Caecilian
Caecilian | |
---|---|
Oscaecilia ochrocephala (Caeciliidae) | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Class: | Amphibia |
Subclass: | Lissamphibia |
Order: | Gymnophiona Müller, 1832[1] |
Subgroups | |
Synonyms[1] | |
|
Caecilians (
Modern caecilians are a
The study of caecilian evolution is complicated by their poor fossil record and specialized anatomy. Genetic evidence and some anatomical details (such as pedicellate teeth) support the idea that frogs, salamanders, and caecilians (collectively known as lissamphibians) are each others' closest relatives. Frogs and salamanders show many similarities to dissorophoids, a group of extinct amphibians in the order Temnospondyli. Caecilians are more controversial; many studies extend dissorophoid ancestry to caecilians. Some studies have instead argued that caecilians descend from extinct lepospondyl or stereospondyl amphibians, contradicting evidence for lissamphibian monophyly (common ancestry). Rare fossils of early gymnophionans such as Eocaecilia and Funcusvermis have helped to test the various conflicting hypotheses for the relationships between caecilians and other living and extinct amphibians.
Description
Caecilians' anatomy is highly adapted for a burrowing lifestyle. In a couple of species belonging to the primitive genus Ichthyophis vestigial traces of limbs have been found, and in Typhlonectes 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 like Caecilia thompsoni, with lengths up to 1.5 m (5 ft), resemble snakes. Their tails are short or absent, and their cloacae are near the ends of their bodies.[8][9][10]
Except for one lungless species,
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 the coelom 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 family Typhlonectidae are aquatic, and the largest of their kind. The representatives of this family have a fleshy fin running along the rear section of their bodies, which enhances propulsion in water.[16]
Skull and senses
Caecilians have small or absent eyes, with only a single known class of photoreceptors, and their vision is limited to dark-light perception.[17][18] 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. Many bones of the skull are fused together: the maxilla and palatine bones have fused into a maxillopalatine in all living caecilians, and the nasal and premaxilla bones fuse into a nasopremaxilla in some families. Some families can be differentiated by the presence of absence of certain skull bones, such as the septomaxillae, prefrontals, an/or a postfrontal-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, a mesethmoid bone may be visible in some species, wedging into the midline of the skull roof.[19][20][21]
All caecilians have a pair of unique sensory structures, known as
The ringed caecilian (Siphonops annulatus) has dental glands that may be homologous to the venom glands of some snakes and lizards. The function of these glands is unknown.[22]
The middle ear consists of only the stapes bone and the oval window, which transfer vibrations into the inner ear through a reentrant fluid circuit as seen in some reptiles. Adults of species within the family Scolecomorphidae lack both a stapes and an oval window, making them the only known amphibians missing all the components of a middle ear apparatus.[23]
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.[20]
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][24]
Skin
Their skin is smooth and usually dark, but some species have colourful skins. Inside the skin are
Milk provisioning
Recent research, as documented in the journal Science, 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 amphibian Siphonops 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 the Siphonops annulatus species from cacao plantations in Brazil's Atlantic Forest and filmed them with their altricial hatchlings in the lab. The mothers remained with their offspring, which suckled on a white, viscous liquid from their cloaca, experiencing rapid growth in their first week. This milk-like substance, rich in fats and carbohydrates, is produced in the mother's oviduct epithelium's hypertrophied glands, similar to mammal 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 between egg-laying and live birth.[27][28][29]
Distribution
Caecilians are native to wet, tropical regions of
In 2021, a live specimen of Typhlonectes natans, a caecilian native to Colombia and Venezuela, was collected from a drainage canal in South Florida. It was the only caecilian ever reported in the wild in the United States, and is considered to be an introduction, perhaps from the wildlife trade. Whether a breeding population has been established in the area is unknown.[32][33]
Taxonomy
The name caecilian derives from the Latin word caecus, meaning "blind", referring to the small or sometimes nonexistent eyes. The name dates back to the taxonomic name of the first species described by Carl Linnaeus, which he named Caecilia 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.[20] In 2012, a tenth caecilian family was newly described, Chikilidae.[34][35] This classification is based on a thorough definition of monophyly based on morphological and molecular evidence,[36][37][38][39] and it solves the longstanding problems of paraphyly of the Caeciliidae in previous classifications without an exclusive reliance upon synonymy.[20][40] There are 219 species of caecilian in 33 genera and 10 families.
Family | Image | Taxon author | Genera | Species | Common name | Geographic range |
---|---|---|---|---|---|---|
Caeciliidae | Caecilia subnigricans |
Rafinesque, 1814 | 2 | 47 | Common caecilians | Central and South America (Bolivia north to Costa Rica). |
Chikilidae | Kamei et al., 2012 | 1 | 4 | Northeast Indian caecilians | Northeast India and Bangladesh, with possible occurrences in Myanmar. | |
Dermophiidae | Geotrypetes seraphini |
Taylor, 1969 | 4 | 15 | Neotropical caecilians | Equatorial Africa (West Africa, Tanzania, Kenya), Central and South America (Colombia north to Mexico). |
Indotyphlidae )
|
Grandisonia sechellensis |
Lescure, Renous & Gasc, 1986 | 7 | 24 | Indo-African caecilians | Equatorial Africa (Cameroon, Ethiopia), the Seychelles, western India (Western Ghats). |
Herpelidae | Boulengerula taitana |
Laurent, 1984 | 2 | 10 | African caecilians | Equatorial Africa (Nigeria south to the Democratic Republic of the Congo, Kenya south to Malawi, possible occurrences in Angola and Zambia). |
Ichthyophiidae | Ichthyophis kodaguensis |
Taylor, 1969 | 2 | 57 | Asian tailed caecilians | South and Southeast Asia (western India north to Nepal, east to the Philippines, southern China and Indonesia). |
Rhinatrematidae | Epicrionops sp. |
Nussbaum, 1977 | 3 | 14 | American tailed caecilians | Northern South America (northernmost Brazil west to Venezuela, Colombia, Ecuador, and Peru). |
Scolecomorphidae | Scolecomorphus kirkii |
Taylor, 1969 | 2 | 6 | Buried-eyed caecilians | Equatorial Africa (Cameroon, Tanzania, Malawi, Mozambique). |
Siphonopidae | Microcaecilia dermatophaga |
Bonaparte, 1850 | 5 | 28 | South American caecilians | South America (Colombia south to northern Argentina, Paraguay, and southernmost Brazil). |
Typhlonectidae | Typhlonectes natans |
Taylor, 1968 | 5 | 14 | Aquatic caecilians | South America (Colombia and Venezuela south to northern Argentina and Uruguay). |
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 as Amazops.[41][42][43]
Gymnophiona |
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Evolution
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 the Paleocene, was not discovered until 1972.[44] 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 and batrachians (including frogs and salamanders) diverged from one another during the Carboniferous. This leaves a gap of more than 70 million years between the presumed origins of caecilians and the earliest definitive fossils of stem-caecilians.[45][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 the
Chinlestegophis, a stereospondyl temnospondyl from the Late Triassic Chinle 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 of Lissamphibia 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.[45] However, affinities between Chinlestegophis 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 of Chinlestegophis and gymnophionans among tetrapods.[50]
A 2023 paper by Kligman and co-authors described Funcusvermis, 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 by Chinlestegophis, 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 to Gerobatrachus.[4]
Behavior
Reproduction
Caecilians are the only order of amphibians to use internal insemination exclusively (although most salamanders have internal fertilization and the
About 75% of caecilians are
The egg-laying herpelid 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 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.[53][54][55] This form of parental care, known as maternal dermatophagy, has also been reported in two species in the family Siphonopidae: Siphonops annulatus and Microcaecilia 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.[56][57]
Herpele squalostoma caecilians vertically transmit the mother's microbiome 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 like chytridiomycosis.[58][59]
Diet
Caecilians are considered as generalist predators.[60] While caecilians are generally carnivorous, their diet differs between taxa. The stomach contents of wild caecilians include primarily soil ecosystem engineers[61] like
Cultural significance
As caecilians are a reclusive group, they are only featured in a few human myths, and are generally considered repulsive in traditional customs.
In the folklore of certain regions of India, caecilians are feared and reviled, based on the belief that they are fatally venomous. Caecilians in the Eastern Himalayas are colloquially known as "back ache snakes",[64] while in the Western Ghats, Ichthyophis tricolor is considered to be more toxic than a king cobra.[65][66] Despite deep cultural respect for the cobra and other dangerous animals, the caecilian is killed on sight by salt and kerosene.[65] These myths have complicated conservation initiatives for Indian caecilians.[65][64][66]
Crotaphatrema lamottei, a rare species native to Mount Oku in Cameroon, is classified as a Kefa-ntie (burrowing creature) by the Oku. 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.[67]
South American caecilians have a variable relationship to local cultures.
See also
References
- ^ a b Frost, Darrel R. (2019). "Gymnophiona Müller, 1832". Amphibian Species of the World: an Online Reference. Version 6.0. American Museum of Natural History. Retrieved 11 September 2019.
- PMID 22974987.
- ^ .
- ^ PMID 36697827.
- ^ .
- ^ Wilkinson, M. (2012). Caecilians. Current Biology, 22(17), R668-R669.
- ^ An Anatomical Feature of Caecilian Amphibians: The Lengthening of the Body and Organs
- ISBN 978-0-7167-0020-3.
- ISBN 978-3-89432-434-6.
- ^ a b c d Chisholm H, ed. (1911). . Encyclopædia Britannica (11th ed.). Cambridge University Press.
- ^ "Atretochoana eiselti". Natural History Museum. Retrieved 22 February 2012.
- ^ Mader D (June 1995). "Reptilian Anatomy". Reptiles. 3 (2): 84–93.
- ^ The Ecology and Behavior of Amphibians
- ^ Caecilians - Current Biology
- ^ ISBN 978-0-12-178560-4.
- ^ Piper R (2007). Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals. Greenwood Press.
- .
- PMID 20889838.
- ^ Taylor EH (1969). "Skulls of Gymnophiona and their significance in the taxonomy of the group". The University of Kansas Science Bulletin. 48 (15): 585–687.
- ^ S2CID 86301415.
- ISSN 0024-4066.
- PMID 32621800.
- ^ Hearing and Sound Communication in Amphibians
- PMID 18482905.
- PMID 32994727.
- PMID 10398514.
- ISSN 0362-4331. Retrieved 11 March 2024.
- PMID 38452095.
- PMID 38452082.
- ^ Rathor H (5 May 2016). "Kutnjema Mareka Jiv Sarpa Hoina". Katipur. Kantipur News. Archived from the original on 22 February 2017. Retrieved 5 May 2016.
- ^ James D. Gardner, Jean-Claude Rage, The fossil record of lissamphibians from Africa, Madagascar, and the Arabian Plate, Palaeobiodiversity and Palaeoenvironments 96(1):1-52 · March 2016
- ^ "Worm-Like, Limbless Amphibian Known As Caecilians Spotted in Florida For The First Time". Conservation.Reefcause.com. 3 August 2021. Retrieved 4 August 2021.
- ISSN 2332-4961.
- PMID 22357266.
- ^ "New amphibian family found in India". CBC News. Associated Press. 21 February 2012.
- PMID 15336675.
- PMID 20525595.
- PMID 19577653.
- PMID 22328568.
- S2CID 86140137.
- PMID 21723399.
- PMID 24480323.
- ^ "AmphibiaWeb - Amazops amazops". amphibiaweb.org. Retrieved 24 January 2021.
- S2CID 4260251.
- ^ PMID 28630337.
- S2CID 4342438.
- S2CID 205212809.
- S2CID 129144343.
- PMID 20399871.
- PMID 30631641.
- ISBN 978-1-4822-8014-2.
- ^ Caecilians - Cell Press
- S2CID 4327433.
- ^ Vince G (12 April 2006). "'Yummy mummy' worms feed their skin to offspring". New Scientist.
- PMID 18618577.
- PMID 18547909.
- PMID 23483926.
- ^ Quaglia S (18 October 2023). "These Amphibians Have a Taste for Their Mom's Skin". The New York Times. Retrieved 22 October 2023.
- PMID 37189209.
- JSTOR 24908573. Retrieved 15 March 2024.
- JSTOR 24908573. Retrieved 15 March 2024.
- JSTOR 24908573. Retrieved 15 March 2024.
- ISBN 978-0-08-053163-2.
- ^ a b Sathyabhamu, Das Biju, Rachunliu G Kamei, David Gower, & Mark Wilkinson (2009) "Conservation of Caecilians in the Eastern Himalayas Region" Critical Ecosystem Partnership Fund Project Report. pp. 1–22.
- ^ a b c K. Ramachandran & Oommen V. Oommen (August 2008) "Deep-rooted myths and their impact on the population of gymnophionan amphibians among the inhabited areas of Kerala, India" FrogLog v. 88. pp 3–5.
- ^ ISBN 978-0-226-11600-6.
- ^ Doherty-Bone TM, Ndifon R, Gower DJ (2011). "Traditional indigenous perspectives on soil-dwelling vertebrates in Oku, Cameroon, with special reference to the caecilian Crotaphatrema lamottei". Herpetological Bulletin. 116: 19–24.
External links
- Media related to Gymnophiona at Wikimedia Commons
- Data related to Gymnophiona at Wikispecies