Caecilians are the weird and oft-forgotten cousins of frogs and salamanders. It now seems that they are stranger still, and have a pretty unusual defensive method in which they make poisonous mucus from their butt.
It’s not uncommon for amphibian skins to be covered with mucus or poison glands. But in most cases, these are used to secrete goo to help in gas exchange. In other words, it helps them to breathe through their skin. But researchers noticed that on the long and slender burrowing caecilians, all was not as it seemed.
Studying Siphonops annulatus, a species of caecilian from Brazil, the team found that the creature had a concentration of enlarged mucus-producing glands not only on their head, but also around their butts. They realized that the amphibians were producing huge amounts of the slick goo from their heads to line their holes and make it easier for them to slip and slide through the soil.
But it turns out that glands in different regions of the caecilian seem to have been put under different evolutionary pressure. The glands change as you move along the amphibians’ body, from making slippery mucus at the head to making poisonous mucus in the rear, which it leaves in its burrow in case any predators are following behind.
“We know of no other amphibian with this high concentration of mucous glands,” said Edmund “Butch” Brodie, Jr, co-author of the research, published in Scientific Reports. “In other terrestrial amphibians, mucous is mainly related to the uptake of oxygen. Here, in caecilians, it's obviously used in locomotion.”
“The poison glands, resulting from a different selective pressure, provide another defense from predators,” Brodie continued. “In addition to chemical defense, the tail acts as a 'plug,' blocking the tunnel and further deterring predators.”
Often confused for snakes, earthworms, or things of a more unsavory nature, nothing much about caecilians could be considered usual. Having evolved a long slender body to burrow around under the surface of the Earth where they hunt for insects, much of their morphology is highly adapted to their subterranean lifestyle, although some have also become aquatic.
Their skulls are basically fused into one piece of bone with the jaw recessed underneath it filled with some seriously ferocious-looking teeth. This means that the amphibians can ram their head into the ground, helped by their strong piston-like muscles that run the length of their bodies.
Their eyes have been reduced to such a degree that they are now covered over with skin, and can only sense light and dark, while their main sense of smell is helped by a pair of tentacles that they can protrude from between their eyes and nostrils.
Now they can add poisonous butt mucus to that growing arsenal.