What Are Caecilians?


If you have ever seen a caecilian, you may know they look like a giant earthworm or snake, but did you know they are amphibians?

Caecilians are long-bodied, limbless amphibians with almost no tail and are often mistaken for giant earthworms due to similar segmented rings around their bodies. There are ten families with about 200 species and can be found around the world. However, they are rarely seen as they live underground.

If you want to know more about caecilians, please read on.

Caecilians
Dick Culbert Flickr CC by 2.0

Caecilians lack a fossil record, with the only fossil being a single vertebra from the Paleocene (65 million years ago) found in Brazil, but their biology hints at their evolution.

Caecilians belong to the class Amphibia and are related to salamanders and frogs. Caecilians’ bones, teeth, fat bodies, and other structures show many resemblances.

Caecilians lost their limbs, and some lost their tails early in their evolution. As they adopted more of an underground lifestyle, their features changed. They grew large skulls that housed small eyes, something seen in other underground animals such as moles.

They can be found in large numbers in Ghana, the tropics, and Central America, although they are more scarce elsewhere. Although the aquatic species of South America are occasionally seen in fishermen’s nets, they mostly stay underground, infrequently emerging from their burrows.

Caecilians range in size, with the largest coming from Colombia. Caecilia thompsoni grows up to 1.5 meters (4 ft 11in). In contrast, the smallest, Idiocranium of West Africa grows to just 7cm (2.7in.)

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Diet

Most caecilians are opportunistic feeders that prey on earthworms, termites, grasshoppers, and locusts. Their diet depends on their environment. Afrocaecilia feeds primarily on termites, while Dermophis feeds on easily found worms. Some larger species feed on small lizards and rodents, such as the Mexican burrowing caecilian (Dermophis mexicanus.)

Caecilians usually wait for their food to approach them before grabbing it in their strong jaws. They have two rows of teeth on the upper jaw and either one or two on the bottom. The teeth are sharp and curved inwards, allowing them to grip their prey as they eat it.

Behavior

Caecilians are burrowing animals that can be found in soil or on the bottom of a body of water. Their skulls are large and bony, allowing them to dig with their heads. The skin is fused to the bones so that it doesn’t stretch or split while exploring.

Caecilians move using hydrostatic locomotion. They grip using their rear ends, and forward movement occurs as the body curves move against the soil or water. This is similar to how worms move, although the segmental rings around the body do not seem to help it move.

The skin is smooth and slimy, and toughened with keratin. The inner layer is densely packed with mucus glands and several poison glands. Their secretions can be highly poisonous to predators and even humans. This may make them the only amphibians with a toxic bite.

Scales, more similar to fish scales than reptile scales, can be found in the segmental rings of many species. However, evolution seems to be phasing these out. While primitive species have them down their bodies, most modern species have none.

Caecilians are categorized as having a worm-like body form and covered eyes. The eyes are covered by skin or skin and bone, adhering to the covered layers. Many species lack eye muscles, and the eye lens cannot move. Caecilians are not blind, and the eyes are used to detect light.

Another sense that caecilians rely on is smell. Caecilians have a small tentacle that develops during metamorphosis on the upper jaw behind each nostril. This transports chemical messages from the environment to the nasal cavity.

The caecilian tentacle is unique among vertebrates and uses the same muscle used in other amphibians to retract the eye.

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Caecilians

Reproduction

Internal fertilization occurs in all caecilians. As in reptiles, birds, and mammals, the male extrudes the rear part of his cloaca and inserts it into the female, transferring sperm to the female, where fertilization occurs. Caecilians will either lay eggs or give birth to live young.

Little is known about how they find a mate or courtship due to their underground habitat. However, they have been seen dancing together before mating.

Many caecilians lay eggs underground near streams. As they hatch, they enter the water, only becoming terrestrial after metamorphosis. Other species will lay their eggs underground and, after metamorphosis, will hatch into an adult before appearing above ground.

Caecilians have a mechanism that only some frogs and salamanders have when it comes to live birth. After the yolk supply is exhausted, half of the species will retain the developing young in the female’s oviducts throughout metamorphosis, nourishing the young with milk from glands in the oviducts.

Caecilians are born with adult teeth, which help them feed on the milk. Gills are used to exchange nutrients with the mother through the circulating blood.

Gestation lasts 9—11 months, with the young being fed by secretions most of the time. A clutch contains between 7 to 20 young. While they are born small, their mass grows quickly.

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Classification

Ten caecilian families are distinguished by various characteristics, including their body forms, bones, and muscles and how they reproduce. Because these amphibians are so unknown, there are no common names for the families or most species.

Ichthyophiidae – 2 genera, 57 species; South and Southeast Asia

The Ichthyophiidae family includes species with multiple rings in each body segment, vertebrae extending beyond the anus, a large number of skull bones, and an egg-laying mode of reproduction with free-living larvae.

Rhinatrematidae – 3 genera, 14 species; South America

The Rhinatrematidae shares the characteristics of Ichthyophiidae, but they have fewer skull bones and different musculature.

Caeciliidae – 2 genera, 43 species; South and Central America

Caeciliidae has fewer skull bones and only one or two rings per body segment. They can either lay eggs or be live-bearing.

Typhlonectidae – 5 genera, 14 species; South America

The live-bearing Typhlonectidae have one ring per body segment and a slight to moderate dorsal fin. They are aquatic and the only caecilians that are not terrestrial borrowers.

Scolecomorphidae – 2 genera, 6 species; Africa

The Scolecomorphidae also have only one ring per segment. They are livebearers and have a reduced number of skull bones, including the absence of the stapes, the middle ear bone.

Dermophiidae – 4 genera, 14 species; Africa, Central, and South America

Dermophiidae is the only caecilians that give birth to live young and have secondary rings around the body.

Herpelidae – 2 genera, 10 species; Africa

Herpelidae is from Africa and includes ten known species.

Chikilidae – 1 genus, 4 species; India

Chikilidae is found in India and is the most recent family of caecilians. They are distinct as they do not enter metamorphosis.

Siphonopidae – 5 genera, 28 species; South America

Siphonopidae is found in Central and South America and lays eggs. They are excellent borrowers and have large skulls to push through the earth.

Grandisoniidae (formerly Indotyphlidae) – 7 genera, 24 species; Seychelles, India, Africa

Found in Seychelles, Africa, and India, Grandisoniidae is made up of 24 species. Some species give birth to live young while others lay eggs.

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