Survival Story: The Unlikely Amphibian That Defied Its “Apocalypse”

Caecilians are unique limbless amphibians that bear a wormlike or snake-like form. Credit: Marco Mancuso

Scientists have unveiled the remarkable venom immunity in caecilians, a group of legless amphibians. This immunity is thought to be an evolutionary development to resist venomous elapid snakes, giving an intriguing look into the evolutionary dance between predators and prey.

A global group of researchers has exposed an extraordinary resistance to snake venom in a surprising species – the limbless amphibians known as caecilians.

The study was headed by Associate Professor Bryan Fry of the University of Queensland. He claimed that their research serves as a robust model to illustrate the basic evolutionary principle of predator-prey relationships.

“Through our study, we have provided a classic case of how one predatory factor can initiate an evolutionary chain reaction where the same resistance strategy evolves independently in various lineages of a species,” stated Dr. Fry.

Caecilians predominantly lead secretive lives buried in soil or streambeds, making them some of the least recognized amphibians. Credit: Marco Mancuso

Elapid Snakes: A Predatory Force

“In this context, the main predatory force was the emergence of the elapid snakes, such as cobras and coral snakes, defined by the evolution of a new venom delivery method through their hollow, fixed, syringe-like fangs.

“Even though they are quite elusive, caecilians resemble worms in their movement and speed and fell easy prey to cobras and other snakes, which used their fangs to immobilize them and consume them later.

“The level of devastation was so severe that elapids were virtually feasting on caecilians, leading to the rapid proliferation of elapid snakes across Africa, Asia, and the Americas.

“The caecilian’s knack to persist and adapt in the face of these pressures is akin to a film narrative – the survivors of an apocalypse fighting back by altering their chemical makeup.”

Study Approach and Discoveries

The researchers investigated caecilian species from all known families worldwide, including species in the Seychelles islands untouched by elapid snakes.

The study’s primary author, Marco Mancuso from the Amphibian Evolution Lab at Vrije Universiteit Brussel, explained that the research required using tissue collections to sequence a part of the neuromuscular receptor in caecilians targeted by toxins in snake venom.

“We demonstrated that the immunity to elapid snake venom neurotoxins has evolved at least 15 times – an unparalleled feat,” stated Mr. Mancuso.

“One fascinating validation of our theory was that the caecilians in the Seychelles islands were not immune to snake venom, which aligns with the fact that elapid snakes never reached those islands.

“It’s a remarkable example of survival of the fittest under such intense selection pressure, where those less sensitive to the venom and those with mutations that made them entirely immune were the ones who repopulated the earth post the elapid snake invasion.”

Evolutionary Defense Tactics

According to Dr. Fry, caecilians managed to achieve this unprecedented venom immunity through three different biological methods.

“One method is setting up a sort of barrier that obstructs the toxins from reaching receptors that would ordinarily trigger a lethal response,” he stated.

“Another method of resistance is modifying the physical form of the receptor. As the toxins have evolved to be like keys that fit into the lock-like receptor, altering the shape means the toxin no longer fits.

“Lastly, caecilians employ an electromagnetic ‘weapon’ which reverses the charge during this toxin-receptor interaction.

“The positive-to-positive charge repulsion grows exponentially the closer the objects come together, similar to trying to push two magnets together.

“This region of the receptor is typically negatively charged, so snake toxins have evolved with a positive charge to facilitate binding. The mutation where the receptor is now positively charged like the toxins, repels the toxins electrostatically.”

Academic Implications of the Findings

Dr. Fry mentioned that while the results will not translate into immediate practical benefits for humans, such as new antivenoms, the findings do have the advantage of illustrating a critical evolutionary interaction to future scientists in an engaging manner.

“The concept of animals preying on others, and the prey evolving to evade their predators, is always enthralling, especially to young minds just stepping into the world of science.”

The research is published in the International Journal of Molecular Sciences.

Reference: “Resistance Is Not Futile: Widespread Convergent Evolution of Resistance to Alpha-Neurotoxic Snake Venoms in Caecilians (Amphibia: Gymnophiona)” by Marco Mancuso, Shabnam Zaman, Simon T. Maddock, Rachunliu G. Kamei, David Salazar-Valenzuela, Mark Wilkinson, Kim Roelants
 and Bryan G. Fry, 12 July 2023, International Journal of Molecular Sciences.
DOI: 10.3390/ijms241411353

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