquito larvae are encountered. They frequently will keep a pond free of the pestiferous insects during the aquatic phase of their life.

Today, we hear of the routine cloning of various animals. This is the creation of a new individual from a non-reproductive cell of another, resulting in a clone with the exact genetic complement of one who donated the cell. Most do not know that the first successful cloning was accomplished with a frog in the 1970s by Gurdon of Oxford University. Gurdon used a cell from the intestine of a tadpole to create a carbon copy of the tadpole’s parent. So, the lowly pollywog has some claim to fame.

Dr. Robert Hedeen is a former resident of Maryland’s eastern shore and resided in the Chicago area from 1960 to 1971. He is a retired professor emeritus of biological sciences in the University of Maryland system. He has published more than 30 scientific papers, has written numerous magazine articles, and is the author of two books on the natural history of the Chesapeake Bay.

From the Jan. 11-17, 2006, issue

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