Texas blind salamander has optic nerves but no real eyes

September 22, 2016 • 1:30 pm

This is the kind of post I originally intended to go on this site. When I started this website, I thought that every few weeks I’d publish a bit of new (or old) evidence for evolution, supporting Why Evolution Is True, which was a new book in 2009. Well, as you see, things kind of got out of hand. . .

But here is some information and links imparted by reader Charleen about the famous Texas blind salamanderEurycea rathbuni. As you might suspect from its name, it lives in dark underground abysses, caves, and artisian wells, and has retained into adulthood its juvenile gills. It’s also lost pigmentation, as is the case for many cave-dwellers. Finally, it’s endangered, its distribution limited to the Balcones Escarpment near San Marcos, Hays County, Texas You can find this animal only in the green area below; it’s been listed as endangered since 1967. As the U.S. Fish and Wildlife Service (USFWS) notes:

The Texas blind salamander has been listed as endangered since 1967. It remains vulnerable, says San Marcos ARC director, Dr. Ken Ostrand. “They are impossible for scientists to sample underground, so we collect them in nets when they pop out in wells and springs, young ones, too small to fight currents,” said Ostrand. The young go into captivity at San Marcos ARC where they are held in refugia as a guard against potential harm that could come in the wild. Ostrand says their habitats, which he describes as a ‘limestone honey-combed sponge,’ are quick to recharge with surface precipitation, which could be accidentally laden with unwanted chemicals or spills. Preserving Texas blind salamanders in captivity is a security measure.


Here’s what they looks like:



Behold it in its habitat; the first salamander shows up at 3:44. What weird creatures they are!

The USFWS has just put out an information sheet about the species, first discovered in the 1890s, giving notes about its morphology, behavior, and adding that 135 of these beasts are being kept at the San Marcos Aquatic Resources Center.

What’s relevant for our purposes is the salamander’s vestigial eyes, which appear to be small black dots of pigment below the skin, accompanied by a vestigial optic nerve that doesn’t appear to carry any impulses to the brain. As the USFWS notes:

Recent research on the salamander has yielded other useful information. San Marcos ARC scientists collaborated with Texas State University faculty on a study of eye development in the Texas blind salamander and two other salamander species that live in the Guadalupe watershed: the San Marcos salamander and Barton Springs salamander, both of which are sighted animals that live near sunlight.  Both are held in refugium at the ARC as well.

The research revealed that the blind salamanders retained a vestigial optic nerve with no eyes while the other species had well-developed eyes with structures for focus and variable light adaptation. The findings will be published in a peer-reviewed scientific journal, and have already yielded a master’s thesis.

It’s hard to imagine anything but evolution that would explain the presence of these vestigial eyes and nerves. Here are three photos I found of the pigment-spot “eyes,” which, like many vestigial traits, vary in their degree of development (e.g., human wisdom teeth and ear muscles).




The last one has no visible eyespots at all.  Now I could write some more about eye loss and how it gives evidence for evolution, but I’ll ask you, the readers, to answer two questions:

  1. If you were a creationist trying to show that the vestigial eyes were not evidence for evolution, what would you say? I can think of at least three responses.
  2. Assuming, as is certainly the case, that the eyes have become vestigial via evolutionary processes, how do you think it happened? We don’t know for sure, of course, but I can think of at least three ways.

Put your answers below; there is no prize save the joy of thinking.

76 thoughts on “Texas blind salamander has optic nerves but no real eyes

  1. 1. But they’re still just salamanders. Evolution would be a completely different kind.

    2. That’s just loss of information, probably as a result of the Fall. Evolution would be gain of information.

    3. That’s just front-loaded capacity to adapt. Evolution is something else.

    Same as yours?

      1. Ha! That means you missed one of mine. I’m going to suppose

        4. The eyes are really fully functional, just like the human appendix and coccyx and whale pelvises.

    1. I can only think of two things to add besides just goddidit:

      Those ‘vestigial’ eyes and nerves do have some function. We just haven’t figured out what it is.

      A common creator used a common toolkit, and so made the salamanders with eye spots and optic nerves because that’s the way he likes them. (I guess that is close to goddidit).

    1. Me, too, lessbutnotlast. Quite happy with “things kind of out of hand” here on W E I T !

      On salamanders ? They are cool.
      I know squat about them.
      Even less than a creationist “knows” about ’em.

      I just do know, however (& ‘ve told this knowledge repeatedly and by way of this work [https://goo.gl/ltTUwO] to my darling granddaughters), that: our great – grandmama to the bagazillionth nth power, why, she’s a fine – looking one she was !


    1. I’m guessing you mean in the world of this salamander it no longer requires the eye, no longer uses it, therefore his body does not use the energy to continue having the eye. It can be used for other purposes that are needed?

      1. Yes. It’s well known that the brain is a very expensive organ, in terms of calories, blood supply, and so on. It kind of follows that neurons are expensive. In total darkness, any mutation that selected for no eyes and the associated neurons would be advantageous, given no other very harmful effects.

        It appears to me that these salamanders are a transitional species, and that given time the optic nerves would disappear entirely.

        It would be interesting to know if they could adapt to a lighted environment by redeveloping eyes. The genes are still be there in the population, but turned off. It’s telling that there’s a large variation in the development of vestigial eyes.

        1. As to your speculation that the eye’s functionality could reemerge – I would think that’s improbable. In a light environment, competitors and predators would make this little guy’s life miserable. The loss of vision in darkness, on the other hand seems an easier transition to make.

        2. It is surely a mistake of hindsight, which nature does not have, to think of anything as a ‘transitional species’? There are synchronous species, that is members of a (potential) breeding species community like living humans or ostriches, at a specific point in time, then there are diachronous species, that is species stretching through time, so in a line of descent or ascent, like say mammoths or tyrannosaurs.

          1. If we find a species in the process of colonizing a novel environment, partially but not yet fully adapted to it, I don’t see a problem with calling that a transitional species.

            Natural selection has no foresight, but that doesn’t prohibit us from making predictions about the probable direction of adaptation in response to observable selection pressures.

  2. 1) For question 2, in the absence of persistent selection to maintain vision, alleles needed for eye form and function could have been lost by genetic drift, particularly since these animals are likely to have been in chronically small populations? Mutations could have freely accumulated that altered eye development.
    2) There could be some small resource “savings” to not producing eyes, so there could have been direct selection for reduced investment in eyes and allocation to other needs.
    3) In the cave environment, eyes could have been a directly detrimental trait – sources of infection – so that again, with no need of vision selection could play a role in reducing them.
    4) (doubtful) Loss of eyes could be a pleiotropic response to changes in other traits in response to the cave environment, again in the absence of selection for maintaining them.

    1. Good answers.

      Still, I can’t help feeling there ought to be a way of working spandrels into the explanation, given that these salamanders live near San Marcos.

      1. Isn’t the optic nerve in these salamanderes a spandrel if it (say) keeps two tissues apart that now need to be apart? I’m making something up – I am just guessing that it in fact could be – and assuming that the optic nerve was part of an adaptation to begin with.

    2. Given that blindness and lack of pigment are characteristics shared by many different cave dwelling species I am inclined to think that in the cave environment there is a selection pressure against them. That would chime with either your example (2) or example (3).

    3. Your #1 & #2 were what I was going to say, but you beat me to it. I hadn’t thought of #3, but that seems like another likely influence.

  3. “Gawd dun did it” is usually good enough answer for most, isn’t it?

    I can say that seeing the (almost completely) blind Missouri grotto salamander in a wild cave in southern Mo was one of the high points in my life, along with seeing a blind crayfish in the flesh (in the chitin?) in the same cave. I wish I could have gotten a pic but cave humidity requires a special camera set up for decent shots that aren’t fogged up with steam from your body. Cheers!

    1. I wish I could have gotten a pic but cave humidity requires a special camera set up for decent shots that aren’t fogged up with steam from your body.

      Well, technique, but not a lot of dedicated kit, unless you count the ammo box for getting the camera down t’ole and still working.
      I’ve never been terribly successful with cave photography, but my dad does (did) a fair bit more. Essentials are a camera with a “bulb” exposure setting, and assistants who can sit quietly in the dark and take instruction to let off their flashes on cue. Slave flash triggers are a useful addition, but technique is more important. Specifically to let the steam dissipate. Getting the flash metres away from the optical axis of the camera is another standard technique. Since you’ve got indefinitely large supplies of dark, you don’t need to muck around with synchronising the shutter and the flash.

      1. I saw the kit used by another member of our group, and it was far more complex and professional than I could ever use or afford, but yes, I get what you are saying. However, at the time, I didn’t even own an iPhone. Had I owned solid waterproof camera I would have attempted some pics, but even that was a bit out of reach for me (I did have access to an ancient Pentax but that wouldn’t have survived the water, mud, muck, and belly-crawls). I suppose I should have prefaced the statement with the fact that, through my own failures and issues, I’ve been a member of the working poor single parent brigade for the last 18 years, so pretty much anything was out of financial reach. Not that I had the technique to do it anyway…

        1. I do have a “proper” waterproof camera (rated to 40m ; I’ve taken it to 35m and it ain’t wet yet ; the housing cost pretty much the same as the mid-range (£200 ~ $/€ 250) digital camera, and won’t fit any other cameras in that range. But if you keep you eyes peeled on diving websites, they’re not as eye-watering as a Nikonos.
          However, unless you’re atually diving, you don’t need a waterproof camera.

          that wouldn’t have survived the water, mud, muck, and belly-crawls)

          The traditional solution in Britain is a military “ammo box”. About 80mm x 150mm x 250mm with a rubber seal held in place by a meaty clip. That provides the water- and ud- proofing, but you need some degree of padding inside if you’re going to drop it. So you don’t do that.
          Before I hung up my fins, I used the same boxes to transport my gags (breathing demand valves) into the dive site.
          There is a joke (in Britain) that “caves are tight, but cavers are tighter”. Here you go – http://www.denbigharmysurplus.co.uk/collections/ammo-boxes-and-storage/products/50-cal-ammo-box – £20 a pop, which would get you and the kid a meal at a pub. Last a lifetime – human, not the product’s.

  4. I like these posts, and the others as well.

    My answers:

    1. Vestigial eyes give the animal the beauty reflecting G*d’s design.

    2. E.g. loss-of-function mutation(s) in the eyeless (Pax6) gene.

    1. A mutation in Pax6 doesn’t work; in that case there would be no eye remnant at all. Pax6 must be active in the early embryo. Something(s) downstream must be implicated.

      1. I disagree. Pax6 has been initially “cought” by causing eye defects – not altogether missing eyes – in Drosophila, mouse and human (in human, its first name was Aniridia).

    1. Or to put it another way “we cannot know the mind of God” – that’s always a good way of shutting down any tricky arguments.

  5. A mutation occurs that renders eyes less functional. Then a slight change in the environment occurs, causing the salamanders with good eyes to fair less well, reducing their numbers. Or perhaps there are diseases that afflict full-eyed salamanders that bud-eyed salamanders don’t get, making full eyes more costly.

    I wonder what happens in the brains of creatures with eyes who nearly never see?

    Ohhhh, okay, so every now and then, there’s a flood and the salamanders surface above ground. If the eyes can detect light but the creatures haven’t developed the neurological patterning for what to do with light information, maybe they’d react less optimally above ground than they would if they relied on the same methods for sensing their environmental cues beneath the surface. So maybe the salamanders without functional eyes find their way back underground easier than ones with real eyes. I don’t know. But maybe there is a cost to having functional eyes.

    But also what happens in the brains of animals who can sense light but are virtually never in light, as compared to the brains of animals without the ability to sense light?

    1. Regarding eyes, brains and costs, a major cost of vision other than the eyes themselves is the brain volume necessary to process the signals from the eyes. I don’t know how sophisticated salamander vision is, but brains are metabolically expensive.

      Giving up sight could free up brain capacity for other tasks, perhaps more processing power for other sensory signals. Or it could lead to a loss of brain volume / function that would reduce the metabolic load, perhaps more than the loss of the eyes themselves.

      1. Giving up sight could free up brain capacity for other tasks, perhaps more processing power for other sensory signals.

        Looking at those very oxygenated gills, I see lots of potential for picking up chemical signals from the water, as well as current information and possibly vibration/ sound of prey/ predators in the water.

    2. Do you even need something to actively cull good eyes from the population? I think genetic drift alone could do it. There must be lots of different mutations that will move them towards eyelessness. With no active selection to keep eyes functional the population will eventually pass around the various eyeless mutations.

      I think that wooden boats make a good analogy. They are constantly mutating (rotting and corroding) in ways that make them less well suited for being boats. Without constant selective pressure applied by their owners they will eventually evolve into something that bears a passing resemblance to a boat, but which is better suited to being a fish habitat. No active selection in the direction of being a fish habitat is needed.

  6. I’m wagering that there are costs involved in the development of eyes from egg to adult. Any mutations that happen along that eliminate or reduce eye development will free up resources for other purposes. So in a cave, advantage goes to the guys not wasting their time developing useless organs.

    Creationists would say that there is no reason god would have created eyes for an underground dweller and we just haven’t figured out the divine purpose for the vestigial nerves.

    1. Don’t question God’s ways!

      Now why would I waste my time doing that? There is a whole plethora of other god-squaddies – battalions of them – willing to question your god’s ways.
      Nothing like sowing a little dissent and strife amongst the godly. To misquote Cromwell, “Let them kill themselves and let god sort them out.”

  7. God knew they’d never need eyes so he made sure to not give them any so they could avoid the possibility of being injured since eye injuries are painful and especially pointless in the dark. It’s just like how lava lizards are made of rock with a high melting point so they can survive in that similarly difficult environment.

    Ok, all I really wanted to post here is how much I love their little stick legs. I am always delighted by lizards.

    1. O, geckzilla, this is funny ! But I truly do dig your “God knew they’d never need” – Creationist “reasoning” !

      I mean: .That. is one which The Goddies can spew forth, er use, for a whole rascally / passel – ly Basketful of Those Deplorable UNexplainables !


    2. Geckzilla, they are not actually lizards, but rather amphibians, also possibly neotenous so that they retain the gills which would not normally be present in the adult form.

      But regarding their legs, they also belong to a group where other members have been seen to regenerate amputated limbs. Now if it can be shown that the others pray *really* hard for the unfortunate one…

      1. Yeah, I used the word lizards in the same way many people use the word bug. A moth, though not a hemipteran, can be a bug. Even a spider or a millipede can be a bug.

        So, yes, I am aware they are amphibians, but they also fit a very general and casual “lizard” type category. If I was writing formally I would, of course, be more specific and hopefully appear less of a dumbass.

  8. An answer to question 2:

    (a selection-based answer related to post #4 concerning eyes as a detrimental trait) When the Salamanders still had eyes, they may have lived mostly near the entrances to caves. Short stays deeper in the cave may have provided protection but salamanders with fully functional eyes would have been inherently drawn to the light and prefer to remain at the entrance of the cave. Salamanders with dysfunctioanl eyes due to mutations may have been more likely to go deeper into caves and stay longer as signals from light would not stimulate them as strongly to remain near the cave entance. Going deeper into caves for longer may have provided access to niche with less competition for nutrients, less predation from natural enemies at the surface, or microbial disease at the surface. So functional eyes may have been selected against because living at the cave entrance was somehow more challenging than the inner cave.

    1. Perhaps a group of salamanders from a population that spent part of its time underground became trapped, or partially trapped, underground. By partially trapped I mean that perhaps gene flow between the underground group and the parent population was not completely cut off but merely significantly reduced.

      1. Yes, from being part of a population continuum to developing in isolation. I wonder if they can breed with other salamanders? I would guess direr post Ice Age conditions have left them more isolated than being trapped in the cave – the cave as a refugia…?

  9. Creationist: The vestigial eyes are there since the Fall from grace. And my inner Jesus compels me to say: “For judgment I came into this world, that those who do not see may see, and those who see may become blind.” John, 9:39.

    Evolutionist: Organs that fall into disuse still require energy to build and maintain, and they can still reduce fitness if they are injured. So natural selection would select for mutations that reduce the organ, thereby saving energy for reproduction and reducing risk from injury.

  10. 1.No way Noah and the missus would let anything that nasty on the Ark.

    2.I’m from Texas and I never saw one of these.

    3. Evolutionists lie all the time.

    Therefore the thang don’t exist.

  11. When Jesus comes back and makes the earth new, the salamanders will have their eyes restored, proclaiming, “I once was blind, but now I see”.

  12. Ways that functional eyes can become vestigial in cave creatures:
    1)Eyes are selected against because eye disease and other pathological conditions tend to infect salamanders with better developed eyes. Eyes, besides being of no use in a cave, are delicate and pose an unnecessary risk factor.
    2)Selection favors salamanders with smaller eyes because scarce energy and chemical resources normally used in eye construction and maintenance is freed to increase egg output or survival in lean periods.
    3)Random mutations deactivating genes responsible for eye development may be neutral and by genetic drift, given enough time, will become fixed as pseudogenes.

    Reasons that vestigial eyes in cave salamanders is not evidence for evolution:
    1)Eyelessness is good intelligent design for cave life.
    2)Vestigial optic nerves cannot be assumed to lack a function.
    3)”My thoughts are nothing like your thoughts,” says the LORD. “And my ways are far beyond anything you could imagine.” Isaiah, 55:8. [I think this might be an argument for evolution.]
    (The work is my own, excepting the Isaiah reference)

  13. Reminds me of the axolotyl…a charming little creature living in ONE Mexican lake. Obviously, it was somehow trapped there. Eyes were no longer needed and those with vulnerable eyes didn’t survive as well as those with none.

    Or…you can use W. Benson’s argument and simply say that Santa Claus took them away. Why not? His ways are not ours to imagine…..

  14. From the map, that looks to be the Hill Country, pert’near LBJ’s old stomping grounds (and family namesake) Johnson City.

    That’d make the salamander the second blind critter to come outta there. (“None are so blind as he who keeps sending troops to Gen. Westmoreland.”)

  15. 1. God made a wonderously diverse world. He even thought to give salamanders that he’d just started making eyes on, pretty flowery red gills instead
    2. I don’t know but Franks answer sounds best to me.

  16. Question 1

    A. Sin and free will (you may think this makes no sense, but you haven’t read the third chapter of Hezeziah in the original Hebrew while standing on your head on a Thursday).
    B. Satan is trying to trick us.
    C. God can do anything he likes. Checkmate atheists.

    Question 2
    Eyes are easily injured. If they provide no benefit to survival as in a dark cave, individuals born without eyes or with skin covering the eyes have an advantage.

  17. 1. Creationists say, “We don’t know, but we sincerely thank the USFWS for not only protecting but also showing such selfless fascination for one of God’s creatures. We are donating some money to help save these wonderful little…”

    What? Ah…. okay,

    1a) Those aren’t eyes, they’re decoration
    1b) The eyes do see
    1c) Before the Fall they used to wear little spectacles.

  18. 1 a/ god gave them the eyes ‘he’ wanted them to have
    b/ the fact that some have eyes & some do not shows that it is the sort of variation that humans have for hair colour rather than being evolved
    c/ god made them & then sightless salamanders sought the dark

    2 a/ there was no use in an eye in the total darkness so salamanders that were trapped in a lightless environment did not have that trait selected for.
    b/ the salamanders without eyes had some additional advantage, however slight, such as the areas of the visual cortex given over to some other sense.
    c/ the ancestral salamander had poor vision anyway & the ones not in caves have improved their vision while these ones stayed pretty much the same.

  19. The fact that this salamander retains its gills and it is blind could imply a state more akin to an embryo (the correct terms elude me)and may reflect the principle of parsimony. A creature in the environments described lacking the genes to fully mature could be just as successful mating as a more adult type.
    The triumph of the runt of the litter?

  20. “The Creator” moves in mysterious ways.

    Who are we puny humans to analyze God’s motives [please ignore my catechism]?

    How do you know? My opinion is just as valid as yours!

  21. 1) creationist response
    a)We don’t know if it’s useful or not, just because we didn’t see the function doesn’t mean there isn’t one
    b) God’s creation is elegant, so he used the same building blocks
    c) That’s micro evolution, just like you can breed dogs to be smaller or bigger, you could breed eyeless salamander “by accident” in nature.
    d) funny how evolutionists cherry pick the evidence, when here you can clearly see these red appendices that themselves disprove all these arguments about loss of pigmentation in caves being a result in evolution etc…

    2) how did it happen
    a) eyes are vulnerable, smaller eyes were selected for until they closed completely
    b) view is not important in the cave, but skin sensitivity is, more skin = better
    c) easier to slip in narrow places with an eyeless head

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