Penis snake?

August 12, 2012 • 5:14 am

This newly rediscovered species is being touted as a “penis snake”, but it’s neither penis nor snake. It is in fact a caecilian, and since you’re interested in biology (why else are you here?), you’ll need to know a bit about them. I found, via website io9, a description of a new caecelian species that happens to be the largest tetrapod (land-dwelling vertebrate) without any lungs.  This one had actually been previously described from a couple of pickled specimens, but new work has found its natural habitat and has even led to observations of them alive in the wild. (See a more informative description of caecelians at National Geographic.)

But first, what is a caecilian? Before I did a bit of digging, I’d always thought they were simply legless salamanders, but that turns out to be untrue.  True, like salamanders they are carnivorous amphibians and have species that are either fully aquatic or terrestrial (but bound to moist habitat). They’re found on three continents and central America, though caecelians (of which there are 187 species) are tropical. They’re also the only group of amphibians (the order Apoda, which means “no feet”) that reproduces entirely through internal insemination.

But they are not salamanders: they constitute a monophyletic group (i.e., a group in which all modern descendants go back to one common ancestor) that split of from the ancestors of modern salamanders around 150 million years ago. They are in fact a “sister group” of salamanders. Here’s where they fall on the tetrapod family tree:

So they ain’t salamanders, but they’re close to them. Anyway, a new paper by Marinus Hoogmoed et al. (free download) from Bol. Mus. Para. Emílio Goeldi. Cienc. Nat., Belém, describes several new specimens of Atretochoana eiselti from Brazil, and compares them to the known pickled ones.  This turns out to be the largest tetrapod that does not have lungs: one female specimen was a meter long and weighed 570 grams (a pound is 454 grams).  Here’s a photo:

Lunglessness is known in several species of salamanders and one species of frogs; breathing occurs by diffusion of oxygen through the skin.

And the head, showing the nostrils, mouth, and numerous little teeth:

The other finding of note is that, despite previous speculations that this species was limited to cold, fast-flowing streams (perhaps because such waters are highly oxygenated, helping an animal without lungs to breathe), this species was found along riverine beaches and in lakes and pools in tropical forest, including muddy mangrove pools. Apparently it gets enough oxygen to survive in those habitats; the mystery is how an animal as large as this gets sufficient oxygen solely through its skin.

Here are two more caecilians from National Geographic:

The wormlike amphibian Scolecomorphus vittatus sometimes moves over leaf litter in the forests of Tanzania. Since color in amphibians is usually a sign of toxicity, researchers think the caecilian’s bright stripe could be a warning that, if eaten, the species creates a burning taste in the mouth.
A native to Kenya’s Taita Hills, the caecilian Boulengerula taitanus looks a dull blue-grey from a distance. But up close and from the sides, bright vertical stripes become noticeable.

As for their evolutionary origins, here’s a candidate for something close to the common ancestor of caecelians: Eocaecelia micropoda, which has tiny limbs, clearly on the way out. It was described in the journal Nature in 1993:

This reminds me of an ancestral snake: snakes evolved from lizards by gradual diminution of the limbs, and this looks very much like an ancestral snake.

h/t: Grania Spingies


Hoogmoed, M. S., A. O. Maciel, and J. T. Coragem. Discovery of the largest lungless tetrapod, Atretochoana eiselti (Taylor, 1968) (Amphibia: Gymnophiona: Typhlonectidae),in its natural habitat in Brazilian Amazonia. Bol. Mus. Para. Emílio Goeldi. Cienc. Nat., Belém, 6: 241-262

39 thoughts on “Penis snake?

  1. I caught one in Colombia and didn’t know what it was until I showed it to a herpetologist. Do Caecilians and legless lizards have rudimentary pelvic girdles?

    1. Most caecilians have lungs, though these don’t. I’m not sure whether any caecilian breathes through its nostrils rather than its mouth, but all of them use their nostrils for olfaction, which is undoubtedly the case for this species.

  2. I thought salamanders (caudata) were now regarded as a sister group to frogs (anura), in the clade batrachians, rather than to caecilians; Nature 453, 515, 2008, and Cannaella et al, Amphibians (Lissamphibia) in Time Tree of Life, OUP 2009, 353. Or has opinion shifted again since then?

    1. You are correct, Paul. Caecilians are the outgroup. That’s not the only problem with this cladogram: “Reptiles” as the sister group of birds? Birds are deeply nested (no pun intended) within Reptilia (Sauropsida) as sister to crocodilians.

  3. Another animal to be fashioned into full-scale size and shape and sent to the Netherlands Ark for permanent exhibit.

    1. I think the Netherlands have metre-long, half kilogram rubber representations of these already. At least, that woman in a window in the back streets of Groningen (pronounced so that the audience need to clean their spectacles) had something similar, but it was a bit dark to see exactly what. She must have been a herpetology student.

        1. It certainly gave the place an appropriate atmosphere. 77% nitrogen, 21% oxygen, 1% argon and 1% red-lit clouds of hashish smoke. “Hallejluleahbubble!” said Fat Freddy Freekowtski, before getting burned again.

  4. Outstanding that you posted this while ~1700 folks from all over the world are meeting in Vancouver for the World Congress of Herpetology, which includes an entire session on caecilians–thanks!

  5. Nitpick: definition of monophyly – “i.e., a group in which all modern descendants go back to one common ancestor)”

    Strictly speaking, that would be true of any group of organisms you choose to put together.

    Better?: a named group that contains all of the modern descendants of their most recent common ancestor.

    Birds as the sister group of reptiles? Not so much, but I guess it might be confusing to make a branch with reptiles + birds?

    1. More correctly, as you suggest, “a group in which all modern descendants go back to one common ancestor” AND that includes all the descendants of that ancestor, i.e. a clade. which is why all the best people classify humans as apes; otherwise apes are not monophyletic. But Jerry knows that as well as anyone and looks like he was just being too elliptical.

      It’s usual to include both surviving (“crown”) and extinct (“stem”) members in a clade, but I think that stem/crown is an unhelpful distinction.

  6. Weird. No source in Portuguese I’ve ever seen calls it a penis snake, they call it “cobra mole” (“soft/limp snake”). Maybe it was a translation error?

  7. No lungs? Between this post and the post on self-fertilization, I continue to be amazed at how some vertebrates radically alter what seem to be fundamental aspects of the standard body plan.

  8. According to Wikipedia the caecilians are now placed in the order Gymnophiona and the Clade Apoda. I’m not sure what the difference is since the order and the clade seem to be identical.

    1. Thanks, Sam. Good question.

      But it is at least clear now that frogs and salamanders are indeed close rto each other than they are to caecilians. Ref 16 from that Wikipedia article, Molecular Phylogenetics and Evolution Volume 56, Issue 2, August 2010, Pages 554–561 says “The monophyly of each extant amphibian order and the sister group relationship between frogs and salamanders (Batrachia hypothesis) are all strongly supported. Dating analyses (all methods and calibration schemes used) suggest that the origin of extant amphibians (divergence between caecilian and batrachians) occurred in the Late Carboniferous, around 315 Mya, and the divergence between frogs and salamanders occurred in the Early Permian, around 290 Mya”

      This matters to me because salamanders have a walk-on (swim-on? Waddle-on?) part in my next book.

    2. Pyron 2011 in Systematic Biology restricts Gymnophiona to the crown group. There is a Jurassic form, Eocaecilia, that has limbs and digits that lies outside this crown group, but would be in the total group (ironically named Apoda).

      1. Got it. Thanks. DOI:10.1093/sysbio/syr047

        Presumably the vestigial limbs have some function, or they’d have gone by now.

        Three examples, then, of near-total limb loss – caecilia, snakes, and slow-worm lizards. Any others? (as opposed to limb modification and hind limb loss in aquatic mammals)

        1. Paul,

          Besides Anguis (and its relative the glass lizard, Ophiosaurus) and Serpentes (snakes), there are two other squamate lineages in which limbs were lost, Amphisbaenia and Dibamidae. There was a recent paper (forgot citation) showing squamates are genetically “set up” so that limb loss is more likely than in other lineages.

          1. Thanks. I’ve found a very helpful student-level review and other stuff largely from the same lab. But nothing that I (I’m just a chemist) can immediately identify as showing that ‘squamates are genetically “set up” so that limb loss is more likely than in other lineages’, a conclusion that would interest me greatly, so if you come across it please post.

            1. In finding this article, I found I had forgotten that many skinks are limbless too. It’s within skinks that this study took place. We also read that limblessness in squamates has arisen 53 times!

              Not much on genetic basis of limb loss from a cursory glance. Maybe there was something else.

              Skinner et al. 2008. BMC Biology.
              Rapid and repeated limb loss in a clade of scincid lizards.

            2. Thanks for posting the review. And I see I also left out the pygopods, which I would have never even heard of were it not for the most excellent Tetrapod Zoology blog.

  9. Could you please explain “tiny limbs, clearly on the way out”? The way you put it it seems that there’s a plan and a destination.
    Couldn’t the tiny limbs just as well be on their way back or just stay the way they are?

    1. I can’t speak for Jerry, but for tiny limbs to stay just as they are, there’d have to be some positive selective pressure keeping them that way. This seems implausible since the limbs as pictured aren’t really big enough to be functional.

      There could conceivably be some selective pressure pushing them to larger size, but I’m guessing that process would be fairly rapid, and the chance of finding them in this barely functional transitional state correspondingly small.

      In contrast, the selective pressures acting on nonfunctional limbs would presumably be much smaller and take longer to eliminate them. So if we find limbs in this state, it’s much more likely they’re on the way out than staying put or making a comeback.

      That’s my inexpert guess anyway.

  10. Actually, most salamanders don’t have lungs. All members of the family Plethodontidae share this trait and comprise about 70% of known salamander species. Plethodontids are so numerous that their combined biomass outweighs that of all other vertebrates in some habitats!

    One of the more bizarre features of caecilians is a pair of retractable, chemoreceptive tentacles just in front of the eyes. In the family Scolecomorphidae the eyes are covered by bone, but are connected to the tentacles and can actually be extruded entirely outside the skull through the tentacle openings when needed.

  11. Damn! … and I thought you were going to talk about the talking snake in the Garden of Eden. Everyone knows it was a penis…

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