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A tadpole or polliwog (also spelled pollywog) is the Larva in the biological life cycle of an amphibian. Most tadpoles are fully Aquatic animal, though some species of amphibians have tadpoles that are terrestrial. Tadpoles have some fish-like features that may not be found in adult amphibians, such as a lateral line, and swimming . As they undergo metamorphosis, they start to develop functional for breathing air, and the diet of tadpoles changes drastically.
A few amphibians, such as some members of the frog family Brevicipitidae, undergo direct development i.e., they do not undergo a free-living larval stage as tadpoles instead emerging from eggs as fully formed "froglet" miniatures of the adult morphology. Some other species hatch into tadpoles underneath the skin of the female adult or are kept in a pouch until after metamorphosis. Having no hard skeletons, it might be expected that tadpole would not exist. However, traces of have been preserved and fossil tadpoles have been found dating back to the Middle Jurassic.
Tadpoles are eaten as human food in some parts of the world and are mentioned in various Folklore from around the world.
As a frog tadpole matures it gradually develops its limbs, with the back legs growing first and the front legs second. The tail is absorbed into the body using apoptosis. Lungs develop around the time as the legs start growing, and tadpoles at this stage will often swim to the surface and gulp air. During the final stages of metamorphosis, the tadpole's mouth changes from a small, enclosed mouth at the front of the head to a large mouth the same width as the head. The intestines shorten as they transition from a herbivorous diet to the carnivorous diet of adult frogs.
Tadpoles vary greatly in size, both during their development and between species. For example, in a single family, Megophryidae, length of late-stage tadpoles varies between and . The tadpoles of the paradoxical frog ( Pseudis paradoxa) can reach up to , the longest of any frog, before shrinking to a mere snout-to-vent length of .
While most anuran tadpoles inhabit , , , and other small bodies of water with slow moving water, a few species are adapted to different environments. Some frogs have terrestrial tadpoles, such as the family Ranixalidae, whose tadpoles are found in wet crevices near streams. The tadpoles of Micrixalus herrei are adapted to a fossorial lifestyle, with a muscular body and tail, eyes covered by a layer of skin, and reduced pigment. Several frogs have stream dwelling tadpoles equipped with a strong oral sucker that allows them to hold onto rocks in fast flowing water, two examples being the Indian purple frog ( Purple frog) and the tailed frogs (Tailed frog) of Western North America. Although there are no marine tadpoles, the tadpoles of the crab-eating frog can cope with brackish water.
Some anurans will provide parental care towards their tadpoles. Frogs of the genus Afrixalus will lay their eggs on leaves above water, folding the leaves around the eggs for protection. Female Pipa frogs will embed the eggs into their backs where they get covered by a thin layer of skin. The eggs will hatch underneath her skin and grow, eventually leaving as either large tadpoles (such as in Pipa parva) or as fully formed froglets ( Pipa pipa). Female marsupial frogs (Hemiphractidae) will carry eggs on her back for various amounts of time, with it going as far as letting the tadpoles develop into tiny froglets in a pouch. Male African bullfrogs ( African bullfrog) will keep watch over their tadpoles, attacking anything that might be a potential threat, even though he may eat some of the tadpoles himself.
Males of the Emei mustache toads ( Leptobrachium boringii) will construct nests along riverbanks where they breed with females and keep watch over the eggs, losing as much as 7.3% of their body mass in the time they spend protecting the nest. Male midwife toads ( Midwife toad) will carry eggs between their legs to protect them from predators, eventually releasing them into a body of water when they are ready to hatch. Poison dart frogs (Dendrobatidae) will carry their tadpoles to various locations, usually phytotelma, where they remain until metamorphosis. Some female dart frogs such as the strawberry poison dart frog ( Oophaga pumilio) will regularly lay unfertilized eggs for the developing tadpoles to feed on.
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