Over the next few days I’ll post a few “holiday snaps” from my week-long trip to Texas: beasts, boots, and BBQ. Today we have a very rare animal I was privileged to see in central Texas, the Texas blind salamander (Eurycea rathbuni). It’s a cave-dwelling animal, of course, and so its eyes are either absent or vestigial and, like many cave beasts, it’s lost its pigment. Highly endangered, it’s endemic to the underground caves found in the Edwards Aquifer around San Marcos, which is itself about 40 miles south of Austin. It’s also completely aquatic, and so breathes through external gills, which it can regenerate if damaged (a Federal wildlife guy told me that he once had a salamander whose gills were completely ablated, and it managed to breathe through its skin until the gills grew back).
They can grow up to about six inches, and eat invertebrates. You can see a nice film of them here (note that they give the species name of Typhlomolge rathbuni; the genus name has been updated).
Here’s a picture of one I took at the San Marcos National Fish Hatchery and Technology Center, where I was kindly given a tour. You can see the vestigial eyes, nearly completely covered with skin. Only the left eye is visible: variable and asymmetric expression of vestigial traits is common. The eyes are much more visible in young salamanders, and then gradually disappear as the skin grows over them.
I need hardly point out that the presence of nonfunctional, vestigial eyes in an animal descended from ancestors that had functioning eyes is pretty good evidence for evolution (see chapter 3 of Why Evolution is True).
Here’s a close-up of the head and gills. This individual has no visible eyes (variable expression among individuals!). I was told that the large swellings at the base of the head contain neurons for processing olfactory information: this beast lives by smelling rather than seeing. This guy also has a bit more pigment than the one above. (I say “guy,” but I don’t know the sex. The only way to sex these things is to hold them up against a strong light and look for either eggs or sperm ducts.)
Phylogenies show that, like all cave animals, the blind salamander descended from a sighted species that lived aboveground. Why do cave animals lose their eyes? There are several explanations. One is that, since they’re not useful in the dark, mutations that make them degenerate are not selected against. The other two explanations involve positive selection. Eyes are easily damaged and infected, and if you don’t need them, individuals with mutations reducing the eyes are less likely to risk injury/infection.
Alternatively, eyes take metabolic energy and bodily substance to build; if you don’t need them, you may have higher fitness if you divert that energy away from eyes and toward other structures or functions that enhance reproduction. And of course all three factors could operate together. I don’t think we know in any case the precise evolutionary forces that led loss of eyes (or pigmentation) in a cave species. The hypotheses are reasonable, though I don’t know of any experimental test (one would be to look at the genes for eye or pigment loss and see if the DNA sequence shows a signature of positive selection, though that test can be misleading).
Here are the eggs photographed in water (and slightly out of focus). You can see the tiny newts developing.
And finally, my friend Jim Bull, a UT Austin professor, built a birdhouse in his backyard. It was taken over by a screech owl, who, at twilight, would poke his head out of the hole for an hour or so. Really cute!
This is probably an eastern screech owl (Megascops asio), but I can’t tell from the photo. Both eastern and western screech owls are sympatric in some parts of Texas. This one, like most owls, hunts at night, and probably poked his head out each evening to scope out the surroundings.