Caecilians are limbless amphibians that, to the inexperienced eye, can be quickly misinterpreted for snakes. Though caecilians are just distantly associated to their reptilian cousins, scientists in a research study appearing today (July 3, 2020) in the journal iScience explain customized glands discovered along the teeth of the ringed caecilian (Siphonops annulatus), which have the exact same biological origin and potentially comparable function to the venom glands of snakes. If more research study can verify that the glands consist of venom, caecilians might represent the earliest land-dwelling vertebrate animal with oral venom glands.
Caecilians are strange animals, being almost blind and utilizing a mix of facial arms and slime to browse their underground tunnels. “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,” states senior author Carlos Jared, a biologist and Director of the Structural Biology Lab at the Butantan Institute in São Paulo. “Because caecilians are one of the least-studied vertebrates, their biology is a black box full of surprises.”
“It is while examining the mucous glands of the ringed caecilian that I stumbled upon a never before described set of glands closer to the teeth,” states very first author Pedro Luiz Mailho-Fontana, a post-doctoral trainee in the Structural Biology Lab at the Butantan Institute.
What Mailho-Fontana discovered were a series of little fluid-filled glands in the upper and lower jaw, with long ducts that opened at the base of each tooth. Using embryonic analysis, he discovered that these oral glands stemmed from a various tissue than the slime and toxin glands discovered in the caecilian’s skin. “The poisonous skin glands of the ringed caecilian 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,” states Mailho-Fontana. This marks the very first time glands of this kind have actually been discovered in an amphibian.
Researchers presume that the ringed caecilian might utilize the secretions from these snake-like oral glands to disarm its victim. “Since caecilians have no arms or legs, the mouth is the only tool they have to hunt,” states co-author Marta Maria Antoniazzi, an evolutionary biologist at the Butantan Institute. “We think they trigger their oral glands the minute they bite down, and specialized biomolecules are included into their secretions.
An initial chemical analysis of the oral gland secretions of the ringed caecilian discovered high activity of phospholipase A2, a typical protein discovered in the contaminants of poisonous animals. “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,” states Mailho-Fontana. In truth, the biological activity of phospholipase A2 discovered in the ringed caecilian was greater than what is discovered in some rattlesnakes. Still, more biochemical analysis is required to verify whether the glandular secretions are hazardous.
If future work can confirm the secretions are hazardous, caecilian oral glands might suggest an early evolutionary style of oral venom organs. “Unlike snakes which have couple of glands with a big bank of venom, the ringed caecilian has lots of little glands with small quantities of fluid. Perhaps caecilians represent a more primitive kind of venom gland advancement. Snakes appeared in the Cretaceous most likely 100 million years earlier, however caecilians are far older, being approximately 250 million years of ages,” Jared states.
Very couple of groups of land-dwelling vertebrates have serpent-like bodies, and this research study recommends there may be a connection in between a limbless body strategy and the advancement of a poisonous bite. “For snakes and caecilians, the head is the sole unit to explore the environment, to fight, to eat, and to kill,” states Antoniazzi. “One theory is that perhaps these necessities encourage the evolution of venom in limbless animals.”
Read Fang’tastic: They May Not Be Snakes, however These Amphibians Have a Venomous Bite for more on this research study.
Reference: “Morphological Evidence for an Oral Venom System in Caecilian Amphibians” by Pedro Luiz Mailho-Fontana, Marta Maria Antoniazzi, Cesar Alexandre, Daniel Carvalho Pimenta, Juliana Mozer Sciani, Edmund D. Brodie Jr. and Carlos Jared, 3 July 2020, iScience.
This work was supported by the National Council for Scientific and Technological Development (CNPq), São Paulo Research Foundation (FAPESP), and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brazil (CAPES).