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Caecilians: Unusual, Interesting, and Perhaps Venomous Amphibians

Linda Crampton is a writer and teacher with an honors degree in biology. She loves to study nature and write about living things.

This caecilian is named Ichthyophis kodaguensis and was photographed in India.

This caecilian is named Ichthyophis kodaguensis and was photographed in India.

Interesting and Unusual Amphibians

Caecilians are intriguing animals. They look like worms or snakes, but they are actually amphibians. They live in tropical areas and are often hard to find. The terrestrial ones live underground or in leaf litter. The aquatic species are found in freshwater lakes or streams. Scientists have known for some time that the animals are poisonous. Recent evidence suggests that they may be venomous as well.

A poisonous organism injures other creatures when they eat it or touch it. A venomous one hurts another individual by biting or stinging it. Researchers have found what appear to be venom glands in the mouth of caecilians. They have also found that the secretion from the glands contains chemicals found in snake venom. They haven't yet demonstrated that the secretion kills the caecilian's prey, but the research is continuing.

Some people might think that this female Caecilia pulchraserrana is an earthworm until they notice its eye.

Some people might think that this female Caecilia pulchraserrana is an earthworm until they notice its eye.

Physical Features of Caecilians

Caecilians are a fascinating but poorly studied group of animals. They need to be more widely investigated in order for scientists to reach conclusions about the group as a whole. The facts that have been discovered so far are very interesting, though some of them may apply to only certain species.

External Characteristics

Unlike other amphibians, caecilians have no limbs. Their head bears eyes, nostrils, short tentacles, and a mouth. The mouth contains small teeth that have a needle-like tip. The animals have rings, or annuli, on the surface of their body. This feature makes their body appear to be segmented and sometimes gives the impression that they are earthworms.

Size and Color

Caecilians may be as short as four inches or as long as five feet. If the longer individuals are seen and their annuli are not clearly visible, they can be mistaken for a snake. The animals are often black, brown, or grey and may have yellow or orange patches on their body. Some animals are an attractive blue or purple color and may have pink patches.

Chordate Features

Caecilians are vertebrates while earthworms are invertebrates. Unlike the case in an earthworm, the interior of a caecilian's body is not segmented. In addition, the interior contains organs and structures found in vertebrates (or more specifically, chordates), including a skull and a backbone. Caecilians have no appendicular skeleton (shoulder and arm bones, pelvic girdle and leg bones) because they don't have limbs.

A view of a Bombay caecilian (Ichthyophis bombayensis) that shows one of its tiny white tentacles just above its mouth

A view of a Bombay caecilian (Ichthyophis bombayensis) that shows one of its tiny white tentacles just above its mouth

Sense Organs

Vision

The eyes of a caecilian are often covered by skin. They contain a light-sensitive layer called a retina, as our eyes do. Our retina contains rods and cones. The rods are used at night and provide black and white vision. The cones provide color vision. A caecilian's eyes contain rods but no cones.

Caecilians are thought to be able to tell the difference between light and dark but to be unable to see color or form an image. This may be an area that requires more research. A lens has been found in the eyes of some caecilian species. In our eyes, the lenses focus light rays on the retina, and the optic nerve (which caecilians possess) then sends a signal to the brain. The brain creates an image.

Tentacles

The amphibians have a small tentacle on each side of their body between the eye and the nostril. In the Bombay caecilian shown below, the white tentacle is positioned close to the upper lip. The tentacles detect the presence of certain chemicals.

Hearing

Caecilians have no external ears, but they do have the semicircular canals found in the chordate inner ear and can almost certainly hear some sounds. Some species have components of the middle ear as well.

Other Senses

Sense organs on the skin of the amphibians respond to touch. Taste buds have been found in the pharynx of some species. The animals can probably detect vibrations beyond sound ones and can perhaps detect additional stimuli.

Internal Organs

Like other amphibians and us, the internal organs of caecilians follow the chordate pattern. Despite their narrow, worm-like appearance, the animals have many of the same organs and body structures that we do. There are some variations in the structure and function of caecilian organs compared to the comparable ones in us. In addition, their size and shape are usually different.

The animals have a nervous system consisting of a brain, a spinal cord, and other nerves. They also contain a circulatory system consisting of a heart and blood vessels. As in other amphibians, the heart contains two atria and one ventricle. Our heart has two atria and two ventricles. Caecilians have kidneys for the removal of waste from the blood.

The digestive tract consists of the esophagus, stomach, and intestine. The animals have a liver, gall bladder, spleen, and pancreas. The undigested food is collected in the cloaca. This chamber receives material released by the digestive, excretory, and reproductive tracts and then releases it into the outer environment.

Many caecilians that have been studied have two lungs. The right lung is functional, but the left one is vestigial (reduced in size and nonfunctional). The skin is probably important for gas exchange. A few aquatic caecilians without lungs have been discovered.

Reproduction

The reproductive features of the amphibians are intriguing. As is the case for other features of caecilians, further research is needed to discover more details.

Fertilization and Birth

Fertilization in caecilians is internal. Females have ovaries, oviducts, and a uterus. The male has an appendage called a phallodeum, which he uses to insert sperm from his testes into the female's cloaca.

The females of some species lay eggs and then coil their body around them to protect them. The species are said to be oviparous because they produce eggs that hatch outside the female's body. In other species, the eggs hatch inside the mother and then emerge. The species are said to be ovoviviparous (having eggs that hatch within the body followed by the birth of live young) or simply viviparous (giving birth to live young).

Feeding on the Mother's Skin

In at least some oviparous species, the young animals feed on skin cells of their mother, as shown in the photo above and the video below. The youngsters tear strips of skin from the surface of their mother. The process is known as dermatophagy. Researchers have discovered that the cells in the strips are richer in lipids than normal skin cells, which probably provides nutrition for the youngsters. The mother doesn't seem to suffer from the bites of her offspring and soon produces a new skin layer.

Feeding on the Uterine Lining

Some ovovivparous species have been found to feed in the uterus after hatching and eating the egg yolk. The youngsters eat the lining of the uterus, which nourishes them. The lining is said to be quickly replaced. The uterus secretes a liquid called uterine milk, which also nourishes the young caecilians.

A Possibly Venomous Mouth Secretion

Researchers have discovered sac-like structures at the base of the teeth in both the upper and lower jaws of a caecilian named Siphonops annulatus. They say that the sacs develop from the dental lamina, which is the tissue that produces the teeth. The sacs are in the same location as the venom glands in snakes and are produced from similar tissue.

The researchers have also discovered that the sacs contain glands that produce a secretion containing mucus, lipids, and proteins that act as enzymes. The enzymes are similar to ones commonly found in snake venom and include the following:

  • gelatinolytic and caseinolytic enzymes that break down specific proteins
  • fibrinogenolytic enzymes that break down a protein called fibrinogen, which is involved in the blood clotting process
  • hyaluronidase that breaks down hyaluronic acid
  • phospholipase A2 that breaks down phospholipids, which are an essential component of cell membranes

The scientific name in the quote below represents a South American rattlesnake, a highly venomous species. The rattlesnake is much bigger than a caecilian and would likely inject a larger quantity of venom into its prey.

Phospholipase A2 activity was also detected in all preparations tested and was higher than that detected in Crotalus durissus terrificus venom.

— Pedro Luiz Mailho-Fontana et al, via iScience (a Cell Press journal)

Sending the Secretion into the Prey's Body

Unlike snakes, the species described above has no slots or grooves in its teeth to deliver the secretion into the prey's body. The researchers discovered that when the caecilians in their experiment were about to attack their prey, a viscous liquid appeared around their teeth. In addition, when the scientists gently compressed the jaws of an animal, the viscous liquid also appeared. The liquid is thought to be the secretion produced by the dental glands. It might provide lubrication, but it may have another function. The pressure on the jaws as a caecilian clamps its jaws on its prey may enable the destructive enzymes to enter the animal during a bite.

Further Investigations Are Needed

After exploring the dental glands in S. annulatus, the researchers found the glands in two additional species of caecilians. In Typhlonectes compressicauda, an aquatic caecilian, the glands were present only in the upper jaw. Though the evidence that the animals are venomous is quite compelling, it isn't conclusive. Further research is necessary.

One problem is that the researchers haven't yet demonstrated that the enzymes from the dental glands kill the caecilian's prey or contribute to its death. The potentially dangerous enzymes are apparently present in the amphibian's mouth as it attacks (though this needs to be confirmed), but this doesn't necessarily mean that they hurt the prey. The amount of secretion that enters the prey's body and the effects of its enzymes on the prey are unknown. Some substances are dangerous if they enter an animal at a high concentration but not if they enter it at a low one.

Another situation that the scientists would like to clarify is the specific version of the enzymes that are present in the amphibian's secretion. There are multiple types of gelatinolytic and caseinolytic enzymes, for example. It would be interesting to know which ones are present in the amphibian's secretion and to know how they might affect the prey.

Animals That Are Worth Investigating

I think that caecilians are intriguing animals that are worth investigating. The similarities between their dental gland secretion and the one produced by the venom glands of snakes are interesting. There are still some questions that need to be answered, however. Researchers need to discover whether the dangerous components of the amphibian's secretion are sufficiently concentrated to affect their prey and whether enough of the secretion enters the prey to subdue it. The results of future studies of the order Gymnophiona should be interesting in more ways than one.

References

  • Caecilian information from the San Diego Zoo
  • Gymnophiona entry from the Encyclopedia Britannica (written by a herpetologist)
  • Burrowing behavior of caecilians from the University of Washington
  • Facts about the aquatic caecilian (Typhlonectes natans) from the Detroit Zoo
  • Aquatic caecilians without lungs from the U.S. National Library of Medicine and The Royal Society Publishing (including a description of internal caecilian organs)
  • A new species of skin-feeding caecilian discovered from PLOS ONE
  • Book excerpts about caecilians from ScienceDirect
  • "Morphological Evidence for an Oral Venom System in Caecilian Amphibians" from the iScience journal, Cell Press

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2020 Linda Crampton

Comments

Linda Crampton (author) from British Columbia, Canada on October 09, 2020:

Thanks for the visit, Liza. They are unique animals, as you say. I'm looking forward to future discoveries about them.

Liza from USA on October 09, 2020:

Your article on caecilians drew my attention to this intriguing animal, as I have never heard nor seen. They are unique. I agree that they are worth to be investigated and do more research on them. Thank you for sharing the well informative reading article, Linda.

Linda Crampton (author) from British Columbia, Canada on October 07, 2020:

Hi, Eman. I appreciate your visit and your comment.

Eman Abdallah Kamel from Egypt on October 07, 2020:

Very useful and informative article. Thank you, Linda, for sharing

Linda Crampton (author) from British Columbia, Canada on October 07, 2020:

Hi, Denise. I think the world of nature is fascinating. There's so much to explore and there are so many facts that need to be understood.

I appreciate your comment, as always. Blessings to you.

Miebakagh Fiberesima from Port Harcourt, Rivers State, NIGERIA. on October 07, 2020:

Linda, I agreed. Welcomed.

Denise McGill from Fresno CA on October 07, 2020:

You keep coming up with intriguing creatures I never heard of and didn't know existed. How do you do it? Just when you think you know all there is to know about the world, you find you know nothing at all. It is amazing how much there still is to study. These "giant earthworms" give me the creeps. I wouldn't want to meet one without knowing more about it.

Blessings,

Denise

Linda Crampton (author) from British Columbia, Canada on October 07, 2020:

Thanks for commenting, Fran. I enjoy observing caecilians and exploring their features, but I can understand why some people wouldn't like to get close to one.

Linda Crampton (author) from British Columbia, Canada on October 07, 2020:

Thank you very much for the kind comment, Eric.

fran rooks from Toledo, Ohio on October 07, 2020:

Again Linda, an informing, detailed article about caecilians. I hope I never run into one. Guess I'm squeamish. So much to learn about them yet but they are interesting.

Eric Caunca from Maharlika on October 07, 2020:

I always love your articles. You inspire me to write similarly to your work.

Linda Crampton (author) from British Columbia, Canada on October 07, 2020:

Hi, Peggy. Yes, that video was amazing. I was very happy when I found it! Thank you for the kind comment.

Linda Crampton (author) from British Columbia, Canada on October 07, 2020:

Thank you for the visit and the comment, Ankita. Caecilians are interesting animals to explore.

Linda Crampton (author) from British Columbia, Canada on October 07, 2020:

Hi, Mary. Yes, they can look like very much like earthworms. They are said to live in tropical parts of the world, but there's a lot that is unknown about them. They may have more surprises for us!

Peggy Woods from Houston, Texas on October 07, 2020:

You always provide such fascinating information, and reading your article about Caecilians added to my knowledge today about things in nature that I never knew existed. That video showing the babies eating the skin of their mother was amazing.

Linda Crampton (author) from British Columbia, Canada on October 07, 2020:

Hi, Heidi. Caecilians are curious animals. There's a lot that is unknown about them. I'm glad that that the mouth secretion has been discovered. That's the most interesting discovery about the animals that I've seen in a long time. Thanks for the comment.

Linda Crampton (author) from British Columbia, Canada on October 07, 2020:

Hi, Dora. I like caecilians, even though they can resemble snakes. It will be interesting to discover how dangerous their secretion is. Thanks for the visit and the comment.

Linda Crampton (author) from British Columbia, Canada on October 07, 2020:

It sounds like you had an interesting experience, Miebakagh. Thank you for the visit and the comment.

Linda Crampton (author) from British Columbia, Canada on October 07, 2020:

Hi, Raymond. They do seem to lack some features that are useful for us. It will be interesting to see what else is discovered about them. Thanks for the comment.

Linda Crampton (author) from British Columbia, Canada on October 07, 2020:

Hi, EK. Thanks for the visit. I know caecilians may not be appealing animals for everyone, but I like them.

Ankita B on October 07, 2020:

It was fascinating to know about Caecilians. This was an interesting read.

Mary Norton from Ontario, Canada on October 07, 2020:

This is very interesting. I know more about caecilians now. I hope we get more information on its venom and poison. They look like earthworms. I wonder if the lakes in Canada have it.

Heidi Thorne from Chicago Area on October 07, 2020:

If I would see one without your explanation, I would've said it was a mini snake of some sort. Lesson: Don't go into the wild without Linda as a guide! :)

Thanks for sharing yet another corner of the animal kingdom with us!

Dora Weithers from The Caribbean on October 07, 2020:

Another fascinating creature to study. I hope that I never get close enough to figure out whether or not it's a snake. The fact that it injects venom is enough to make it scary. Thanks for bring us another marvel in nature.

Miebakagh Fiberesima from Port Harcourt, Rivers State, NIGERIA. on October 07, 2020:

Many persons have told me that the cacillians are minor snakes changing from earth worm to snakes! I disagree. I had an encounter with a spices last year while digging ground to fix a poll for a cloth line at the back side of my house where few grass sprout. I would have had took it to the agriculture Dept but I killed it. Cutting the head off prove hard with a shovel, not until I use a knife. The creature appear dark-brown. In my biology lessons, I study worms, frogs, and many other vertebrates and in-vertebrates. But I go further in studing snakes. Thanks for the comparative analysis of the human and the cacillian structure. It is a wonderful study.

Raymond Philippe from The Netherlands on October 07, 2020:

Never heard of Caecilians before (that I can remember). They seem to have missed out on several of the goodies that were handed out like a good hearing and eyesight. Anyway, this was an interesting read and well researched. Thanks for sharing!

EK Jadoon from Abbottabad Pakistan on October 07, 2020:

Well explained article, Alicia. But whenever I see any creature like snake; it makes my hairs stand on end. LOL...

Linda Crampton (author) from British Columbia, Canada on October 06, 2020:

Thanks, Nithya. I always appreciate your visits.

Linda Crampton (author) from British Columbia, Canada on October 06, 2020:

Thank you for the comment, Helna. I think amphibians are interesting organisms.

Nithya Venkat from Dubai on October 06, 2020:

I learned a lot about Caecilians, so much of information well researched and presented. Thank you for sharing.

Helna on October 06, 2020:

Very interesting information about Amphibians. Thanks for sharing. I don't like Amphibians.

Thanks

Linda Crampton (author) from British Columbia, Canada on October 06, 2020:

Thank you very much for such a kind comment, Anupam.

Anupam Mitu from MUMBAI on October 06, 2020:

Oh God So much of interesting information.

I loved watching Discovery and National Geographic for quite long. Reading your article I had the similar experience.

Thank you for this.