Caecilians (/sɪˈsɪliən/;New Latin for 'blind ones') are a group of limbless,worm-shaped or snake-shapedamphibians, with either small eyes or no eyes, comprising the orderGymnophiona. They mostly live hidden in soil or in streambeds, making them some of the least familiar amphibians. Modern caecilians live in the tropics ofSouth andCentral America,Africa, and southernAsia. Caecilians feed on small subterranean creatures, such asearthworms. The body is noodle-like and often dark in colour, and the skull is bullet-shaped and strongly built. Caecilian heads have several unique adaptations, such as fused skull and jaw bones, a two-part system of jaw muscles, andchemosensory tentacles between the eyes and nostrils. The skin is slimy, with ringlike markings or grooves, and in some species hides scales underneath.[2]
Modern caecilians are aclade, theorderGymnophiona/ˌdʒɪmnəˈfaɪənə/ (orApoda/ˈæpədə/), one of the three living amphibian groups alongside Anura (frogs) and Urodela (salamanders). Gymnophiona is acrown group, encompassing all modern caecilians and all descendants of their last common ancestor. There are more than 220 livingspecies of caecilian classified in 10families.Gymnophionomorpha is a recently coined name for the correspondingtotal group which includes Gymnophiona as well as a few extinctstem-group caecilians (extinct amphibians whose closest living relatives are caecilians but are not descended from any caecilian).[3][4] Some palaeontologists have used the name Gymnophiona for thetotal group and the old name Apoda for the crown group.[5] However, Apoda has other even older uses, including as the name of a genus ofbutterfly, making its use potentially confusing and best avoided. The clade's name 'Gymnophiona' comes fromAncient Greek γυμνος (gumnos), meaning "naked", and ὄφις (óphis), meaning "snake", as the caecilians were originally thought to be related to snakes and to lack scales.[6]
The study of caecilian evolution is complicated by their poor fossil record and specialized anatomy. Genetic evidence and some anatomical details (such aspedicellate teeth) support the idea that frogs, salamanders, and caecilians (collectively known aslissamphibians) are each other's closest relatives. Frogs and salamanders show many similarities todissorophoids, a group of extinct amphibians in the orderTemnospondyli. Caecilians are more controversial; many studies extend dissorophoid ancestry to caecilians. Some studies have instead argued that caecilians descend from extinctlepospondyl orstereospondyl amphibians, contradicting evidence for lissamphibianmonophyly (common ancestry). Rare fossils of early gymnophionans, such asEocaecilia andFuncusvermis, have helped to test the various conflicting hypotheses for the relationships between caecilians and other living and extinct amphibians.
X-ray showing the skeleton ofTyphlonectes (Typhlonectidae)
Caecilians' anatomy is highly adapted for a burrowing lifestyle. In a couple of species, belonging to the primitive genusIchthyophis, vestigial traces of limbs have been found, and inTyphlonectes compressicauda the presence of limb buds has been observed during embryonic development, remnants in an otherwise completely limbless body.[7]This makes the smaller species resemble worms, while the larger species likeCaecilia thompsoni, with lengths up to 1.5 m (5 ft), resemble snakes. Their tails are short or absent, and theircloacae are near the ends of their bodies.[8][9][10]
Except for one lungless species,Atretochoana eiselti,[11] all caecilians havelungs, but also use their skin or mouths foroxygen absorption. Often, the left lung is much smaller than the right one, an adaptation to body shape that is also found in snakes.[12]
Their trunk muscles are adapted to pushing their way through the ground, with the vertebral column and its musculature acting as a piston inside the outer layer of the body wall musculature, which is closely attached to the skin.[13] By contracting the outer layer of muscles it squeezes thecoelom and generates a strong hydrostatic force that lengthens the body.[14] This muscle system 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.[15] Caecilians in the familyTyphlonectidae are aquatic, and the largest of their kind. The first fossil discovered belonging to an extant family of caecilians,Ymboirana acrux, is from family Typhlonectidae.[16] The representatives of this family have a fleshy fin running along the rear section of their bodies, which enhances propulsion in water.[17]
Caecilians have small or absent eyes, with only a single known class ofphotoreceptors, and their vision is limited to dark-light perception.[18][19] Despite their restricted vision, caecilians display circadian rhythms in response to the photoperiod and tend to demonstrate more surface activity at night.[20] Unlike other modern amphibians (frogs and salamanders), the skull is compact and solid, with few large openings between plate-like cranial bones. The snout is pointed and bullet-shaped, used to force their way through soil or mud. In most species, the mouth is recessed under the head, so that the snout overhangs the mouth.[10]
The bones in the skull are reduced in number compared to prehistoric amphibian species. Caecilians have one of two skull types, stegokrotaphic or zygokrotaphic. In stegokrotaphic skulls the squamosal covers the temporal region and the jaw closing muscles while in zygokrotaphic skulls the squamosal only partially covers the temporal region.[21] Many bones of the skull are fused together: themaxilla andpalatine bones have fused into a maxillopalatine in all living caecilians, and thenasal andpremaxilla bones fuse into a nasopremaxilla in some families. Some families can be differentiated by the presence or absence of certain skull bones, such as theseptomaxillae,prefrontals, an/or apostfrontal-like bone surrounding the orbit (eye socket). The braincase is encased in a fully integrated compound bone called the os basale, which takes up most of the rear and lower parts of the skull. In skulls viewed from above, amesethmoid bone may be visible in some species, wedging into the midline of the skull roof.[22][23][24]
All caecilians have a pair of unique sensory structures, known astentacles, located on either side of the head between the eyes and nostrils. These are probably used for a secondolfactory capability, in addition to the normal sense of smell based in the nose.[15]
The middle ear consists of only thestapes bone and theoval window, which transfer vibrations into the inner ear through a reentrant fluid circuit as seen in some reptiles. Adults of species in the familyScolecomorphidae lack both a stapes and an oval window, making them the only known amphibians missing all the components of a middle ear apparatus.[26]
The lower jaw is specialized in caecilians. Gymnophionans, including extinct species, have only two components of the jaw: the pseudodentary (at the front, bearing teeth) and pseudoangular (at the back, bearing the jaw joint and muscle attachments). These two components are what remains following fusion between a larger set of bones. An additional inset tooth row with up to 20 teeth lies parallel to the main marginal tooth row of the jaw.[23]
All but the most primitive caecilians have two sets of muscles for closing the jaw, compared with the single pair found in other amphibians. One set of muscles, the adductors, insert into the upper edge of the pseudoangular in front of the jaw joint. Adductor muscles are commonplace in vertebrates, and close the jaw by pulling upwards and forwards. A more unique set of muscles, the abductors, insert into the rear edge of the pseudoangular below and behind the jaw joint. They close the jaw by pulling backwards and downwards. Jaw muscles are more highly developed in the most efficient burrowers among the caecilians, and appear to help keep the skull and jaw rigid.[15][27]
Their skin is smooth and usually dark, but some species have colourful skins. Inside the skin arecalcitescales. Because of these scales, the caecilians were once thought to be related to the fossilStegocephalia, but they are now believed to be a secondary development, and the two groups are most likely unrelated.[10] Scales are absent in the familiesScolecomorphidae andTyphlonectidae, except the speciesTyphlonectes compressicauda where minute scales have been found in the hinder region of the body.[28] The skin also has numerous ring-shaped folds, or annuli, that partially encircle the body, giving them a segmented appearance. Like some other living amphibians, the skin contains glands that secrete a toxin to deter predators.[15] The skin secretions ofSiphonops paulensis have been shown to havehemolytic properties.[29]
Recent research, as documented in the journalScience, has shed light on the behavior of certain species of caecilians. These studies reveal that some caecilians exhibit a phenomenon wherein they provide their hatchlings with a nutrient-rich substance akin to milk, delivered through a maternal vent. Among the species investigated, the oviparous nonmammalian caecilian amphibianSiphonops annulatus stood out, indicating that the practice of lactation may be more widespread among these creatures than previously thought. As detailed in a 2024 study, researchers collected 16 mothers of theSiphonops annulatus species from cacao plantations in Brazil'sAtlantic Forest and filmed them with theiraltricial hatchlings in the lab. The mothers remained with their offspring, which suckled on a white, viscous liquid from theircloaca, experiencing rapid growth in their first week. This milk-like substance, rich infats andcarbohydrates, is produced in the mother'soviductepithelium's hypertrophied glands, similar tomammal milk. The substance was released seemingly in response to tactile and acoustic stimulation by the babies. The researchers observed the hatchlings emitting high-pitched clicking sounds as they approached their mothers for milk, a behavior unique among amphibians. This milk-feeding behavior may contribute to the development of the hatchlings'microbiome and immune system, similar to mammalian young. The presence of milk production in caecilians that lay eggs suggests an evolutionary transition betweenegg-laying andlive birth.[30][31][32]
In 2021, a live specimen ofTyphlonectes natans, a caecilian native toColombia andVenezuela, was collected from a drainage canal in SouthFlorida. It was the only caecilian ever reported in the wild in the United States, and is considered to be anintroduction, perhaps from thewildlife trade. Whether a breeding population has been established in the area is unknown.[35][36]
The namecaecilian derives from the Latin wordcaecus, meaning "blind", referring to the small or sometimes nonexistent eyes. The name dates back to the taxonomic name of the first species described byCarl Linnaeus, which he namedCaecilia tentaculata.[10]
There has historically been disagreement over the use of the two primary scientific names for caecilians, Apoda and Gymnophiona. Some palaeontologists prefer to use the name Apoda to refer to the "crown group", that is, the group containing all modern caecilians and extinct members of these modern lineages and the name Gymnophiona to refer to the total group, that is, all caecilians and caecilian-like amphibians that are more closely related to modern groups than to frogs or salamanders. However, Apoda been used for groups of fishes and of sea cucumbers and is the name of a genus of moth, and its continued use in caecilian taxonomy is potentially confusing and unhelpful.
A classification of caecilians by Wilkinson et al. (2011) divided the living caecilians into 9 families containing nearly 200 species.[23] In 2012, a tenth caecilian family was newly described,Chikilidae.[37][38] This classification is based on a thorough definition of monophyly based on morphological and molecular evidence,[39][40][41][42] and it solves the longstanding problems ofparaphyly of theCaeciliidae in previous classifications without an exclusive reliance upon synonymy.[23][43] There are 219 species of caecilian in 33 genera and 10 families.
South America (Colombia and Venezuela south to northern Argentina andUruguay).
The most recent phylogeny of caecilians is based on molecular mitogenomic evidence examined by San Mauro et al. (2014), and modified to include some more recently described genera such asAmazops.[44][45][46]
Eocaecilia, an Early Jurassic amphibian commonly considered one of the oldest (stem-group) caecilians
Little is known of the evolutionary history of the caecilians, which have left a very sparse fossil record. The first fossil, a vertebra dated to thePaleocene, was not discovered until 1972.[47] Other vertebrae, which have characteristic features unique to modern species, were later found in Paleocene and Late Cretaceous (Cenomanian) sediments.[5]
Phylogenetic evidence suggests that the ancestors of caecilians andbatrachians (including frogs and salamanders) diverged from one another during theCarboniferous. This leaves a gap of more than 70 million years between the presumed origins of caecilians and the earliest definitive fossils of stem-caecilians.[48][4]
Prior to 2023, the earliest fossil attributed to a stem-caecilian (an amphibian closer to caecilians than to frogs or salamanders but not a member of the extant caecilian lineage) comes from theJurassic period. This primitive genus,Eocaecilia, had small limbs and well-developed eyes.[49] In their 2008 description of the Early Permian amphibianGerobatrachus,[50] Anderson and co-authors suggested that caecilians arose from theLepospondyl group of ancestraltetrapods, and may be more closely related toamniotes than to frogs and salamanders, which arose fromTemnospondyl ancestors. Numerous groups of lepospondyls evolved reduced limbs, elongated bodies, and burrowing behaviors, and morphological studies on Permian and Carboniferous lepospondyls have placed the early caecilian (Eocaecilia) among these groups.[51] Divergent origins of caecilians and other extant amphibians may help explain the slight discrepancy between fossil dates for the origins of modern Amphibia, which suggestPermian origins, and the earlier dates, in theCarboniferous, predicted by some molecular clock studies of DNA sequences. Most morphological and molecular studies of extant amphibians, however, supportmonophyly for caecilians, frogs, and salamanders, and the most recent molecular study based on multi-locus data suggest a LateCarboniferous–Early Permian origin of extant amphibians.[52]
Chinlestegophis, astereospondyl temnospondyl from the Late TriassicChinle Formation of Colorado, was proposed to be a stem-caecilian in a 2017 paper by Pardo and co-authors. If confirmed, this would bolster the proposed pre-Triassic origin ofLissamphibia suggested by molecular clocks. It would fill a gap in the fossil record of early caecilians and suggest that stereospondyls as a whole qualify as stem-group caecilians.[48] However, affinities betweenChinlestegophis and gymnophionans have been disputed along several lines of evidence. A 2020 study questioned the choice of characters supporting the relationship,[3] and a 2019 reanalysis of the original data matrix found that other equally parsimonious positions were supported for the placement ofChinlestegophis and gymnophionans among tetrapods.[53] In 2024,Chinlestegophis was consistently recovered as a sister taxon ofRileymillerus within various positions ofStereospondyli outsideLissamphibia based on phylogenetic analyses and revisions.[54]
A 2023 paper by Kligman and co-authors describedFuncusvermis, another amphibian from the Chinle Formation of Arizona.Funcusvermis was strongly supported as a stem group caecilian based on traits of its numerous skull and jaw fragments, the largest sample of caecilian fossils known. The paper discussed the various hypotheses for caecilian origins: the polyphyly hypothesis (caecilians as lepospondyls, and other lissamphibians as temnospondyls), the lepospondyl hypothesis (lissamphibians as lepospondyls), and the newer hypothesis supported byChinlestegophis, where caecilians and other lissamphibians had separate origins within temnospondyls. Nevertheless, all of these ideas were refuted, and the most strongly supported hypothesis combined lissamphibians into a monophyletic group of dissorophoid temnospondyls closely related toGerobatrachus.[4]
Caecilians are the only order of amphibians to use internal insemination exclusively (although most salamanders have internal fertilization and thetailed frog in the US uses a tail-like appendage for internal insemination in its fast-flowing water environment).[15] The male caecilians have a long tube-likeintromittent organ, the phallodeum,[55] which is inserted into the cloaca of the female for two to three hours. About 25% of the species areoviparous (egg-laying); the eggs are laid in terrestrial nests rather than in water and are guarded by the female. For some species, the young caecilians are alreadymetamorphosed when they hatch; others hatch as larvae. The larvae are not fully aquatic, but spend the daytime in the soil near the water.[15][56]
About 75% of caecilians areviviparous, meaning they give birth to already-developed offspring. The foetus is fed inside the female with cells lining theoviduct, which they eat with special scraping teeth. Some larvae, such as those ofTyphlonectes, are born with enormous externalgills which are shed almost immediately.
Microcaecilia dermatophaga (Siphonopidae) mother and hatchlings. This species is one of several caecilians in which maternal dermatophagy has been observed.
The egg-layingherpelid speciesBoulengerula taitana feeds its young by developing an outer layer of skin, high in fat and other nutrients, which the young peel off with modified teeth. This allows them to grow by up to 10 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 mothers.[57][58][59] This form of parental care, known as maternal dermatophagy, has also been reported in two species in the familySiphonopidae:Siphonops annulatus andMicrocaecilia dermatophaga. Siphonopids and herpelids are not closely related to each other, having diverged in the Cretaceous Period. The presence of maternal dermatophagy in both families suggest that it may be more widespread among caecilians than previously considered.[60][61]
Herpele squalostoma caeciliansvertically transmit the mother'smicrobiome to their offspring through maternal dermatophagy. In comparison to other amphibians, the extended parenting of caecilians can provide beneficial bacteria and fungi, but this transmission risks the spread of diseases likechytridiomycosis.[62][63]
Caecilians are considered as generalist predators.[64] While caecilians are generallycarnivorous, their diet differs between taxa. The stomach contents of wild caecilians include primarily soil ecosystem engineers[64] likeearthworms,termites,lizards,moth larvae, andshrimp. Some species of caecilians will opportunistically consume newbornrodents,salmon eggs, andveal in laboratory conditions, as well as vertebrates such asscolecophidian snakes,lizards, smallfish, andfrogs.[65][66]
As caecilians are a reclusive group, they are featured in only a few humanmyths and are considered repulsive by many cultures.
In thefolklore of certain regions of India, caecilians are feared and reviled, based on the belief, mostly incorrect, that they are fatally venomous. While some species of caecillians have venomous bites, they are not thought to be fatal to adult humans. Caecilians in theEastern Himalayas are colloquially known as "back ache snakes",[67] while in theWestern Ghats,Ichthyophis tricolor is considered to be more toxic than aking cobra.[68][69] Despite deep cultural respect for the cobra and other dangerous animals, the caecilian is killed on sight by salt and kerosene.[68] These myths have complicated conservation initiatives for Indian caecilians.[68][67][69]
Crotaphatrema lamottei, a rare species native toMount Oku inCameroon, is classified as a Kefa-ntie (burrowing creature) by theOku. Kefa-ntie, a term also encompassing native moles and blind snakes, are considered poisonous, causing painful sores if encountered, contacted, or killed. According to Oku tradition, the ceremony to cleanse the affliction involves a potion composed of ground herbs, palm oil, snail shells, and chicken blood applied to and licked off of the left thumb.[70]
South American caecilians have a variable relationship to local cultures.[69] Theminhocão, a legendary worm-like beast inBrazilian folklore, may be inspired by caecilians.Colombian folklore states that the aquatic caecilian,Typhlonectes natans, can be manifested from a lock of hair sealed in a sunken bottle. In southernMexico andCentral America,Dermophis mexicanus is colloquially known as the "tapalcua", a name referencing the belief that it emerges to embed itself in the rear end of any unsuspecting person who chooses to relieve themself over its home. This may be inspired by their tendency to nest in refuse heaps.[69]
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