Caecilians are limbless, worm-like (vermiform) amphibians native to the tropical regions of South and Central America, Africa, and southern Asia. Their name stems from the Latin word caecus, meaning ‘blind,’ referring to the extremely small, almost vestigial, and non-existent eyes.
Their dark-colored cylindrical-shaped bodies are complemented with a tough skull. These amphibians are members of the order Gymnophiona, one of the living amphibians, along with Anura (frogs) and Urodela (salamanders). Gymnophiona, being a crown group, consists of all modern caecilians and the descendants of their last common ancestor.
More than 220 extant species are in this group, classified into 10 families. They usually prefer hiding in soil or streambeds and are well-adapted for burrowing.
They vary greatly in size, with the largest Caecilia thompsoni being nearly 5 feet (2.4 meters) long, while the smallest Idiocranium russell is only 3.5 inches (90 millimeters).
Their limbless, cylindrical bodies with segmented skin and reduced or absent eyes give them a worm-like appearance in the smaller species, while the larger ones look more like snakes. However, some primitive species belonging to the genus Ichthyophis retain vestigial limbs, while in Typhlonectes compressicauda, limb buds are observed only during embryonic development.
The rear orifice, or the cloaca in caecilians, is present near the posterior end of their bodies, followed by a short tail. Their skulls, skin, and sensory organs are well-adapted for their burrowing lifestyle.
Unlike frogs and salamanders, caecilians have compact brains and a sharp, bullet-like snout (overhanging the mouth) that helps them navigate through soil or mud. There are a few large openings between the plate-like cranial bones. Many of these bones are fused in caecilians. For example, the maxilla and palatine bones have fused into a maxillopalatine, whereas the nasal and premaxilla bones sometimes merge into nasopremaxilla. The caecilian families are often differentiated based on the presence or absence of some skull bones, such as the septomaxillae, prefrontals, or a postfrontal-like bone surrounding the eye socket. The skull is encased in a compound bone called the os basale.
The lower jaw comprises two characteristic components: the anterior, tooth-bearing pseudodentary, and the posterior pseudoangular (bearing the jaw joint and muscle attachments). The main row of teeth at the margin of the pseudodentary is backed by an inset of about twenty teeth.
Most caecilians (except primitive ones) possess two sets of muscles for closing the jaw. The adductor muscles insert into the upper edge of the pseudoangular, whereas the abductor muscles enter the rear edge of the pseudoangular. While the adductors pull the jaws upwards and forward, the abductors close the jaw by pulling backward and downwards (antagonistic).
Although caecilians’ skin is superficially dark and smooth, it has an underlying layer of calcite scales. These scales are usually absent in the families Scolecomorphidae and Typhlonectidae except for the species Typhlonectes compressicauda, which contains tiny scales in the hind region of its body.
The skin also bears repeated ring-shaped folds, or annuli, that give these amphibians a segmented appearance.
Scientists have forever debated the naming of the Caecilian order. While some use the term Apoda to refer to this crown group (all modern caecilians and extinct members), others prefer addressing it as Gymnophiona (all caecilians and caecilian-like amphibians more closely related to modern groups than to frogs or salamanders). Since the name Apoda is also used for groups of fishes and sea cucumbers and refers to a moth genus, this term is discarded for ambiguity.
In 2011, Wilkinson et al. initially classified all caecilians into nine families containing nearly 200 species; however, in 2012, a tenth family of NorthEast Indian caecilians (Chikilidae) was added. The present classification scheme establishes the monophyly of caecilians based on morphological and molecular evidence, refuting the paraphyly of the family Caeciliidae in previous classifications.
Currently, there are 219 species of caecilians listed under 33 genera and 10 families.
The lack of sufficient fossil records makes it difficult for scientists to study caecilians. However, certain anatomical features, like pedicellate teeth, suggest that frogs, salamanders, and caecilians are most closely related. While frogs and salamanders show similarities to dissorophoids, a group of extinct amphibians in the order Temnospondyli, caecilians are believed to have descended from extinct lepospondyl or stereospondyl amphibians. Based on the few fossils recovered, the timeline of caecilian evolution could be sketched as follows.
These amphibians are indigenous to humid, tropical regions of Southeast Asia (India, Bangladesh, Nepal, and Sri Lanka), parts of East and West Africa, the Seychelles Islands in the Indian Ocean, and Central and South America.
Exceptions
Wild caecilians are usually carnivorous, feeding on small invertebrates like earthworms, termites, lizards, moth larvae, and shrimp. However, some are opportunistic feeders, consuming newborn rodents, salmon eggs, and veal. These amphibians also prey on vertebrates such as frogs, snakes, lizards, and small fishes.
While pushing through the ground, the trunk muscles move the vertebral column like a piston. When the outer layer of body muscles contracts, the coelom is squeezed, producing a hydrostatic force that straightens the body. This mechanism allows the hind part of the body to remain fixed, pushing the head forward, followed by the rest of the body waving up. When underwater, this mechanism also helps them swim like eels.
They are the only amphibians that inseminate females internally using a long tube-like intromittent (sperm-releasing) organ, the phallodeum. The males insert this organ into the female’s cloaca for around two to three hours, following which they release their sperm into her body.
After the egg fertilizes internally, it gives rise to the embryo, which continues developing inside it. About 25% of caecilians lay eggs (oviparous) in terrestrial nests and guard them after laying. While most species hatch as larvae, some become completely developed (metamorphosis) during hatching. The larvae can live on land and in water (amphibious), spending most of their daytime in moist soil around water bodies. Some larvae, like those belonging to the Typhlonectes, are born with conspicuously large external gills, which they shed almost immediately.
The remaining 75% of caecilians give birth to live young (viviparous). In this case, the fetus develops inside the oviduct, nutritioned by its lining cells.
The mothers of Boulengerula taitana have evolved a unique parental care strategy. They develop a highly nutritious layer of skin enriched with fats and protein, which the young peel off (maternal dermatophagy). They consume the skin only at night every three days and grow up to 10 times their weight in a week. Congo caecilian (Herpele squalostoma) mothers pass beneficial microbiomes (bacteria and fungi) down to their offspring (vertical transmission) through this specialized form of care.
The females of some species, like Siphonops annulatus, provide lactation to the young by producing a milk-like substance rich in fats and carbohydrates from their cloaca. When the young suckles on the mother, the hypertrophied glands in the oviduct epithelium (homologous to mammary glands) produce this nutritious substance similar to mammalian milk. The young grow rapidly in their first week, consuming such a diet.
Although snakes are considered the primary predators of caecilians, hawks, turtles, spiders, and ants also feed on these amphibians.
The IUCN Red Data List contains around 172 caecilian species, of which around 114 are data deficient (DD). Of the remaining, Grandisonia brevis is Endangered (EN), while Boulengerula niedeni is Critically Endangered (CR) due to increasing habitat loss.
In some areas, humans mistake them for snakes, accidentally killing them for safety.
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