What is a caecilian? The ringed caecilian, explained

Picture an amphibian with no legs, a snake-shaped body marked by pale grooves, and a life spent in soil. Now add this: after her eggs hatch, the mother feeds her young a fluid that researchers call milk. That is the ringed caecilian (Siphonops annulatus), a South American animal that most people have never heard of. There is a reason for that. Scientists from the Natural History Museum and the Zoological Society of London say caecilians are little studied because most species live buried in soil.

Caecilians are the third big group of amphibians, next to frogs and salamanders. They are also the group where our knowledge is thinnest, and the ringed caecilian is no exception. A few strong studies exist, including a 2024 one whose authors include Carlos Jared of Brazil’s Instituto Butantan, and they are remarkable. Beyond them, a lot of basic questions have no published answer in the sources we read.

This article explains what a caecilian is, introduces the ringed caecilian, and follows its two best-known stories: a mother’s skin and milk, and glands at the base of its teeth that may or may not be venom. Where the sources stop, we say so. They stop earlier than we would like.

What is a caecilian?

Caecilians make up the amphibian order Gymnophiona. They are limbless, snake-shaped animals with burrowing habits, found across the tropics. A 2014 release from the Natural History Museum says they superficially look like a cross between a worm and a snake. They are neither. A 2020 paper describes them as jaw-feeders with a strong bite and curved, sharp-pointed teeth, and says they diverged from the ancestors of frogs and salamanders about 250 million years ago.

How many kinds are there? The answer depends on the year you ask. A 2013 release said nearly 200 species. The Natural History Museum marked the 200th named species in 2014 and predicted that about 100 more might exist. The 2020 paper counts 214 species in 10 families, and a 2024 article from Froglife says more than 220. The count keeps climbing as new species are described.

Caecilians also reproduce in more than one way. The Froglife article says about 20 percent of species give birth to live young, and that egg-laying species often show parental care. That is one source, so treat the 20 percent as a rough figure. And a 2013 release said more than two thirds of caecilian species were listed as Data Deficient on the IUCN Red List, which means there is too little information to say whether they are threatened.

Meet the ringed caecilian

The ringed caecilian gets its name from the grooves (called annuli) that circle its body. Most of the numbers below come from a 2011 taxonomic treatment of the species in a paper on Brazilian Amazonian caecilians. Because that treatment draws on older descriptions, we credit it each time.

A 2008 paper in Biology Letters describes the species as having the broadest distribution of any terrestrial caecilian, a superlative we could not check against a second source. The 2011 treatment adds that in Brazil it has been recorded in the Caatinga, Atlantic Forest, Cerrado and Amazonia. It also cites an older study that linked this wide range to lower moisture needs than many other caecilians, and another that found specimens under decomposing oil palm trunks, including in grazed pastures. The first report that it lays eggs, from 1899, described a female with a clutch under a tree trunk that had been moved while a pasture was being plowed.

A mother’s skin, and then her milk

The best-known thing about the ringed caecilian is how its young eat. In 2008, a team that included Mark Wilkinson studied six family groups, each with one mother and 8 to 16 young when caught. They watched the animals closely, checking every 15 to 60 minutes over 96 hours. The hatchlings were about 40 millimeters long, unpigmented, with heads much wider than their bodies.

The young peel off the outer layer of their mother’s skin using multi-cusped teeth. A 120 mm youngster had 44 teeth in its lower jaw, while a 40 mm hatchling had six. The mothers were paler than other adults, and their skin cells were enlarged and packed with fat droplets. The team also saw young gather around the mother’s vent and drink a fluid released there. The authors did not know what that fluid was.

In 2024, Pedro Mailho-Fontana and colleagues answered that question in Science. The fluid is a milk, rich in fats and carbohydrates, made in glands in the walls of the oviduct and released from the vent. Offspring drank it several times a day, apparently in response to touch and sound from the young, for roughly two months, and the State Museum of Natural History Stuttgart’s release agrees that the young consumed milk for over two months. A press release from the study says this kind of parent-to-offspring signaling is not known in any other amphibian. Milk was long thought to be a mammal specialty, but the release notes that spiders and other non-mammals are known to make similar nutrients.

Glands at the base of the teeth: venom or not?

A 2020 paper in iScience looked inside the ringed caecilian’s jaws. It found glands near the lips, each with a single duct. In the upper jaw the ducts open only at the base of the outer row of teeth. In hatchlings, the glands develop from the same tissue that forms teeth. The secretion showed high phospholipase A2 activity, an enzyme also found in some venoms, and the paper reports that in its tests this activity was higher than in the venom of a South American rattlesnake.

Does that make the ringed caecilian venomous? The authors are careful. They suggest that caecilians may have evolved a way to inject toxins through their teeth early in their history, but they call it a suggestion. They say more evidence is needed, including toxicity data, before anyone can say whether caecilians are venomous. They saw a thick secretion on the teeth when an animal opened its mouth before biting, and watched it hold prey against the ground and rotate its body. For caecilians in general, the paper lists earthworms, underground arthropods and larger prey such as frogs, lizards and snakes. This article is not a guide to handling wild animals, and caecilians are best left alone.

What we know and what we do not

Common mistakes and mix-ups

Mix-up: a caecilian is a worm. The 2020 paper lists earthworms among the prey of caecilians in general, and caecilians have a jawful of teeth. They are vertebrates and amphibians.

Mix-up: a caecilian is a snake. Snakes such as the Boa constrictor are reptiles. Caecilians belong to the same amphibian class as the Red-backed salamander and the Axolotl.

Myth: only mammals make milk. The 2024 authors call the fluid milk, and the release notes that spiders also make similar nutrients. Mammals did not patent the idea.

Mistake: venom means danger to people. Whether the secretion is toxic is unconfirmed, and the sources we read report no harm to humans.

Conservation status

The ringed caecilian is listed as Least Concern on the IUCN Red List of Threatened Species. That is the category as distributed by GBIF, the Global Biodiversity Information Facility, and we checked it on November 9, 2026. Least Concern means the species was evaluated and does not meet the criteria for a threatened or near-threatened category.

That label stands out in a group with so many Data Deficient species: enough was known about this one to evaluate it. None of the sources we read names specific threats to the ringed caecilian. They do show a wide range and some tolerance of dry conditions and grazed land. Amphibian chytrid fungus is one concern for the group: the 2013 release reported it in nearly 30 percent of more than 200 wild-caught caecilians from 29 species, and some infected animals died. The summary we read does not say which species were sampled. We have no confirmed population trend for the ringed caecilian.

Categories are reviewed and can change, and the copy GBIF distributes can trail the IUCN website by some months, which is why we print the date we checked. GBIF’s copy does not give the year of the IUCN assessment, so we do not state one.