Amphibians
Caecilian
Gymnophiona · also called rubber eel
Most people know that amphibians come in two kinds, the frogs and the salamanders. There is a third. Caecilians are limbless, ringed, mostly blind amphibians that spend their lives burrowing through tropical soil, and they are so rarely seen that even many biologists have never encountered one alive.
Overview
The caecilians make up the amphibian order Gymnophiona, roughly 220 species, found across the wet tropics of the Americas, Africa, and southern Asia. They range from a few centimetres to about 1.5 m, they have no legs, and their bodies are marked with grooved rings that make them look like big earthworms. Most burrow through soil and leaf litter hunting earthworms and termites; a few are fully aquatic.
Built for the dark
A caecilian has tiny eyes, often skin-covered and nearly useless, and a compact bullet-shaped skull for pushing head-first through soil. To find its way it uses smell, touch, and a feature no other animal has — a pair of small retractable tentacles between the eye and the nostril that carry its chemical senses out ahead of the head.
Extreme parental care
About half of caecilian species lay eggs and half give birth to live young. In several egg-laying species the mother grows a thick, fat-rich outer skin that the hatchlings peel and eat with special hooked baby teeth, a behaviour called skin-feeding that provides most of their early nutrition. A 2020 study also reported venom-like glands at the base of caecilian teeth, which would make them the only venom-biting amphibians.
Status
Caecilians as a group are Not Evaluated. Many species are too poorly known to assess, most seem secure in intact tropical soils, and a few narrow-range species are threatened by farming and habitat loss.
Caecilian: common questions
Is a caecilian a snake, a worm, or an amphibian?
It is an amphibian, and not closely related to snakes or worms at all. Caecilians make up one of the three living orders of amphibians, the other two being the frogs and toads and the salamanders and newts. They have lost their legs over evolutionary time and taken on a long, ringed, limbless body that superficially resembles a large earthworm or a small snake, which is a case of different animals converging on the same shape for burrowing. Inside, a caecilian is unmistakably an amphibian, with moist glandular skin, a larval stage or direct development, and the same basic body chemistry as a frog. Most people have never seen one because they live hidden in tropical soil.
How does a blind animal find food underground?
Caecilians have very small eyes, often covered by skin or even bone, and in the darkness of the soil good vision would be useless anyway. Instead they rely on smell, on touch through the whole body, and on a feature unique to the group, a pair of small tentacles that can be pushed out from a groove between the eye and the nostril on each side. These tentacles carry the animal's chemical senses out ahead of the head, effectively letting it taste and smell the soil it is about to push into, so it can track an earthworm or a termite trail by scent while moving forward blind.
Do baby caecilians really eat their mother's skin?
In some species, yes, and it is one of the strangest known forms of parental care. The mother of certain egg-laying caecilians develops an unusually thick outer skin layer, packed with fat and nutrients, in the weeks before her eggs hatch. The newly hatched young, which are equipped with rows of specialised hooked and spoon-shaped baby teeth, then repeatedly peel and consume the outer layer of her skin, which she regrows every few days. Over the first weeks of life the young get most of their nourishment this way, more than doubling in length, before they leave to feed independently. Live-bearing species have an internal equivalent, with the fetus scraping nutritious tissue from the lining of the oviduct.
Are caecilians dangerous to people?
No. Caecilians are shy, secretive burrowers that pose no threat to humans. They have no venomous fangs of the kind that could injure a person, they do not constrict, and they will try to escape rather than confront anything large. Some produce distasteful or mildly toxic skin secretions as a defence against predators, so it is sensible to wash your hands after handling one, and a large caecilian can give a firm nip with its teeth if grabbed. The 2020 report of venom-like glands at the tooth bases concerns tiny quantities used on soil prey, not anything relevant to humans. For people, the main significance of caecilians is as soil predators that help control pest invertebrates.