Researchers Discover Venomous Glands in Caecilians, the Legless Amphibians Resembling Snakes
In a groundbreaking study published on July 3 in the journal iScience, researchers have described the presence of special glandular structures along the teeth of the ringed caecilian (Siphonops annulatus), exhibiting biological origins and functions similar to the venom glands found in snakes. If further investigations confirm that these glands contain venom, caecilians may become the oldest terrestrial vertebrates known to possess venomous glands in their mouths.
Siphonops annulatus curled up on a forest floor. (Photo: Sci Tech Daily).
Caecilians are peculiar creatures, nearly blind, and they use a combination of facial tentacles and mucus to navigate through underground burrows. “This species secretes two types of compounds; the tail secretes venom while the head produces mucus that aids in their movement underground,” said biologist Carlos Jared, director of the Structural Biology Laboratory at the Butantan Institute in São Paulo, Brazil. “Since caecilians are among the least studied vertebrates, their biological structure is a black box filled with surprises.”
Pedro Luiz Mailho-Fontana, a postdoctoral researcher in the Structural Biology Laboratory and the lead author of the study, found a series of small glands filled with fluid in the upper and lower jaws of the caecilian, along with long ducts at the base of the teeth. Through embryonic analysis, Mailho-Fontana discovered that the glands in the mouth originated from tissue distinct from the venom and mucus glands on the skin of caecilians.
Mailho-Fontana and colleagues suspect that caecilians might utilize secretions from the mouth glands to immobilize prey. Given that caecilians lack limbs, their mouths are their only tool for hunting. According to Marta Maria Antoniazzi, an evolutionary biologist at the Butantan Institute and a co-author of the study, caecilians may activate the mouth glands when biting and integrate specialized biomolecules into their secretions.
Preliminary chemical analysis of secretions from the mouth glands of caecilians revealed significant activity of phospholipase A2, a protein commonly found in animal venoms, with even higher levels than some rattlesnake species. This suggests that caecilians may represent a more primitive form of venom gland evolution. Snakes emerged during the Cretaceous period around 100 million years ago, but caecilians date back around 250 million years.
This also indicates that it represents a more primitive evolutionary form of venom glands. Scientists are still unclear about how the mucus from these venomous glands operates and are awaiting further chemical analyses.
Caecilians Are the Only Amphibians with Internal Fertilization
The male caecilian possesses a penis-like organ that is inserted into the female’s cloaca during mating, a process that lasts from two to three hours. Approximately 25% of caecilians are oviparous (egg-laying), while 75% are ovoviviparous (giving birth to live young); the eggs they produce are crystal-clear. Similar to snakes, these creatures also use their mouths to hunt, with earthworms, mollusks, small snakes, frogs, and lizards being part of their diet.
