A team of scientists from Brazil and the United States discovered the specialized oral glands in the jaws of the ringed cecilia (Siphonops annulatus), a snake-shaped amphibian related to frogs and salamanders.
The ringed cecilia (Siphonops annulatus) Image credit: Carlos Jared.
Neither snakes nor worms, the cilia (order Gymnophiona) are limbless amphibians found in the tropical climates of Africa, Asia, and America.
These creatures are aquatic or live in burrows of their own creation. They are nearly blind and use a combination of slime and facial tentacles to navigate their underground tunnels.
Due to their distribution and excavation habits, the Cecilias consist of one of the least known vertebrate groups.
“We think of amphibians (frogs, toads, and the like) as basically harmless,” said Professor Edmund ‘Butch’ Brodie, Jr. of Utah State University, co-author of the study.
“We know that several amphibians store unpleasant and poisonous secretions in their skin to deter predators. But knowing that at least one can inflict mouth injuries is extraordinary. “
“These animals produce two types of secretions: one is found mainly in the tail which is poisonous, while the head produces mucus to help crawl across the ground,” added lead author Dr. Carlos Jared, Director of the Laboratory of Structural Biology 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.”
“Examining the ringed cecilia’s mucous glands, I stumbled upon a never-before-described set of glands closer to the teeth,” said first author Dr. Pedro Luiz Mailho-Fontana, a postdoctoral researcher at the Structural Biology Laboratory of the Butantan Institute.
In the study, the researchers analyzed the morphology of the ringed Cecilian head and found a series of small, fluid-filled glands in the upper and lower jaw, with long canals that opened at the base of each tooth.
Using embryonic analysis, they discovered that these oral glands originated from a different tissue than slime and the poisonous glands found in the skin of the Cecilia.
“The poisonous glands in the skin of the ring-shaped zillies of the epidermis, but these oral glands develop from dental tissue, and this is the same source of development that we find in the poisonous glands of reptiles,” he said. Dr. Mailho-Fontana.
“This is the first time that glands of this type have been found in an amphibian.”
Structure of the upper jaw and lower jaw of the ringed cecilium (Siphonops annulatus): (A) the ringed cecilium; (B) upper jaw; (C) lower jaw; (D) head after partial skin corrosion showing digitally enhanced teeth related glands in green; (E) section of the upper labial region showing the glands and glandular ducts. Black arrows (row of outer teeth); white arrow (row of internal teeth); arrowheads (cavities that accommodate teeth when the mouth is closed). Abbreviations: A – tongue, S – skin, E – eye, Tentacle, Oe – oral epithelium. Image credit: Mailho-Fontana et al, doi: 10.1016 / j.isci.2020.101234.
Scientists think that ringed cecilium can use the secretions of these snake-like oral glands to incapacitate their prey.
“Since the Cecilias don’t have arms or legs, the mouth is the only tool they have to hunt,” said co-author Dr. Marta Maria Antoniazzi, an evolutionary biologist at the Butantan Institute.
“We believe they activate their oral glands the moment they bite, and specialized biomolecules are incorporated into their secretions.”
In a preliminary chemical analysis of ringed cecilia oral gland secretions, the authors found high activity of phospholipase A2, a common protein found in toxins from poisonous animals.
“The protein phospholipase A2 is rare in non-poisonous species, but we find it in the venom of bees, wasps, and many types of reptiles,” said Dr. Mailho-Fontana.
In fact, the biological activity of phospholipase A2 found in this species was greater than that found in some rattlesnakes. Still, more biochemical analysis is needed to confirm whether the glandular secretions are toxic.
If future work can verify that secretions are toxic, the Cecilian oral glands may indicate an early evolutionary design of the organs of oral venom.
“If we can verify that the secretions are toxic, these glands could indicate an early evolutionary design of the organs of the oral venom,” said Professor Brodie.
“They may have evolved in the cilia before the snakes.”
“Unlike snakes that have few glands with a large bank of venom, ringed cecilia has many small glands with small amounts of fluid,” said Dr. Jared.
Perhaps the caecilians represent a more primitive form of evolution of the poisonous glands. Snakes appeared in the Cretaceous probably 100 million years ago, but the cecilias are much older and are approximately 250 million years old. “
The discovery is reported in an article in the magazine. iScience.
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Pedro Luiz Mailho-Fontana et al. Morphological evidence of an oral venom system in Cecilio amphibians. iScience, published online July 3, 2020; doi: 10.1016 / j.isci.2020.101234
