Researchers have discovered snake-like venom glands in an amphibian for the first time – along the teeth of the little studied caecilian.
If further research can confirm that the glands do indeed contain venom, they say, the finding would push the evolution of a venom system in vertebrates back to 150 million years before snakes, though in a more primitive form.
Snakes, spiders and scorpions protect themselves by injecting venom using teeth, stingers and thorns. Amphibians, on the other hand, are not considered venomous; rather they protect themselves passively with poison glands on their skin that make predators sorry they tried to eat them.
Caecilians are limbless amphibians and in Brazil are actually known as blind snakes. They have been around for 250 million years, with 214 species strewn across tropical South America, Africa, Seychelles, India and South Asia, but relatively little is known about them as they live underground.
They require skill and strength to collect, according to Carlos Jared, from Brazil’s Butantan Institute, who has long been intrigued by their peculiar characteristics.
“Studies on caecilians can lead to very interesting discoveries,” he says, “which basically reflects the high degree of creativity that nature offers through adaptive processes leading to the conquer of new environments.”
These include loss of limbs, stretching of the body, tentacles, internal fertilisation and other adaptions to their underground burrows.
Jared’s group has a long-standing relationship with the Cocoa Research Centre with a shade-grown cocoa plantation in Brazil’s Atlantic Rainforest, giving them access to an ecological hotspot of caecilians.
Previously, they discovered that, while other amphibians have poison glands all over their body, a caecilian’s glands are clearly segregated at each end.
They have a large concentration of mucous glands in their head to help them slide along through the soil and an “amazing poison accumulation” in their tail, which Jared says is “formed by very large glands with ducts able to quickly direct poison to the surface”.
This makes sense, he adds, as the tail is unprotected inside a tunnel and likely evolved to poison a predator creeping up on it from behind or to block the entrance to a hastily formed burrow.
Probing further into the head morphology of the ringed caecilian (Siphonops annulatus), his colleague Pedro Mailho-Fontana noticed mucous glands around the lips that, although similar to those in the skin of the head, had clear differences.
Notably, the ducts were inside the mouth, and embryological evidence showed they originated from dental tissue.
“Incredibly, the origin of these caecilian glands, in fact, is very similar to that of the snake venom glands,” says Jared, “which also come from the dental lamina.”
Biochemical analysis revealed that the glands’ secretions contain proteins including phospholipases A2, enzymes commonly found in the venom of other animals, including snakes.
The venom needs to be further investigated but Jared speculates that caecilians’ dental gland secretion likely enables them to both poison and lubricate their prey, making their dinner easier to ingest.
So, these clever little quirks of nature appear to have evolved passive and active forms of defence – and attack, in the case of feeding on prey including worms, termites, frogs and lizards.
“From this study we can say that Amphibia class has acquired a special status in the study of toxicology,” says Jared.
The study also involved researchers from Brazil’s Sao Francisco University and Utah State University, US. The findings are published in the journal iScience.
Natalie Parletta is a freelance science writer based in Adelaide and an adjunct senior research fellow with the University of South Australia.
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