Smithsonian (EUA) online

This Worm-Like Amphibian May Pack a Venomous Bite

Publicado em 09 julho 2020

Smooth-bodied, legless amphibians called caecilians look like giant earthworms with mouths full of sharp teeth, and, according to new research, they may be the only amphibians known to possess a venomous bite, reports Katherine J. Wu for the New York Times.

The nearly 200 known species of caecilians are found in the tropics the world over and are so adapted to their underground lifestyle (most are burrowers but some are aquatic) that the tiny eyes of some species are entirely covered by skin. Some are just a few inches long, but the giant caecilian of Colombia (Caecilia thompsoni) can reach lengths of up to five feet, reports Jason Bittel for National Geographic. Because these strange creatures mainly live underground, they are seldom seen and poorly understood.

The new paper, published last week in the journal iScience, describes glands found inside the mouths of caecilians that appear to secrete saliva suffused with venomous enzymes, according to National Geographic. This would coat the creatures’ fangs with venom, a delivery system also seen in venomous lizards such as the gila monster (Heloderma suspectum) but different from the hypodermic needle-style fangs of rattlesnakes and cobras, reports Christie Wilcox for Science News. Yet just like in snakes, the caecilian’s toxin-producing glands arise from dental tissue.

A preliminary chemical analysis indicated that saliva samples from two ringed caecilians (Siphonops annulatus) contained enzymes that belong to a group called A2 phospholipase that is present in the venom of wasps, scorpions and snakes, per National Geographic. The researchers studied the physical structure of the glands in four euthanized specimens and took an even closer look at two of the specimens with an electron microscope. Carlos Jared, an evolutionary biologist at the Butantan Institute in São Paulo, Brazil and co-author of the study, tells National Geographic that more analysis is required to confirm that the saliva is venomous.

Scientists categorize animals as venomous or poisonous based on whether a toxin is actively injected (venom) or whether the victim has to swallow, inhale or absorb it (poison) to experience the toxin’s ill effects. Many species of snake and a few lizards use venomous bites to subdue their prey, but until now amphibians, with two potential exceptions, were only known to deploy poison secreted through glands in their skin to deter predators.

Caecilians evolved some 100 million years before snakes, and if the legless amphibian’s bite is indeed venomous it could make them the oldest known venomous creatures on Earth, according to the New York Times.

Marta Antoniazzi, an evolutionary biologist at the Butantan Institute and co-author of the study, tells Science News that the superficial similarities between snakes and caecilians may actually help explain why both groups evolved a venomous bite. “We think it has to do with this fact that they have similar bodies,” she says. Without arms or legs to help bring down prey, caecilians and snakes may have both needed to find another way to kill.

But Shab Mohammadi, an evolutionary biologist at the University of Nebraska-Lincoln who wasn’t involved in the study, tells the New York Times more study is needed to determine whether the caecilian’s spit is toxic and capable of taking down the insects and worms they prey upon.

Kevin Arbuckle, an evolutionary biologist at Swansea University who also was not involved in the study, tells National Geographic that the enzymes the study found in caecilians’ glands don’t necessarily imply toxicity. He says animals can possess A2 phospholipase enzymes in their saliva without being venomous.

The authors tell Science News they are working on follow-up studies of the glands and their secretions to help determine whether these mysterious, slithering amphibians earn their venom bona fides.