Caecilian
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 "Caecilian" | ||||||||
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Rhinatrematidae |
The Caecilians are elongated, segmented amphibians that belong to the order Gymnophiona or Apoda ("without legs"). They resemble earthworm or snakes in that they lack external limbs.
Amphibians include all the tetrapods (four-legged vertebrates) that do not have amniotic eggs. Despite the lack of external limbs, caecilians, like snakes, are still considered tetrapods because the lack of limbs is considered a derived, secondary characteristic, with the assumption being that they evolved from forms that did have appendages.
The name caecilian means "blind," but most have small eyes. They mostly live hidden in the ground, which makes them the least explored order of amphibians. There are about 200 known caecilian species. There habitat is in tropical and subtropical regions of the world.
Anatomy
Caecilian heads are adapted for burrowing, being strong with highly ossified skulls. All Caecilians share two tentacles at their head, which are probably used for a second olfactory capability in addition to the normal sense of smell based in the nose.
Due to their underground life, the eyes are small and covered by skin for protection, which have led to the misconception that they are blind. However due to the skin cover their visual sense is limited to simple dark-light perception.
Caecilians have degenerate feet, making the smaller species resemble worms, while the larger species with lengths up to 1.5 m resemble snakes. The tail is short and the cloaca is near the end of the body.
The skin of caecilians is smooth and usually dark-matte, but some species have colorful skins. Inside the skin are calcite scales, which suggests that they are related to the fossil Stegocephalia. However the scales are now believed to be a secondary development, and not directly inherited from Stegocephalia. Caecilians are the only amphibians with dermal scales; these scale-like structures are more similar to fish scales than reptile scales. Reptile scales are keratinized folds of skin, whereas caecilian scales are layers of fibers covered by mineralized nodules.
Except for one lungless species—Atretochoana eiselti, only known from a single specimen collected somewhere in South America—all Caecilians have lungs, but also use the skin or the mouth for oxygen absorption. Often, the left lung is much smaller than the right one, an adaptation to body shape that is also found in snakes.
A Swiss researcher Daniel Hofer has found that certain caecilians produce potent skin poisons from specialized poison glands. The poison helps prevent predation and the poison of the bright yellow caecilian of São Tomé Island (Schistometopum thomense) in West Africa kills other animals kept in the same tank within a few days. The chemical contents of caecilian poison have not been well studied. Werner Himstedt has shown that the skin glands of Ichthyophis protects the eggs from germs and fungi.
Distribution
Caecilians are found in most of the tropical regions of South-East Asia, Africa, and South America, except the dry areas and high mountains.
In South America, their distribution extends well into the temperate north of Argentina. They can be seen as far south as Buenos Aires, Argentina, when they are carried by the flood waters of the Parana river coming from farther north.
In Africa, caecilians are found from Guinea Bissau (Geotrypetes) until Northern Zambia (Scolecomorphus). No studies have been made in central Africa, but it is likely that caecilians are found in the tropical rainforests there.
The northernmost distribution is of the species Ichthyophis sikkimensis of Northern India. In South-East Asia, the Wallace-Line is not crossed and they are not found in Australia or the islands in between. Ichthyophis is also found in South China and North Vietnam.
Reproduction
Caecilians are the only order of amphibians that only use internal insemination. The male Caecilians have an external 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, other hatch as larvae. The larvae are not fully aquatic, but spend the daytime in the soil near the water.
Seventy-five percent of the species are viviparous, meaning that they give birth to already developed offspring. The fetus is fed inside the female with special cells of the oviduct, which are eaten by the fetus with special scraping teeth. The egg laying species Boulengerula taitanus, feeds its young by developing a special outer layer of skin, which the young peel off with similar teeth.
Ichthyophis is oviparous and is also known to show maternal care.
Origin of the name
The name Caecilian derives from the Latin word caecus = blind, referring to the small or sometimes non-existing eyes. The name dates back to the taxonomic name of the first species described by Carolus Linnaeus, which he gave the name Caecilia tentaculata.
The taxonomic name of the order, Gymnophiona, derives from the Greek words γυμνος (gymnos, naked) and οφις (ophis, snake), as the caecilians were originally thought to be related with snakes. This order is sometimes called Apoda, which means "without legs."
Taxonomy
Gymnophiona is one of three extant (living) orders of amphibians, the others being Anura (or Salientia), which includes frogs and toads, and Caudata (or Urodela), which includes salamanders and newts, among others.
Taxonomically, the caecilians commonly are divided into 5 or 6 families. The species numbers are approximate and some of these species are identified on the basis of only one specimen. It is likely that not all species have been described yet, and that some of the species described below as different may be combined into one species in future reclassifications. The six families are:
- Beaked Caecilians (Rhinatrematidae) - 2 genera, 9 species
- Fish Caecilians (Ichthyophiidae) - 2 genera, 39 species
- Indian Caecilians (Uraeotyphlidae) - 1 genus, 5 species
- Tropical Caecilians (Scolecomorphidae) - 2 genera, 6 species
- Aquatic Caecilians (Typhlonectidae) - 5 genera, 13 species
- Common Caecilians (Caeciliidae) - 26 genera, 99 species
ReferencesISBN links support NWE through referral fees
- Werner Himstedt, Die Blindwühlen, ISBN 3894324341 (German)
- San Mauro, Diego and David J. Gower, Oommen V. Oommen, Mark Wilkinson and Rafael Zardoya (November 2004). Phylogeny of caecilian amphibians (Gymnophiona) based on complete mitochondrial genomes and nuclear RAG1. Molecular Phylogenetics and Evolution 33: 413-427.
- San Mauro, Diego and Miguel Vences, Marina Alcobendas, Rafael Zardoya and Axel Meyer (May 2005). Initial diversification of living amphibians predated the breakup of Pangaea. American Naturalist 165: 590-599.
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