Study Finds the First Evidence of Venom Glands Similar to Snakes in Caecilian Amphibians

A new study discovered the first evidence of the presence of venom glands similar to snake venom glands in caecilian amphibians. The study was published last July 3 in the iScience journal.

Caecilian is a type of limbless amphibians that can easily be mistaken for a snake. Although they are distantly related to reptiles, researchers have found specialized oral glands that are located along with a caecilian's teeth. This species is Siphonops annulatus, the ringed caecilian. The organ shares a biological origin with those present in snakes, and possibly also has the same function as snakes' venom glands.

More research is needed to confirm if the glands secrete venom; if they do, then the caecilians could be considered the oldest terrestrial vertebrates to having an oral venom gland.

Peculiarly, caecilians are almost blind; they combine the use of facial tentacles with slime to successfully navigate the soil and tunnels under the ground.

Senior author and Structural Biology Laboratory, Butantan Institute, São Paulo director and biologist Carlos Jared said that there are two kinds of secretions made by the animal: the first is mostly in its tail, which produces a toxin, and the second is in the head which produces mucus used in crawling. A lot of the details of caecilian biology is unknown.

First author and Structural Biology Lab postdoctoral student Pedro Luiz Mailho-Fontana said that when he examined the ringed caecilian's mucous glands, he accidentally found glands never described before.

Mailho-Fontana discovered a series of small glands filled with fluid at both upper and lower jaws, which were connected to long ducts which opened at each tooth's base.

Upon embryonic analysis, Mailho-Fontana found how the glands came from a tissue apart from that which produced the poison and slime glands on the skin. The ringed caecilian's skin glands had an epidermal origin, while the oral glands originated from dental tissue, which is the same origin of reptile venom glands. This is the first time that this kind of gland was found in amphibians.

The secretions are suspected to be used for incapacitating prey. Butantan Institute evolutionary biologist Marta Maria Antoniazzi says that since caecilians are limbless, their mouth is their sole tool for hunting. This could be true unless some caecilians can constrict prey, as some snakes do.

Antoniazzi believes that the oral glands are activated upon biting down the prey. The chemical composition of the secretions was found to have a high activity from phospholipase A2, a protein commonly found in venomous animal toxins. It is present in the venom of wasps, bees, and numerous reptiles.

Surprisingly, phospholipase A2's biological activity in Siphonops annulatus was greater than that in some species of rattlesnakes. More biochemical analyses are required to confirm the toxic nature of the secretions.

Verification of the toxicity of these secretions may point to a relatively early evolutionary development of venom-secreting oral organs. Caecilian venom glands may be a primitive form of venom gland. Snakes arrived during the Cretaceous about 100 million years in the past, but caecilians appeared 150 million years earlier than snakes.

The study suggests that a connection might exist between being limbless and having venomous oral glands. According to Antoniazzi, caecilians, and snakes only have their head to explore, fight, kill, hunt, and eat. Such a body plan may encourage venom evolution.