Amphibians
Rubber Eel
Typhlonectes natans · also called rubber eel, black caecilian, aquatic caecilian, Sicilian worm eel
The rubber eel that turns up in aquarium shops is not an eel at all. It is a caecilian — a limbless, nearly blind, air-breathing amphibian from the swamps of Colombia and Venezuela, and the females grow their babies by letting them eat the lining of the womb.
Overview
Rubber eels live in slow rivers, swamps, and marshes of the Magdalena, Cauca, and Maracaibo basins in northern Colombia and Venezuela. They are 40–55 cm, smooth, dark, and ringed with faint grooves, with a small fin along the back. They are caecilians — the third, least-known amphibian group after frogs and salamanders — and hunt worms, larvae, shrimp, and small fish along the bottom.
Built for the dark
The eyes are tiny and skin-covered, good only for telling light from dark. The rubber eel finds prey by smell and touch and with a pair of small retractable tentacles on the head that work as a second nose. It has lungs and must surface every 20–30 minutes to breathe; the right lung is large, the left almost absent.
Feeding on the mother
The female carries young in her oviducts for 6–7 months. The fetuses scrape a rich, milky secretion and cells off the oviduct lining with spoon-shaped baby teeth, growing on maternal tissue until they are born as miniature adults. Land-living caecilians do a version of this too — the young peel and eat the mother’s outer skin.
Status
The rubber eel is Least Concern, common in slow fresh water, and traded modestly for aquariums — where it is often bought by mistake and does poorly without a proper species setup.
Rubber Eel: common questions
Is a rubber eel a real eel, a snake, or a worm?
None of those. The rubber eel is an amphibian — specifically a caecilian, the smallest and most obscure of the three living amphibian orders (the others are the frogs and the salamanders). Caecilians are limbless, which is why they are so often mistaken for eels, snakes, or large earthworms. You can tell it is an amphibian rather than a fish because it has no true fins or scales, has smooth moist skin, breathes air with lungs, and has a distinct amphibian skull and jaw. Unlike a snake it has no scales, no venom, and a ring of grooves around the body, and unlike a worm it has a backbone, a mouth full of small sharp teeth, and eyes, however reduced.
Can the rubber eel see?
Barely. Like most caecilians it has tiny eyes that are covered by skin and, in this species, sometimes by bone, so at best it can distinguish light from dark. This is fine for an animal that evolved from burrowing ancestors and lives in murky, slow-moving, often turbid tropical water. It navigates and hunts instead with a strong sense of smell, touch-sensitive skin, and a pair of unique retractable tentacles on the sides of the head, which pick up chemical cues from prey and the environment and effectively serve as a second nose. Some caecilians can also sense vibration through the ground or water.
Do rubber eels really feed their babies with their own skin?
The related terrestrial caecilians do exactly that, and it is one of the strangest behaviours in the animal kingdom — the mother grows a thick, fatty, nutrient-rich outer skin layer, and the hatchlings peel and eat it off her body using specialised temporary teeth, then wait for it to regrow and repeat, over weeks. The rubber eel does an internal version. It gives live birth, and while the young are developing inside the mother's oviducts, they use their own set of spoon-shaped baby teeth to scrape off a rich secretion and cells from the swollen, glandular oviduct lining, effectively "eating" a maternal tissue to grow, until they are born fully formed. Either way, caecilian mothers feed their offspring with their own bodies.
Are rubber eels good aquarium pets?
They are sold for freshwater aquariums but are a poor choice for most keepers and are frequently bought by mistake by people who think they are getting a small eel or a novelty fish. They are almost blind, so they cannot compete for food with fast tankmates and are easily outcompeted or nipped; they are escape artists that will leave any tank without a tightly sealed lid and dry out; they need clean, warm, well-filtered water and a soft substrate or hiding tubes; and they must be able to reach the surface to breathe. They also do best in groups. Many die within months in unsuitable setups. An experienced keeper with a species tank can keep them well, and they have even bred in captivity.