Caecilian
Overview
The caecilians s are an order
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

 (Gymnophiona) of amphibian
Amphibian
Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...

s that superficially resemble earthworm
Earthworm
Earthworm is the common name for the largest members of Oligochaeta in the phylum Annelida. In classical systems they were placed in the order Opisthopora, on the basis of the male pores opening posterior to the female pores, even though the internal male segments are anterior to the female...

s or snake
Snake
Snakes are elongate, legless, carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. Like all squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales...

s. They mostly live hidden in the ground, making them the least familiar order of amphibians. All extant caecilians and their closest fossil relatives are grouped as the clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

 Apoda
Apoda
*Caecilian*Apoda, moth genus of the Limacodidae family...

. They are mostly distributed in the tropics of South America, Africa, and South Asia. The diet of caecilians is not well known.
Caecilians completely lack limbs, making the smaller species resemble worms, while the larger species with lengths up to 1.5 m (4.9 ft) resemble snakes.
Encyclopedia
The caecilians s are an order
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

 (Gymnophiona) of amphibian
Amphibian
Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...

s that superficially resemble earthworm
Earthworm
Earthworm is the common name for the largest members of Oligochaeta in the phylum Annelida. In classical systems they were placed in the order Opisthopora, on the basis of the male pores opening posterior to the female pores, even though the internal male segments are anterior to the female...

s or snake
Snake
Snakes are elongate, legless, carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. Like all squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales...

s. They mostly live hidden in the ground, making them the least familiar order of amphibians. All extant caecilians and their closest fossil relatives are grouped as the clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

 Apoda
Apoda
*Caecilian*Apoda, moth genus of the Limacodidae family...

. They are mostly distributed in the tropics of South America, Africa, and South Asia. The diet of caecilians is not well known.

Description

Caecilians completely lack limbs, making the smaller species resemble worms, while the larger species with lengths up to 1.5 m (4.9 ft) resemble snakes. The tail is short or absent, and the cloaca
Cloaca
In zoological anatomy, a cloaca is the posterior opening that serves as the only such opening for the intestinal, reproductive, and urinary tracts of certain animal species...

 is near the end of the body.

Their skin is smooth and usually dark-matte, but some species have colorful skins. Inside the skin are calcite
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...

 scales
Scale (zoology)
In most biological nomenclature, a scale is a small rigid plate that grows out of an animal's skin to provide protection. In lepidopteran species, scales are plates on the surface of the insect wing, and provide coloration...

. Because of these scales, the caecilians were once thought to be related to the fossil Stegocephalia
Stegocephalia
Stegocephalia is an old term for early amphibians, comprising all pre-Jurassic and some later extinct large amphibians of more or less salamander-like build...

, but they are now believed to be a secondary development, and the two groups are most likely unrelated. The skin also has numerous ring-shaped folds, or annuli, that partially encircle the body, giving them a segmented appearance. Like other living amphibians, the skin contains glands that secrete a toxin to deter predators. The skin secretions of Siphonops paulensis have been shown to have hemolytic properties.

Caecilians' vision is limited to dark-light perception, and their anatomy is highly adapted for a burrowing lifestyle. They have a strong skull, with a pointed snout used to force their way through soil or mud. In most species, the number of bones in the skull are reduced and fused together, and the mouth is recessed under the head. Their muscles are adapted to pushing their way through the ground, with the skeleton and deep muscles acting as a piston inside the skin and outer muscles. This allows the animal to anchor its hind end in position, and force the head forwards, and then pull the rest of the body up to reach it in waves. In water or very loose mud, caecilians instead swim in an eel-like fashion. Caecilians in the family Typhlonectidae
Typhlonectidae
Typhlonectidae, also known as aquatic caecilians or rubber eels, is a group of Gymnophiona amphibians found in South America.They are viviparous animals, giving birth to young that possess external gills...

 are aquatic as well as being the largest of their kind. The representatives of this family have a fleshy fin running along the rear section of their body, which enhances propulsion in water.

All but the most primitive caecilians have two sets of muscles for closing the jaw, compared with the single pair found in other creatures. These are more highly developed in the most efficient burrowers among the caecilians, and appear to help keep the skull and jaw rigid.

Adapting to their underground life, the eyes are small and covered by skin for protection, which has led to the misconception that they are blind. This is not strictly true, although their sight is limited to simple dark-light perception. All caecilians possess a pair of tentacles, located between their eyes and nostrils. These are probably used for a second olfactory capability
Olfaction
Olfaction is the sense of smell. This sense is mediated by specialized sensory cells of the nasal cavity of vertebrates, and, by analogy, sensory cells of the antennae of invertebrates...

, in addition to the normal sense of smell based in the nose.

Except for two lungless species — Atretochoana eiselti
Atretochoana eiselti
Atretochoana eiselti is a species of amphibian in the Caeciliidae family. It the sole species in its genus, Atretochoana. It is notable for having no lungs, one of only two lungless caecilians....

and Caecilita iwokramae — all caecilians have lung
Lung
The lung is the essential respiration organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located near the backbone on either side of the heart...

s, but also use the skin or the mouth for oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 absorption. Often the left lung is much smaller than the right one, an adaptation to body shape that is also found in snakes.

Distribution

Caecilians are found in wet tropical regions of Southeast Asia, India and Sri Lanka, parts of East and West Africa, the Seychelles
Seychelles
Seychelles , officially the Republic of Seychelles , is an island country spanning an archipelago of 115 islands in the Indian Ocean, some east of mainland Africa, northeast of the island of Madagascar....

 islands in the Indian Ocean and in northern and eastern South America. In Africa caecilians are found from Guinea-Bissau (Geotrypetes) to Southern Malawi (Scolecomorphus), with an unconfirmed record from eastern Zimbabwe. They have not been recorded from the extensive areas of tropical forest in central Africa. In South America they extend through subtropical eastern Brazil well into temperate northern Argentina. They can be seen as far south as Buenos Aires
Buenos Aires
Buenos Aires is the capital and largest city of Argentina, and the second-largest metropolitan area in South America, after São Paulo. It is located on the western shore of the estuary of the Río de la Plata, on the southeastern coast of the South American continent...

, when they are carried by the flood waters of the Paraná River
Paraná River
The Paraná River is a river in south Central South America, running through Brazil, Paraguay and Argentina for some . It is second in length only to the Amazon River among South American rivers. The name Paraná is an abbreviation of the phrase "para rehe onáva", which comes from the Tupi language...

 coming from farther north. The northernmost distribution is of the species Ichthyophis sikkimensis of Northern India. Ichthyophis
Ichthyophis
Ichthyophis, sometimes called the Asian caecilians are a genus of caecilians found in Southeast Asia, southern Philippines, and western Indo-Australian Archipelago....

is also found in South China and North Vietnam. In Southeast Asia, they do not cross Wallace's Line, and they are not found in Australia or the islands in between.

Taxonomy

The most recent classification of caecilians divides the Gymnophiona into 9 families. This classification is based on a thorough definition of monophyla based on the most recent morphological and molecular evidence, and it solves the longstanding problems of paraphyly of the Caeciliidae in previous classifications without an exclusive reliance upon synonymy.
  • Rhinatrematidae
    Rhinatrematidae
    Rhinatrematidae is the family of neotropical tailed caecilians or beaked caecilians. Members of the family are found in the equatorial countries of South America....

     - 2 genera, 11 species; South America
  • Ichthyophiidae
    Ichthyophiidae
    Ichthyophiidae is the family of Asiatic tailed caecilians or fish caecilians. They are found in south-east Asia.They are primitive caecilians, lacking many of the derived characters found in the other families. For example, the mouth is not recessed underneath the head, they possess tails, and have...

     - 3 genera, 50 species; South and Southeast Asia
  • Scolecomorphidae
    Scolecomorphidae
    Scolecomorphidae is the family of tropical caecilians or African caecilians . They are found in Cameroon in west Africa, and Malawi and Tanzania in east Africa. Caecilians are legless amphibians which superficially resemble worms or snakes.Scolecomorphids have only vestigial eyes, which are...

     - 2 genera, 6 species; Africa
  • Herpelidae - 2 genera, 9 species; Africa
  • Caeciliidae
    Caeciliidae
    Caeciliidae is the family of common caecilians. They are found in central and south America, equatorial Africa and India. Like other caecilians, they superficially resemble worms or snakes....

     - 2 genera, 42 species; South and Central America
  • Typhlonectidae
    Typhlonectidae
    Typhlonectidae, also known as aquatic caecilians or rubber eels, is a group of Gymnophiona amphibians found in South America.They are viviparous animals, giving birth to young that possess external gills...

     - 5 genera, 13 species; South America
  • Indotyphlidae - 7 genera, 21 species; Seychelles, India, Africa
  • Siphonopidae - 7 genera, 19 species; South America
  • Dermophiidae - 4 genera, 13 species; Africa, Central and South America


The name caecilian derives from the Latin word caecus, meaning "blind", referring to the small or sometimes non-existent eyes. The name dates back to the taxonomic name of the first species described by Carolus Linnaeus
Carolus Linnaeus
Carl Linnaeus , also known after his ennoblement as , was a Swedish botanist, physician, and zoologist, who laid the foundations for the modern scheme of binomial nomenclature. He is known as the father of modern taxonomy, and is also considered one of the fathers of modern ecology...

, which he gave the name Caecilia tentaculata. The taxonomic name of the order derives from the Greek words γυμνος (gymnos, naked) and οφις (ophis, snake), as the caecilians were originally thought to be related to snakes.

Evolution

Little is known of the evolutionary history of the caecilians, which have left a sparse fossil record. The first fossil, a vertebra dated to the Paleocene
Paleocene
The Paleocene or Palaeocene, the "early recent", is a geologic epoch that lasted from about . It is the first epoch of the Palaeogene Period in the modern Cenozoic Era...

, was not discovered until 1972. The putative earliest fossil known comes from the Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to  Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

 period; its primitive genus, Eocaecilia
Eocaecilia
Eocaecilia micropodia is an extinct species of caecilian from the Early Jurassic Period in Arizona, USA. It shared some characteristics with salamanders and the now extinct Microsauria...

, had small limbs and well-developed eyes. In their 2008 description of the fossil batrachian Gerobatrachus
Gerobatrachus
Gerobatrachus, also referred to as a frogamander, is an extinct genus of amphibamid temnospondyl that lived in the Permian period, approximately 290 million years ago, in the area that is now Baylor County, Texas...

Anderson and co-authors suggested that caecilians arose from the Lepospondyl group of ancestral tetrapods, and may be more closely related to amniotes than to frogs and salamanders, which arose from Temnospondyl ancestors. Divergent origins of caecilians and other living amphibians may help explain the slight discrepancy between fossil dates for the origins of modern amphibia, which suggest Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...

 origins, and the earlier dates, in the Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...

, predicted by molecular clock studies of DNA sequences. Most morphological and molecular studies of living amphibians, however, support monophyly
Monophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...

 for caecilians, frogs, and salamanders, and the most recent molecular study based on multi-locus data suggest a Late Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...

–Early Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...

 origin of living amphibians.

Reproduction

Caecilians are the only order of amphibians that use internal insemination exclusively (although the tailed frog
Tailed frog
The tailed frogs are two species of frogs. The species are part of the genus, Ascaphus is the only taxon in the family Ascaphidae . The "tail" in the name is actually an extension of the male cloaca. The tail is one of two distinctive anatomical features adapting the species to life in fast-flowing...

 in the U.S. does use a tail-like appendage for internal insemination in its fast flowing water environment). The male caecilians have a penis-like organ, the phallodeum, which is inserted into the cloaca of the female for 2 to 3 hours. About 25% of the species are oviparous (egg-laying); the eggs are guarded by the female. For some species the young caecilians are already metamorphosed when they hatch; others hatch as larvae. The larvae are not fully aquatic, but spend the daytime in the soil near the water.

75% of the species are viviparous, meaning that they give birth to already developed offspring. The fetus is fed inside the female with cells of the oviduct
Oviduct
In non-mammalian vertebrates, the passageway from the ovaries to the outside of the body is known as the oviduct. The eggs travel along the oviduct. These eggs will either be fertilized by sperm to become a zygote, or will degenerate in the body...

, which they eat with special scraping teeth.

The egg laying species Boulengerula taitana feeds its young by developing an outer layer of skin, high in fat and other nutrients, which the young peel off with similar teeth. This allows them to grow by up to ten times their own weight in a week. The skin is consumed every three days, the time it takes for a new layer to grow, and the young have only been observed to eat it at night. It was formerly thought that the juveniles subsisted only on a liquid secretion from their mother.

Some larvae, such as those of Typhlonectes
Typhlonectes
Typhlonectes is a genus of amphibian in the Caeciliidae family.It contains the following species:* Typhlonectes compressicauda* Typhlonectes cunhai* Typhlonectes natans...

, are born with enormous external gills which are shed almost immediately. Ichthyophis is oviparous and known to show maternal care, with the mother guarding the eggs until they hatch.

Diet

The diet of caecilians is not well known. Mature caecilians seem to feed mostly on insects and other invertebrates found in the habitat of the respective species. The stomach contents of 14 specimens of Afrocaecilia taitana consisted of mostly undefinable organic material and plant remains. Where identifiable remains were most abundant, they were found to be termite heads. While it was suggested that the undefinable organic material shows that the caecilians eat detritus
Detritus
Detritus is a biological term used to describe dead or waste organic material.Detritus may also refer to:* Detritus , a geological term used to describe the particles of rock produced by weathering...

, others believe these are in fact the remains of earthworm
Earthworm
Earthworm is the common name for the largest members of Oligochaeta in the phylum Annelida. In classical systems they were placed in the order Opisthopora, on the basis of the male pores opening posterior to the female pores, even though the internal male segments are anterior to the female...

s. Caecilians in captivity can be easily fed with earthworms, and worms are also common in the habitat of many caecilian species. Some caecilian mothers may shed a nutrient-rich skin for their offspring to eat.

See also

  • Caecilians of the Western Ghats
    Caecilians of the Western Ghats
    The Western Ghats are home to several species of Caecilians .Caecilians are legless, burrowing amphibians which mostly live in leaf litter, loose soil, under rocks and decaying logs. They are also found in agricultural fields and only surface during the monsoon. The body is elongated and smooth...

  • Minhocão
    Minhocão
    The minhocão is a large earthworm-like cryptid that allegedly exists in the forests of South America.- Appearance :...

    – a cryptid that resembles caecilians
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