Caecilians - The Blind Ones

Usually, when someone mentions amphibians, we picture a frog, maybe a toad or perhaps a salamander… also a newt. But unless you’re a true herpeto-nerd (which is a good thing), you definitely don’t picture something limbless and almost blind, right?

You will, from now on! The amphibians that I am about to present to you truly are limbless, they are subterranean, and they’re called caecilians, which means ‘blind ones’ in Latin. Truly, these creatures don’t really see a lot – some species don’t even have eyes, and those that do can only use them to tell the difference between light and dark.

Human Creativity vs. Limbless Organisms

As it goes with most animal groups, they always have a superstar species among them. And as it sometimes goes with strange limbless animals, at least one caecilian has been compared to a phallus and even called ‘man-aconda’, ‘penis snake’ or ‘floppy snake’. Truly creative.

The species in question is Atretochoana eiselti. You might have guessed that it’s neither a snake, nor a penis. Although its resemblance to an (unrealistically large) anatomical structure of a male human is truly uncanny, there’s something else about this amphibian that should astonish people instead. At 72-81 cm in length, A. eiselti is the largest tetrapod to completely lack lungs, and it is the only lungless caecilian! It was thought that a small caecilian, then Caecilita iwokramae, now Microcaecilia iwokramae, was also lungless, which was later disproved - it has one (right) fully developed lung. Gotta love a healthy amount of confusion in science!

It is unclear why this species is completely lungless – not many specimens have been found and studied thus far. We do know that the species is aquatic and that its skin is responsible for gas exchange between its body and the environment - this is called 'cutaneous respiration'. Hookmoed et al., who most recently studied the species, believe that the wrinkled skin of the amphibian could make respiration more efficient by maximising the skin's surface.

The strange thing is, however, that A. eiselti does not live in cold waters as one would expect (lungless salamanders, its closest completely lungless relatives, do not live in warm waters). Instead, it can be found in warm, fast-flowing waters of the Amazon Basin in Brazil and, perhaps, in Bolivia, too. The logic behind this expectation is obvious: cold water is more oxygen-rich, and that is an important feature for a creature that relies on cutaneous respiration.

In their 2011 study, Hookmoed et al. found and described three live specimens. The scientists could not resolve the question of how exactly this species breathes. It seems that cutaneous respiration does take place, since only two or three cells divide the animal’s blood from the surface of its skin. However, the fact that the animal is truly large makes it a bit hard to believe that the species could get enough oxygen this way. It is possible that they also use buccopharyngeal (i.e. pertaining to the cheeck/mouth and the pharynx), intestinal or even cloacal respiration to get more access to oxygen. Makes me wonder even more as to why man-aconda lost its lungs during its evolutionary journey. Sure, if you’re an animal that lives in deep waters, your lungs could make it harder for you to remain deep, so losing them may solve the buoyancy problem. There's just one thing that goes against this argument: its closest known relative, Typhlonectes compressicauda, also lives in deep waters but still has a pair of well-developed lungs! So what's up with A. eiselti? Is it possible that lunglessness evolved in this species because lungs constrained the evolution of a specialized feeding apparatus, as argued by Wilkins & Nussbaum? We need answers!

They’re Not Fish in the Atlantic, but They Are Slippery

Grabbing a caecilian may be difficult. They’re slimy and slip like a bar of soap. A common species in Argentina, Bolivia, Brazil and Paraguay, Siphonops paulensis, also known as Boettger's caecilian, produces slime that even contains toxins that can damage red blood cells in their would-be predators. Since their skin is very sensitive, just like the skin of other amphibians, and is prone to dermatitis, it’s better to avoid touching them altogether.

Truly Unique

No matter how much you think caecilians look like a penis, you should know that they have a penis-like cloacal structure, phallodeum. And that’s pretty unique among amphibians. Actually, caecilians are the only group of amphibians that strictly reproduce by internal fertilization, unlike e.g. frogs who first release the eggs that become fertilized externally.

And their uniqueness does not stop there!

With currently about 215 species described - the latest being Caecilia pulchraserra, discovered in 2019 - caecilians represent a small group of amphibians that deserve much more than just being compared to male reproductive organs. IUCN lists caecilians as data-deficient.