Amphibians
Tadpole
Anura · also called polliwog, pollywog
A tadpole is a frog that has not built its adult body yet. It hatches as a swimming, plant-grazing, gilled animal with a tail and no legs, spends weeks to years feeding and growing, and then tears the whole design down and reassembles it — legs out, tail in, gut rewired, gills traded for lungs — to walk out onto land.
Overview
Tadpole is the larval stage of an anuran, a frog or toad, so there are roughly as many kinds as there are of the 7,000-odd frog species. A typical tadpole has a rounded head-and-body, a finned swimming tail, internal gills, a long coiled gut for digesting plants, and a sucker-like mouth ringed with tiny horny teeth for scraping algae. They live in fresh water on every continent except Antarctica.
The rebuild
Metamorphosis is driven by thyroid hormone and is one of the most drastic transformations in the animal kingdom. Hind legs appear, then front legs; the tail is dismantled from within by programmed cell death and its materials reused; the gut shortens from herbivore to carnivore length; lungs replace gills; the skull and jaw are rebuilt; and the eyes shift up and forward for vision in air. The animal often stops eating at the peak of the change, living on its own tail.
Racing the pond
Development speed is set by the water. Warmth, food, crowding, and especially drying all adjust the timetable, and tadpoles of temporary pools can accelerate sharply to escape a shrinking puddle. Some species do away with the pond stage entirely, hatching as froglets from land eggs, or brood their tadpoles in the mouth, skin, or back.
A pillar of the pond
Tadpoles often reach huge densities, where their grazing keeps algae in check and their bodies pass energy from plants up to fish, birds, snakes, beetles, and dragonfly larvae. The amphibian chytrid fungus attacks tadpole mouthparts directly, and the larval stage is highly sensitive to silt, pesticides, and fertiliser runoff.
Status
A tadpole is not a species and is not assessed on its own, but it shares the outlook of frogs, of which more than 40 per cent are threatened, and the decreasing trend that goes with that.
Tadpole: common questions
How does a tadpole turn into a frog?
Through metamorphosis, a coordinated rebuild driven by thyroid hormone. Over days to weeks the tadpole grows hind legs, then front legs, while its tail is gradually broken down from within and the materials reused. Internally the changes are even bigger, as the long spiral gut of a plant grazer shortens into a carnivore's gut, gills are replaced by lungs, the eardrum and the adult jaw and skull form, the eyes migrate to the top and front of the head for vision above water, and the skin chemistry changes. The animal often stops feeding during the peak of the change, living on its tail, and emerges as a small froglet that may still have a tail stub.
What do tadpoles eat?
Most tadpoles are vegetarians and generalist feeders. They rasp algae, bacterial films, and decaying plant matter off rocks and leaves using rows of small keratin teeth arranged around a sucker-like mouth, and many also filter tiny particles out of the water with their gill baskets. As they grow, a lot of species add animal matter, scavenging dead insects and even dead tadpoles. Some are specialised predators from the start, such as spadefoot toad tadpoles, which in crowded drying pools develop big jaws and hunt and eat their smaller pond-mates, a fast track to metamorphosis before the water disappears.
How long does the tadpole stage last?
It varies enormously with species and conditions. Tadpoles of frogs that breed in temporary desert or woodland pools can complete the whole stage in under two weeks, racing the drying mud. Typical pond frogs and toads take one to three months over a spring and summer. At the other extreme, bullfrog and green frog tadpoles in cool climates commonly spend one to three years in the water, overwintering as tadpoles and reaching a large size before transforming. Warmth, food, how crowded the pool is, and whether it is shrinking all push the timetable faster or slower.
Do all frogs have tadpoles?
No. A free-swimming tadpole is the ancestral pattern and is still the commonest, but a large minority of frogs have done away with it. Many tropical frogs, including most rain frogs and many leaf-litter species, lay a few large yolk-rich eggs on damp ground or on leaves, and these hatch directly into miniature froglets with no aquatic stage at all. Others keep the tadpole but move it out of open water, brooding the larvae in the mouth, in skin pouches, on the back, or, in the now extinct gastric-brooding frogs, in the stomach. These strategies trade fewer young for better protection from pond predators and drying.