Frogs, toads and salamanders are fairly common sight in our gardens, forests and ponds. However, the chance that you ever encountered a caecilian is much smaller. ​ These mysterious animals only live in tropical regions such as Southeast Asia, Africa and the Amazon rainforest. Like other amphibians, amphibians are vertebrates. Yet you couldn't tell because they’re legless and have a worm-like appearance. Many species don't even have visible eyes! Their appearance is an adaptation to a dark underground existence involving burrowing through soil and leaf litter; most species practically never venture above ground. This is also the main reason why we know very little about their biology.
An international team of scientists led by Kim Roelants of the Vrije Universiteit Brsusel (VUB) and Bryan Fry of the University of Queensland (Australia) investigated another adaptation of these mysterious animals. Like other amphibians, caecilians make easy prey for predators like snakes. Many snakes use strong nerve toxins to subdue their prey. Cobras, mambas and coral snakes are notorious examples. About 30 million years ago, these venomous snakes started diversifying across the glove, mainly colonising the same tropical areas where caecilians live. To investigate how caecilians adapted to the worldwide appearance of these dangerous predators, the scientists analyzed their so-called "nicotinic acetylcholine receptors". These receptors are proteins in the nerve cells of all vertebrates responsible for the transmission of neural stimuli to muscles. The neurotoxins in the venom of cobras and other venomous snakes block the action of these proteins, so that neural stimuli can no longer be transmitted, resulting in paralysis and death. This effect is the main reason why the venom of many tropical snakes is so dangerous to humans: our acetylcholine receptors are very sensitive to neurotoxins.
The research, largely conducted in the VUB's Amphibian Evolution Lab, shows that this is not the case for most caecilians: their acteylcholine receptors have evolved to be resistant to neurotoxins, so that the animals do not become paralyzed and can escape certain death. Remarkably, this resistance has arisen repeatedly (at least 20 times in different species) and always in areas where the risk of predation by venomous snakes is high. In contrast, the researchers find no resistance in species that live in regions where venomous snakes are absent, such as the Seychelles.
The research published in the International Journal of Molecular Sciences, illustrates nicely how the worldwide rise of a new type of predator can set the further evolutionary course of other animal groups. The scientists expect to find similar patterns in a large number of other animals exposed to the danger of venomous snakes. The research was mostly conducted by Marco Mancuso to prepare his Master thesis in the VUB’s biology programme and was published last week in the International Journal of Molecular Sciences.