Gymnophiona, also known as caecilians, are amphibians belonging to one of the three major extant orders within the class Amphibia. The name caecilian comes from the Latin word caecus, which means "sightless" or "blind." Most caecilians are limbless, wormlike amphibians that live underground in humid, tropical regions. They have segmented bodies with annular grooves, and some species have scales. About 180 species exist.

What do Gymnophiona look like?

Gymnophiona , also known as caecilians, are a group of amphibians with a wormlike appearance. The majority of caecilians are limbless and live underground in humid, tropical regions. Their bodies are segmented by annular grooves, and some species have scales. Caecilians have compact skulls with some fused elements, small eyes covered with skin or bone, and curved teeth. A projectable sensory tubercle is located between their eyes and nostrils. There are approximately 180 species of caecilians. Adult sizes range from 10 to 152 cm (4 to 60 inches).

Where do these amphibians live?

Caecilians , belonging to the amphibian order Gymnophiona, are mostly found underground in humid, tropical climates across the globe. These limbless, wormlike amphibians are unfamiliar to most people because of their hidden, subterranean existence. The morphology of caecilians reflects their environment, as they have structural forms that differ the least of the three living amphibian orders. Most caecilians live underground, with similar, specialized morphologies. They are widely distributed in moist soil in tropical parts of the New and Old Worlds.

How do Gymnophiona move and survive underground?

Caecilians , or Gymnophiona, are a group of amphibians known for their limbless, wormlike bodies that allow them to thrive in subterranean environments. Their morphology is specially adapted for this lifestyle; they have compact, bony skulls that act like a spade for tunneling through the soil. To move underground, caecilians use a method called concertina locomotion, where they alternately fold and extend their bodies. The skin is connected to the axial muscles, forming a tough sheath that aids in movement and covers the posterior part of the skull. They locate prey using chemosensory tentacles on their heads, and their strong, recurved teeth help them capture and consume earthworms and other soft-bodied creatures in their underground burrows. Though their vision is limited, their nasal organs are well-developed to compensate in their environment.

How do Gymnophiona reproduce and care for their young?

Gymnophiona, or caecilians, exhibit diverse reproductive strategies. All caecilians are believed to have internal fertilization, which is achieved through the male's copulatory organ. Some species, such as Asiatic ichthyophiids, breed aseasonally, while others, like Ichthyophis glutinossus , mate during the rainy season. Caecilian families like Ichthyophiidae and Rhinatrematidae lay eggs in mud burrows near water sources, with females guarding the clutches. Some species deposit eggs on land, hatching as larvae or small adults. Viviparous species, found in three families, give birth to a few young at a time. Aquatic typhlonectids are viviparous and produce larvae. Caecilian fetuses consume oviduct secretions and epithelial tissues using specialized teeth after exhausting their yolk supply.

What makes Gymnophiona different from other amphibians like frogs and salamanders?

Gymnophiona, or caecilians, stand out from other amphibians like frogs and salamanders due to their unique adaptations to a subterranean lifestyle. Most caecilians have long, limbless, cylindrical bodies with annuli (primary grooves) that encircle their bodies, creating a segmented appearance. Their heads are blunt and bony, with fused skull bones, and their degenerate eyes are covered with bone or skin. These physical characteristics enable them to thrive in their underground habitats. Unlike frogs and salamanders, caecilians move through the soil using concertina locomotion, where they alternately fold and extend their bodies. They also have a specialized chemosensory perception enhanced by a tentacle and a unique axial musculature that allows their skin and superficial muscles to move as a single unit.

Gymnophiona

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Zoom in on the serpentine locomotion of the limbless, amphibious caecilian over soil

South American caecilian (Siphonops annulatus).

Gymnophiona

amphibian

Also known as: Apoda, Gymnophiona, gymnophion

Written by

William E. Duellman

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Top Questions

What is Gymnophiona?
Gymnophiona, also known as caecilians, are amphibians belonging to one of the three major extant orders within the class Amphibia. The name caecilian comes from the Latin word caecus, which means "sightless" or "blind." Most caecilians are limbless, wormlike amphibians that live underground in humid, tropical regions. They have segmented bodies with annular grooves, and some species have scales. About 180 species exist.
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What do Gymnophiona look like?
Gymnophiona, also known as caecilians, are a group of amphibians with a wormlike appearance. The majority of caecilians are limbless and live underground in humid, tropical regions. Their bodies are segmented by annular grooves, and some species have scales. Caecilians have compact skulls with some fused elements, small eyes covered with skin or bone, and curved teeth. A projectable sensory tubercle is located between their eyes and nostrils.

There are approximately 180 species of caecilians. Adult sizes range from 10 to 152 cm (4 to 60 inches).
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Where do these amphibians live?
Caecilians, belonging to the amphibian order Gymnophiona, are mostly found underground in humid, tropical climates across the globe. These limbless, wormlike amphibians are unfamiliar to most people because of their hidden, subterranean existence. The morphology of caecilians reflects their environment, as they have structural forms that differ the least of the three living amphibian orders. Most caecilians live underground, with similar, specialized morphologies. They are widely distributed in moist soil in tropical parts of the New and Old Worlds.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How do Gymnophiona move and survive underground?
Caecilians, or Gymnophiona, are a group of amphibians known for their limbless, wormlike bodies that allow them to thrive in subterranean environments. Their morphology is specially adapted for this lifestyle; they have compact, bony skulls that act like a spade for tunneling through the soil.

To move underground, caecilians use a method called concertina locomotion, where they alternately fold and extend their bodies. The skin is connected to the axial muscles, forming a tough sheath that aids in movement and covers the posterior part of the skull.

They locate prey using chemosensory tentacles on their heads, and their strong, recurved teeth help them capture and consume earthworms and other soft-bodied creatures in their underground burrows. Though their vision is limited, their nasal organs are well-developed to compensate in their environment.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How do Gymnophiona reproduce and care for their young?
Gymnophiona, or caecilians, exhibit diverse reproductive strategies. All caecilians are believed to have internal fertilization, which is achieved through the male's copulatory organ. Some species, such as Asiatic ichthyophiids, breed aseasonally, while others, like Ichthyophis glutinossus, mate during the rainy season.

Caecilian families like Ichthyophiidae and Rhinatrematidae lay eggs in mud burrows near water sources, with females guarding the clutches. Some species deposit eggs on land, hatching as larvae or small adults. Viviparous species, found in three families, give birth to a few young at a time. Aquatic typhlonectids are viviparous and produce larvae. Caecilian fetuses consume oviduct secretions and epithelial tissues using specialized teeth after exhausting their yolk supply.
6 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What makes Gymnophiona different from other amphibians like frogs and salamanders?
Gymnophiona, or caecilians, stand out from other amphibians like frogs and salamanders due to their unique adaptations to a subterranean lifestyle. Most caecilians have long, limbless, cylindrical bodies with annuli (primary grooves) that encircle their bodies, creating a segmented appearance. Their heads are blunt and bony, with fused skull bones, and their degenerate eyes are covered with bone or skin. These physical characteristics enable them to thrive in their underground habitats.

Unlike frogs and salamanders, caecilians move through the soil using concertina locomotion, where they alternately fold and extend their bodies. They also have a specialized chemosensory perception enhanced by a tentacle and a unique axial musculature that allows their skin and superficial muscles to move as a single unit.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Gymnophiona, one of the three major extant orders of the class Amphibia. Its members are known as caecilians, a name derived from the Latin word caecus, meaning “sightless” or “blind.” The majority of this group of limbless, wormlike amphibians live underground in humid tropical regions throughout the world. Because of their relatively hidden existence, caecilians are unfamiliar to the layperson and are not usually considered in discussions about amphibians. They are nevertheless a fascinating group of highly specialized amphibians about which there is still much to be learned.

General features

Size and range

Several species of caecilians in the South American genus Caecilia exceed 1 metre (about 3.3 feet) in total length; the largest known caecilian is C. thompsoni, at 152 cm (about 60 inches). The smallest caecilians are Idiocranium russeli in West Africa and Grandisonia brevis in the Seychelles; these species attain lengths of only 98–104 mm (3.9–4.1 inches) and 112 mm (4.4 inches), respectively.

Distribution and abundance

Caecilians are found in tropical areas throughout the world. Of the 10 known families, 5 occur in the Americas, whereas Africa and mainland Asia harbour 3 families each. Caecilians are also found in Indonesia, Sri Lanka, the Philippines, and the Seychelles. On the Seychelles there are three genera native to the islands, although caecilians are not found on any other islands in the Indian Ocean. No caecilians have been found on Madagascar or New Guinea. Approximately 180 caecilian species are known to exist, and up to 5 species have been found to inhabit the same area in the Amazon rainforest.

Natural history

Breeding behaviour

Information about annual reproductive patterns among caecilians is limited. The breeding period of some Asiatic ichthyophiids seems to be aseasonal or at least without seasonal constraints. At least one species, Ichthyophis glutinossus in Sri Lanka, mates only during the rainy season. Females of viviparous species have a biennial reproductive cycle; the viviparous Dermophis mexicanus in Guatemala mates in the early part of the rainy season, and gestation takes one year.

All caecilians are believed to have internal fertilization. This is achieved by means of the phallodeum, a copulatory organ in males that is modified from the cloacal wall. Eggs of all members of the families Ichthyophiidae and Rhinatrematidae are deposited in burrows in mud that is close to water. The females watch over these clutches, which may hold up to 54 eggs. Upon hatching, the larvae leave the burrows to make their homes in ponds and streams. Some caecilians deposit eggs on land, and in different species these hatch as larvae or small adults. Three families have viviparous species to which usually no more than four young are born at one time. Aquatic typhlonectids are viviparous and produce larvae. The caecilian fetus emerges from the egg membrane as soon as its meagre yolk supply is exhausted; it uses its deciduous teeth, adapted for scraping, to obtain secretions and epithelial tissues from the oviduct lining.

Sea otter (Enhydra lutris), also called great sea otter, rare, completely marine otter of the northern Pacific, usually found in kelp beds. Floats on back. Looks like sea otter laughing. saltwater otters

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Feeding habits

The diet of terrestrial caecilians is mainly earthworms and other soft-bodied prey. Feeding either aboveground or in subterranean burrows, terrestrial caecilians are believed to locate their quarry by means of a chemosensory tentacle on each side of the head. They capture their prey with their powerful recurved teeth, masticate, and swallow. Aquatic caecilians, the typhlonectids, prey on fishes, eels, and aquatic invertebrates.

Form and function

Caecilians have long, limbless, cylindrical bodies that abruptly end behind the cloaca or short tail. Annuli (primary grooves) in the skin encircle the body and form segments; in some taxonomic groups, secondary and tertiary grooves partially circumscribe the body. Within the tissue of the annuli, bony scales of dermal origin usually occur. The heads of caecilians are blunt, and their skulls are bony and compact. Centres of ossification have fused, which has reduced the number of independent cranial bones in caecilians in comparison with anurans and salamanders; for example, a single bone, the os basale, forms both the floor of the braincase and the posterior part of the skull. Teeth are found on all jaw bones, and a palatal series of teeth appears in addition, medial to the maxillary series. A U-shaped facet, which articulates with the quadrate and also has a long retroarticular process that serves as an attachment site for three major jaw muscles, is located on the lower jaw. The vertebral column is made up of an atlas (the first vertebra of the neck) and 95 to 285 trunk vertebrae; no differentiated sacral vertebrae are present. Double-headed ribs are found on all vertebrae except the atlas and the terminal three to six vertebrae. Of the three amphibian orders, only caecilians have an axial musculature in which all the hypaxial components, excluding the subvertebral musculature, form an outer muscular sheath. This sheath, which is anchored to the skin by fibrous connective tissue, is all but disconnected from the vertebral musculature and thereby allows the skin and superficial muscles to move as a single unit. The degenerate eyes are covered with bone or skin. These adaptations make it possible for the caecilian to feed, reproduce, and avoid enemies within their subterranean realm. The features of aquatic caecilians of the family Typhlonectidae are representative of secondary adaptations.

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Gymnophiona

General features

Size and range

Distribution and abundance

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Natural history

Breeding behaviour

Feeding habits

Form and function