Amphibians are known for their skin rich in glands containing toxins employed in passive chemical defense against predators, different from, for example, snakes that have an active chemical defense, injecting their venom into the prey. A new study has shown that amphibian caecilians, including species from the basal groups, have venom glands in their mouth besides having poisonous cutaneous glands as other amphibians do.
These venom glands are located at the base of the teeth and develop out of the dental lamina, the tissue that typically gives rise to teeth, as is the case with snakes’ venom glands.
Pedro Luiz Mailho-Fontana, first author of the paper and a postdoctoral intern at Butantan Institute with a scholarship from São Paulo Research Foundation – FAPESP said, “We were analyzing the mucus glands in the skin of the animal’s head, which it uses to burrow down into the soil when we discovered these structures.”
In 2018, scientists showed that caecilians have mucus glands and many poison glands in the tail’s skin that acts as a passive defense against predators. This system, which is also found in frogs, toads, and salamanders, poisons predators when they bite caecilians.
In this new study, scientists found that caecilians can be venomous too. The term venomous is referred to as organisms that bite or sting to inject toxins, such as snakes, spiders, and scorpions. In contrast, poisonous refers to organisms that deliver toxins when touched or eaten.
In these caecilians, the secretion released by the glands also serves to lubricate a prey so that it is easier to swallow.
Carlos Jared, a researcher at Butantan Institute and principal investigator for the study, said, “Snakes have pouches to accumulate venom, which they inject through fangs when muscles squeeze the pouches. In rattlesnakes and pit vipers, for example, the teeth are hollow like hypodermic needles. In caecilians, gland compression during biting releases the venom, which penetrates the puncture wound. The same goes for lizards like the Komodo dragon and Gila monster.”
In this study, scientists evaluated the morphology of the head of the South American caecilian Siphonops annulatus. They found a series of tooth-related glands whose secretion composition was biochemically examined.
Analysis of these glands’ origin shows that the secretion released from the animal’s mouth while it is biting contains phospholipase A2. Phospholipase A2 is an enzyme commonly found in the venom of bees, wasps, and snakes.
This enzyme is found to be more active in caecilians than in rattlesnakes.
Scientists are now planning to conduct tests using molecular biology techniques to characterize caecilians’ dental gland secretion more precisely and confirm that it is venomous. In the future, they may test any proteins they find to explore possible biotechnological applications such as drug development.
Marta Maria Antoniazzi, also a researcher at Butantan Institute and a co-author of the study, said, “More than new data about caecilians, the study offers important information regarding the evolution of amphibians and reptiles. For snakes and caecilians, the head is the only tool for exploring the environment, fighting, eating, and killing. This may have fueled evolutionary pressure for these limbless animals to develop venom.”
- Pedro Luiz Mailho-Fontana et al, Morphological Evidence for an Oral Venom System in Caecilian Amphibians, iScience (2020). DOI: 10.1016/j.isci.2020.101234