Tadpoles—the Larvae of Frogs and Toads
Tadpoles are the baby version of frogs
Tadpoles are the larval stages of frogs and toads. Amphibians differ from most other vertebrates (higher organisms with a backbone), in that their eggs develop into a larval stage, known as a tadpole. The word comes from the old English words for a toad and a head (poll). Pollywog, another name for a tadpole, also comes from the root 'poll' for head and the old word for wriggle, reflecting the fact that their bodies appear to be completely made up of a head and a tail. Whereas frogs and toads are terrestrial, tadpoles are completely aquatic. They have gills instead of eyes. Their food, and therefore their mouths and digestive systems, is often very different from that of their parents. Tadpoles also have tails, which the adults lack. Frogs are in the order anura, meaning no tail, as opposed to tailed amphibians, like salamanders.
After an initial period of growth, the tadpole must develop into a frog, this process is known as metamorphosis, and involved a huge and rapid remodeling of the tadpole’s body. One of the first steps in metamorphosis is the emergence of limbs. These start off as tiny limb buds which can only be seen under the microscope, in a process very similar to limb development in mammalian embryos, but much later during development, and grow into proper fore- and hind-limbs.
Another major change you can observe during metamorphosis is the disappearance of the tail through apoptosis, programmed cell death. A combination of apoptosis and growth also leads to remodeling of other parts of the tadpole’s body like the head and the gut, resulting in a froglet with very different morphology from the tadpole.
Life cycle of a frog
Most frogs reproduce during the rainy season when ponds are flooded with water. Tadpoles, which often have a very different diet from adults, are a good way of taking advantage of plentiful algae and vegetation in the water, so that the larvae don’t need to hunt for insects when they are very tiny and vulnerable to predators. Eggs are laid by the female and fertilized by the male outside her body. Many species of frog leave their eggs behind, either in water or on vegetation close to the water, and do not take care of the offspring. Eggs are laid en masse in protective jelly. Initially, the embryos absorb their reserves of yolk. Once the embryo has developed into a tadpole, the jelly dissolves and the tadpole wriggles out of its protective membrane
Tadpoles of many species are herbivorous, feeding on plant vegetation. Some tadpoles are practically filter feeders, constantly swallowing water and feeding off the algae. Some frogs have a more specialized diet. Certain poison dart frogs take care of their offspring, once the tadpole is hatched the parent carries it on its back to a flooded bromeliad. The female frog lays unfertilized eggs in the water, which the tadpole eats.
If raising tadpoles native to your region as a science project, make sure you check the laws of your country or state to ensure that collecting them from the wild is legal. Never release non-native species into the wild! When breeding frogs in captivity, it is often necessary to simulate the rainy season by putting the adult frogs in a rain chamber. This stimulates them to breed and once the eggs are laid they need to be taken care off and the tadpoles raised. This can be done in a suitably sized aquarium, the size of which depends on the size and number of tadpoles. Clean water is of the utmost importance when raising tadpoles, it should either be gently filtered or a 50% water change should be carried out daily. If tap water is used it must be dechlorinated with an aquarium water treatment sold for aquarium fish. The temperature of the water will depend on the species. Optimum water hardness also varies depending on the frog. Frogs from Amazon rain forest such as poison dart frogs or certain tree frogs will usually do better in soft water.
As the tadpoles begin to metamorphose it is good to provide them with floating islands, either natural or plastic plants, so they can rest out of the water as they lose their gills and start breathing atmospheric air.
Feeding tadpoles also depends on the species and should be researched for the particular tadpole you are rearing. Boiled lettuce is often recommended for herbivorous species, crushed fish flake, or algae tablets also make good tadpole food.
Direct development: some frogs don't have a tadpole stage
Although the vast majority of frogs and toads go through the tadpole stage, there are exceptions to every rule and there are several families of frogs with direct development from egg to froglet. In some frogs, the eggs are carried by the mother until froglets emerge. One such frog is the very bizarre looking, Suriname toad, Pipa pipa, in which fertilized eggs are rolled onto the frog’s back where they become enclosed in tissue, and develop under the frog’s skin until little froglets emerge at the end of the cycle.