Tadpoles, also known as pollywogs and larvae, are the free larval phase of a frog’s life cycle. What does that mean, “free larval phase”? It means the developing frog can swim around freely without being contained by an egg or a parent’s womb while it develops into an adult frog. This is because frogs are oviparous. Tadpoles develop from fertilized eggs which are left either in water or a foamy froth, left around the egg mass, to provide a necessary home during the larval phase. Most frog species lay their eggs in large masses suspended together by a jelly that provides the nutritional requirements for the egg to develop, much in the same way egg whites from the eggs of birds provide the nutrients for a developing chick. Frog eggs are laid outside of the frog’s body but they don’t have a hard protective shell. So, if you were to look at a developing egg you could see the transparent yolk and the black embryo it surrounds. In fact you would be able to watch the entire development process from egg to tadpole.
Depending on which species of frog it is, the tadpole stage can last from days to years. Some species will even develop from egg to young froglet without ever leaving the egg membrane during that stage of development. Once the egg becomes a tadpole it is in the second phase of development. Tadpoles are fish-looking larvae. They have a pair of gills on each side of their body that they use to breathe in oxygen. They don’t have lungs yet. Their mouths are small and designed for their vegetarian diet of algae and pond scum common in their aquatic home environment. Useful, this abundance of food, because they require a lot of energy for development. They also have a long gut to help them digest the planty meals. On the outside they have a tail and fins that allow them to move around getting food and avoiding predators. The eyes face forward allowing them to see where they are going as they move around their aquatic environment.
Then suddenly the hind limbs start to form. When this happens it signals metamorphosis, the change from larvae to adult frog, has begun. During metamorphosis hormones in the body regulate the change and the tadpole will fast since its digestive system is also transforming. The gills will be lost as lungs develop, but the developing frog will still be able to breathe through its skin. The forelimbs will start to grow sometime after the hind limbs. This makes sense since the hind limbs of a frog need to be well developed to provide the power behind jumping or swimming. As the mouth turns into a jaw bone and the tongue grows, the long gut of the vegetarian digestive system will shrink and the stomach and digestive system will develop to support the adult’s carnivorous diet. Eventually the tail will start disappearing. When the tail is almost gone metamorphosis is almost complete and the once tadpole is now a young froglet.
Works Cited:
Behler, J. L., & Behler, D. A. (2005). Frogs: A chorus of colors. New York, NY: Sterling Publishing Co., Inc.
Beltz, E. (2005). Frogs: inside their remarkable world. Buffalo, NY: Firefly Books Ltd.
Forsyth, A. (2008). Nature of the rainforest: Costa Rica and beyond. Ithaca, New York: Cornell University Press.
Mongabay.com
Rhett A. Butler - San Francisco, CA.
1999-2008.
Mongabay.com
Rhett A. Butler - San Francisco, CA.
1999-2008.