Biologists found oral glands in ringed caecilians – serpent-like amphibians related to frogs able to grow up to 17 inches (43cm) in length. Researchers already understood caecilians have poisonous tails and emit a mucous-like lubricant that enables them to quickly dive underground to escape predators.
But biologists have now discovered tiny fluid-filled oral glands in the upper and lower jaw of the ringed caecilian, which was discovered in Brazil, to incapacitate its prey.
To learn at least one can inflict injury from its mouth is extraordinary
Professor Edmund ‘Butch’ Brodi
Edmund ‘Butch’ Brodi, emeritus professor at Utah State University Department of Biology, said: “We think of amphibians—frogs, toads and the like—as basically harmless.
“We know a number of amphibians store nasty, poisonous secretions in their skin to deter predators.
“But to learn at least one can inflict injury from its mouth is extraordinary.”
The team reported in 2018 the species secreted substances from skin glands at both ends of its snake-like body.
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Concentrated at the head and extending the length of the body, the creature emits a mucous-like lubricant that enables it to quickly dive underground to escape predators.
At the tail, caecilians have glands armed with a toxin, able to act as a last line of chemical defence by blocking a hastily burrowed tunnel from hungry hunters.
Professor Brodie added: “What we didn’t know is these caecilians have tiny fluid-filled glands in the upper and lower jaw, with long ducts that open at the base of each of their spoon-shaped teeth.”
His research colleague Pedro Luiz Mailho-Fontana, noticed the never-before-described oral glands.
Using embryonic analysis, Mailho-Fontana, first author of the paper, discovered the glands — dubbed ‘dental glands’ – originated from a different tissue than the slime and poison glands found in the caecilian’s skin.
He said: “The poisonous skin glands form from the epidermis, but these oral glands develop from the dental tissue, and this is the same developmental origin we find in the venom glands of reptiles.”
The researchers concluded caecilians, equipped with no limbs and only a mouth for hunting, activate their oral glands when they bite down on prey, including worms, termites, frogs and lizards.
The team does not yet understand the biochemical composition of the fluid held in the oral glands.
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Professor Brodie added: “If we can verify the secretions are toxic, these glands could indicate an early evolutionary design of oral venom organs.
“They may have evolved in caecilians earlier than in snakes.”
Carlos Jared of the Butantan Institute in São Paulo, and senior author, said: “Because caecilians are one of the least-studied vertebrates, their biology is a black box full of surprises.
“These animals produce two types of secretions – one is found mostly in the tail that is poisonous, while the head produces a mucus to help with crawling through the earth.”
A preliminary chemical analysis of the oral gland secretions of the ringed caecilian found high activity of phospholipase A2, a common protein found in the toxins of venomous animals.
Mailho-Fontana said: “The phospholipase A2 protein is uncommon in non-venomous species, but we do find it in the venom of bees, wasps, and many kinds of reptiles.”
The biological activity of phospholipase A2 found in the ringed caecilian was higher than what is found in some rattlesnakes.
However, more biochemical analysis is needed to confirm whether the glandular secretions are toxic.
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