Caecilian Amphibian

The caecilians (pronounced as seh-SILL-yens) are a group of amphibians that are limbless, vermiform or serpentine. They look like large worms or slick snakes and live hidden in the ground and in-stream substrates. All the modern caecilians and their closest fossil relatives are grouped as a clade, Apoda, under the larger group Gymnophiona. It also includes more primitive extinct caecilian-like amphibians. They are mostly distributed in the tropics of South and Central America, Africa, and southern Asia and their main diet includes small subterranean creatures such as earthworms. 

Some caecilians are as short as a pencil, while the others are as long as a child. The name “caecilian” is derived from the Latin word caecus, meaning “sightless” or “blind”. 

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Caecilians: Scientific Classification

The Families are:

• Caeciliidae

• Chikilidae

• Herpelidae

• Dermophiidae

• Indothyplidae

• Ichthyophiidae

• Rhinatrematidae

• Scolecomorphidae

• Siphonopidae

• Typhlonectidae

Caecilian Amphibian: Description

The caecilians are the limbless amphibians. The smaller species resemble worms and the larger ones resemble snakes with a length of 1.5 m or 5 ft. Their tail is either short or completely absent. Their cloacae are near the ends of their bodies. The skin is generally smooth and dark, but a few species may have colourful skin. Inside the skin, calcite scales are present. It is because of these scales that the caecilians were thought to be related to the fossil Stegocephalia. Now, it is believed that the development is secondary and the two groups are most likely unrelated. 

Their skin has numerous ring-shaped folds or annuli that partially encircle the body and give them a segmented appearance. Like the other living amphibians, their skin contains glands that secrete a toxin to deter the predators. The skin secretions of the Siphonops paulensis are shown to have hemolytic properties. 

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The caecilians have very small eyes. They can only detect the difference between light and dark. In some species, the eyes are completely covered by skin which is an adaptation suited to a life spent almost entirely underground. They have a strong skull with a pointed stout which is used by them to force their way through soil or mud. In most species, the bones in the skull are reduced in number and are fused with the mouth recessed under the head. Their muscles are adapted for pushing their way through the ground, with the skeleton and the deep muscles acting as a piston inside the skin and outer muscles. This allows the caecilian species 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 loose mud or water, they swim like an eel. 

The limbless amphibian caecilians belong to the family Typhlonectidae. They are aquatic and are the largest of their kind. The representatives of the Typhlonectidae family have a fleshy fin running along the rear section of their bodies, which enhances propulsion in water. They have two sets of muscles for closing the jaw. These are more developed in the most efficient burrowers among the caecilians, and help to keep the skull and jaw rigid. They possess a pair of tentacles, which are located between their eyes and nostrils. These are used for a second olfactory capability, in addition to the normal sense of smell.

The Siphonops annulatus i.e, the ringed caecilian has dental glands that are homologous to the venom glands of some snakes and lizards. The middle ear of the Gymnophiona - caecilian consists of only the stapes and the oval window. Except for one lungless species - the Atretochoana eiselti, all the other caecilians have lungs. They use their lungs as well as skin and mouth for oxygen absorption. Most often, the left lung is much smaller than the right one, which is an adaptation to their body shape. 

Caecilians Life Cycle

The caecilians mate in all seasons and they always practice internal fertilization which is a two to three-hour process. After fertilization, the eggs are deposited either in mud or on land and usually hatch into or are born as immature larvae. About one-quarter of caecilians are hatched from eggs and the rest are born alive. The caecilians, developed oviparously usually hatch as larvae as opposed to adults, in which case undergo metamorphosis to become adults. While the eggs are usually laid on land, most larvae live in the water, where they develop into adults. 

Caecilians: Distribution

The limbless amphibian named caecilian is found in wet, tropical regions of Southeast Asia, India, Bangladesh, Nepal, Sri Lanka, parts of East and West Africa, the Seychelles Islands in the Indian Ocean, Central America, and Northern and Eastern South America. The Geotrypetes are also known as West African caecilians as they are found in the Republic of Guinea-Bissau. The Scolecomorphus or the African caecilians or the buried-eyed caecilians are found in Cameroon in West Africa, and Malawi and Tanzania in East Africa. In South America, they extend through subtropical eastern Brazil as well as into temperate northern Argentina. 

The American range of the caecilian amphibians extends north to southern Mexico. The species of Ichthyophis sikkimensis also known as the Sikkimese caecilians or the Darjeeling caecilians are found in India, Nepal and possibly Bhutan. The species Ichthyophis or the Asian caecilians are found in Southeast Asia, the southern Philippines, and the western Indo-Australian Archipelago. In Southeast Asia, they are found in Java, Borneo, and the southern Philippines.

Caecilian Species: Taxonomy

The name “caecilian” is derived from the Latin word caecus, meaning “sightless” or “blind”. The name dates back to the taxonomic name of the first species Caecilia tentaculata which was described by Carl Linnaeus. There has been a disagreement over the use of the two primary scientific names for the caecilians - Apoda and Gymnophiona. Some specialists use the name Gymnophiona to refer to the "crown group" which includes all present-day caecilians and extinct members of the lineages. 

They use the name Apoda to refer to the total group - all caecilians and caecilian-like amphibians. However, there has always been an argument for the use of the names. 

Wilkinson et al. (2011) divided the caecilians into 9 families and 200 species. After the classification, a tenth caecilian family has been discovered - Chikilidae. 

The classification is based on the definition of monophyly based on morphological and molecular evidence and contains 256 species in 56 genera:

• Rhinatrematidae – 3 genera, 14 species; found in South America

• Ichthyophiidae – 2 genera, 57 species; found in South and Southeast Asia

• Scolecomorphidae – 2 genera, 6 species; found in Africa

• Herpelidae – 2 genera, 10 species; found in Africa

• Chikilidae – 1 genus, 4 species; found in India

• Caeciliidae – 2 genera, 43 species; found in South and Central America

• Typhlonectidae – 5 genera, 14 species; found in South America

• Indotyphlidae – 7 genera, 24 species; found in Seychelles, India, Africa

• Siphonopidae – 5 genera, 28 species; found in South America

• Dermophiidae – 4 genera, 14 species; found in Africa, Central and South America

San Mauro et al. (2014) examined the molecular mitogenomic evidence and modified it to include some more recently described genera such as Amazops.

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Caecilian Species: Evolution

The caecilians have left a very sparse fossil record and hence, we know very little of their evolutionary history. Until 1972, the first fossil, a vertebra -  Paleocene, was not discovered. The other vertebrates having characteristic features unique to modern species were found later in Paleocene and Late Cretaceous sediments. The earliest fossil attributed to a stem-caecilian comes from the Jurassic period. This primitive genus, known as Eocaecilia, had small limbs and well-developed eyes.

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Anderson and co-authors, in the year 2008, suggested that the caecilians arose from the Lepospondyl group of ancestral tetrapods. They may be more closely related to amniotes than to frogs and salamanders, which arose from Temnospondyl ancestors. Numerous groups of the lepospondyls have evolved reduced limbs, elongated bodies, and burrowing behaviours and gradually the modern-day caecilian species took its form. 

Caecilian Species: Diet

The diet of the caecilians is not known. Mostly, the matured caecilians feed on insects and other invertebrates found in their habitat. Their stomach contents consist of 14 specimens of Boulengerula taitana which consists of mostly unidentifiable organic material and plant remains. The caecilians in captivity can be easily fed with the earthworms. 

Caecilians: Surprising Facts

• The caecilians are the only amphibians who have tentacles.

• A few of them are born with short, blunt teeth used to peel off the outer layer of the mother’s thick skin for food - the behaviour called dermatotrophy.

• In the 1990s, the researchers discovered a dead specimen of a caecilian, which had no lungs. Initially, scientists thought that this species might inhabit cold, fast-flowing mountain streams, where the water contains more oxygen, but, last year, they found the lungless caecilian in completely opposite weather - warm, low-lying rivers in the Brazilian Amazon. 

• In February 2012, the team of researchers at the University of Delhi, India announced that they had discovered a new type of caecilian. It includes several species and these amphibians from northeastern India live underground. They vary in colour - from light gray to purple and can grow more than a meter (almost 4 ft) long.