# Why are cave-dwelling fish blind? **URL:** \ **Category:** Factual Questions\ **Created:** [November 16, 2007, 1:17am UTC](https://boards.straightdope.com/t/why-are-cave-dwelling-fish-blind/426627 "2007-11-16T01:17:45Z")\ **Posts on this page:** 12\ **Page:** 3
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [November 19, 2007, 10:48pm UTC](https://boards.straightdope.com/t/why-are-cave-dwelling-fish-blind/426627/41 "2007-11-19T22:48:53Z")
[QUOTE=Darwin’s Finch] And I’d say it’s bizarre that you would make that statement at all, given that I never made such a claim (and at the same time, I wonder why you _don’t_ think it bizarre that such injuries are _the_ explanation…). There is a _huge_ difference between “more common” (which is entirely speculative at this point anyway) and “so common that there are selective pressures against the formation of eyes”. [/quote] You said “So, unless there is experimental evidence out there which indicates that eye-possessing individuals in lightless environment are prone to eye injuries” - Implying (in my reading) that you don’t believe eye-poss4essing individuals would be prone to eye injury in a completely dark environment. That just doesn’at make sense. They’re be prone to all kinds of injuries. > [@](#): > > Surely no one here believes that these fish were suddenly whisked from the surface world to their subterranean caverns, wherein they were forced to survive in an alien world of complete darkness, right? The transition from sighted to sightless was very likely just as gradual as the loss of light in their environment. By the time these animals found themselves in complete darkness, they were likely not “fumbling” at all, having gradually adapted for life in lower-and-lower light environments. Remember all those other adaptations I mentioned that these fish have? Those don’t arise by suddenly throwing sighted fish into a dark pool and letting them bump into stuff, maybe getting lucky enough to find food and/or mates as they go. I wouldn’t imagine there is any single universal scenario in which the adaptation occurs - it’s happened many times over, quite possibly under different circumstances in different cases. And just because it may have happened gradually, that doesn’t mean the plausible progressively increasing risk of eye injury in the progressively reduced lighting conditions is no factor. It doesn’t have to happen to every individual in every generation - it only has to be a very small (but real) increase in risk. You seem to have been saying (and please correct me if this is not the case) that vanishingly small statistical factors would be swamped out by other things over time. I believe the opposite is true - if you’ve got a die that has just a very, very slightly increased chance of rolling a six, the effect will become more noticeable as the number of rolls increases. ---
**Author:** ![CarnalK](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/carnalk/32/486_2.png) [@CarnalK](https://boards.straightdope.com/u/CarnalK)\ **Post date:** [November 19, 2007, 11:40pm UTC](https://boards.straightdope.com/t/why-are-cave-dwelling-fish-blind/426627/42 "2007-11-19T23:40:29Z")
I’m under the impression that most cave fish are blind, not eyeless. > [@](#): > > _[The Cave Biology Research Group (CBRG) at New York University](http://homepages.nyu.edu/~rb4/Rationale.htm):_ > > The Waterfall Climbing Cave Fish of Thailand, Cryptotora thamicola , pictured below, illustrates the reduction in eyes and pigmentation typical of cave fishes (Photo by Denis Belliveau). This is one of only three species of cave vertebrates that completely lacks eyes as an adult. So for **Blake** ’s reasoning to be judged we have to know that fish eyes are as vulnerable to injury and infection as land animals’ and whether the blinded fish eyes are significantly less so. ---
**Author:** ![Patty\_O\_Furniture](https://avatars.discourse-cdn.com/v4/letter/p/96bed5/32.png) [@Patty\_O\_Furniture](https://boards.straightdope.com/u/Patty_O_Furniture)\ **Post date:** [November 19, 2007, 11:54pm UTC](https://boards.straightdope.com/t/why-are-cave-dwelling-fish-blind/426627/43 "2007-11-19T23:54:16Z")
[QUOTE=Ronald C. Semone] Or more to the point, has anyone ever raised generations of sighted fish in a totally dark environment and monitored their development? [/QUOTE] Now that’s just plain cruel 😃 ---
**Author:** ![Darwins\_Finch](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darwins_finch/32/20304_2.png) [@Darwins\_Finch](https://boards.straightdope.com/u/Darwins_Finch)\ **Post date:** [November 20, 2007, 1:49am UTC](https://boards.straightdope.com/t/why-are-cave-dwelling-fish-blind/426627/44 "2007-11-20T01:49:03Z")
[QUOTE=Mangetout] You said “So, unless there is experimental evidence out there which indicates that eye-possessing individuals in lightless environment are prone to eye injuries” - Implying (in my reading) that you don’t believe eye-poss4essing individuals would be prone to eye injury in a completely dark environment. That just doesn’at make sense. They’re be prone to all kinds of injuries. [/quote] “Prone to injury” does not mean “injuries can happen”, it means “a tendency toward injury”, that is, injuries are more common. Are blind people, for example, more prone to eye injury than sighted people? Or are sighted people more prone to eye injury in the dark? As **Blake** cited, most human eye injuries come from preventable industrial accidents, not stumbling around in the dark. So, off hand, I’d say no to those questions. What evidence, then, are you working from that would lead you to believe that eye injuries are more common in blind fish than in sighted fish? > [@](#): > > And just because it may have happened gradually, that doesn’t mean the plausible progressively increasing risk of eye injury in the progressively reduced lighting conditions is no factor. It doesn’t have to happen to every individual in every generation - it only has to be a very small (but real) increase in risk. Risk is meaningless. Natural selection cannot operate on a possibility that something unfortunate might happen. If there is a quantifiable increase in eye injuries, _and_ those individuals who suffer such injuries are less likely to reproduce than those who do not suffer said injuries, _and_ the reason those injuries are lessened in the bulk of the reproducers – or that the injuries are more common in those that don’t reproduce – is due to a heritable trait (specifically, in this case, having smaller or less exposed eyes), _then_ selection can be said to operate. As for the gradual scenario, you don’t seem to be acknowledging the fact that while light decreases (and the “risk” of eye injury supposedly increases), the population is also adapting to low-light conditions making such injuries less likely in the first place (or, at least, no more likely than in previous generations). The only way **Blake** ’s scenario does make sense is if he is positing a sudden change in environment, in which case he’s arguing for something other than natural selection. > [@](#): > > You seem to have been saying (and please correct me if this is not the case) that vanishingly small statistical factors would be swamped out by other things over time. I believe the opposite is true - if you’ve got a die that has just a very, very slightly increased chance of rolling a six, the effect will become more noticeable as the number of rolls increases. Natural selection is all about statistical factors; there are no guarantees. If an individual has even a minute advantage due to a certain trait, that individual has a slightly higher chance of passing that trait onto its offspring. So, no, I am not saying that “vanishingly small statistical factors would be swamped out”. I am saying that risks, unless manifested, are evolutionarily meaningless. Unless there _is_ a quantifiable increase in eye injuries among sighted fish in a dark environment, **Blake** ’s hypothesis doesn’t hold water. Furthermore, unless that injury specifically interferes with reproduction, it is also evolutionarily meaningless, as I mentioned above. If an injury merely blinds, then there is no measurable difference between a fish blinded through injury and one blinded simply because it can’t see in the dark. Both would face the same challenges and there would be no selective pressures against having eyes (or for smaller eyes) in such a case. ---
**Author:** ![Tuckerfan](https://avatars.discourse-cdn.com/v4/letter/t/eada6e/32.png) [@Tuckerfan](https://boards.straightdope.com/u/Tuckerfan)\ **Post date:** [January 10, 2008, 10:19am UTC](https://boards.straightdope.com/t/why-are-cave-dwelling-fish-blind/426627/45 "2008-01-10T10:19:17Z")
A bump, because a [bit of new information has come to light.](http://news.nationalgeographic.com/news/2008/01/080108-cave-fish.html) > [@](#): > > It’s a miracle! Blind cavefish, despite having adapted to their lightless environment for more than a million years, can produce sighted offspring in just a single generation, a new study reveals. > > The ability was discovered when researchers mated fish from distinct populations that had been isolated in separate caves. > > In some cases the first-generation offspring of such unions could see. > > The find shows that the genetic mutations causing blindness are different in different lineages of the fish. > > “Evolution’s palette is varied,” said study author Richard Borowsky of New York University in a statement. > > “Restoration of the ability to see comes in a single generation because the populations residing in different caves are blind for different reasons—i.e., different sets of genes are nonfunctional in the different populations.” I’ll leave it to the experts to decide what this might mean. ---
**Author:** ![GingerOfTheNorth](https://avatars.discourse-cdn.com/v4/letter/g/cdc98d/32.png) [@GingerOfTheNorth](https://boards.straightdope.com/u/GingerOfTheNorth)\ **Post date:** [January 10, 2008, 3:11pm UTC](https://boards.straightdope.com/t/why-are-cave-dwelling-fish-blind/426627/46 "2008-01-10T15:11:57Z")
I listened to an interview with Borowsky last night on CBC Radio. It was fascinating. I see that his work has already been quoted here. It can be heard [here](http://www.cbc.ca/asithappens/latestshow.html). ---
**Author:** ![Santo\_Rugger](https://avatars.discourse-cdn.com/v4/letter/s/e95f7d/32.png) [@Santo\_Rugger](https://boards.straightdope.com/u/Santo_Rugger)\ **Post date:** [January 10, 2008, 4:50pm UTC](https://boards.straightdope.com/t/why-are-cave-dwelling-fish-blind/426627/47 "2008-01-10T16:50:24Z")
> [@](#): > > The study also found that cavefish genetics reflect geography. > > The farther apart two parents came from, the more likely it was their offspring would be able to see. > > This suggests that geographically distant populations are genetically more distant and thus have less overlap in blindness-causing genes. Can somebody explain why it matters how far apart they were, if the populations were all isolated from each other? The only reason I can think of is that, although isolated, geographically close populations pass on similar mutated traits. But that seems too simplistic. ---
**Author:** ![groman](https://avatars.discourse-cdn.com/v4/letter/g/73ab20/32.png) [@groman](https://boards.straightdope.com/u/groman)\ **Post date:** [January 10, 2008, 6:22pm UTC](https://boards.straightdope.com/t/why-are-cave-dwelling-fish-blind/426627/48 "2008-01-10T18:22:12Z")
I apologize if this issue has already been addressed but could it be that perhaps fish individuals that had vision defects or blindness managed to reproduce only by seeking out the relative security of a dark cave – an even playing field with prey and predators? It feels like everybody is implying that the fish that started living in dark caves lost their sight, but it seems to me that caves would be a much better place to survive for individuals with vision defects in the first place. If this sort of sorting was going on in a population, those with genetic eyesight difficulties that did survive would tend to survive in the caves, eventually accumulating a large gamut of various genetic vision defects. Or if for example the fish normally used to spawn in caves but feed outside, the individuals born with vision defects that would venture outside would invariably be at a disadvantage, while inside the cave they had no disadvantage at all. ---
**Author:** ![GingerOfTheNorth](https://avatars.discourse-cdn.com/v4/letter/g/cdc98d/32.png) [@GingerOfTheNorth](https://boards.straightdope.com/u/GingerOfTheNorth)\ **Post date:** [January 10, 2008, 6:35pm UTC](https://boards.straightdope.com/t/why-are-cave-dwelling-fish-blind/426627/49 "2008-01-10T18:35:33Z")
[QUOTE=Santo Rugger] Can somebody explain why it matters how far apart they were, if the populations were all isolated from each other? The only reason I can think of is that, although isolated, geographically close populations pass on similar mutated traits. But that seems too simplistic. [/QUOTE] In the interview I mentioned above, the researcher said that it was that one population will have lost one gene, while another will have lost a different. And of course I can’t remember the term, but what it meant in the case of the sighted offspring was that the strengths of one parent’s genetics complement the weakness of the other parent’s genetics. I suspect, then, that it is two completely separate populations which hadn’t cross-bred. ---
**Author:** ![Santo\_Rugger](https://avatars.discourse-cdn.com/v4/letter/s/e95f7d/32.png) [@Santo\_Rugger](https://boards.straightdope.com/u/Santo_Rugger)\ **Post date:** [January 10, 2008, 7:29pm UTC](https://boards.straightdope.com/t/why-are-cave-dwelling-fish-blind/426627/50 "2008-01-10T19:29:04Z")
[QUOTE=GingerOfTheNorth] In the interview I mentioned above, the researcher said that it was that one population will have lost one gene, while another will have lost a different. And of course I can’t remember the term, but what it meant in the case of the sighted offspring was that the strengths of one parent’s genetics complement the weakness of the other parent’s genetics. I suspect, then, that it is two completely separate populations which hadn’t cross-bred. [/QUOTE] I understand what you’re saying, but I understood the part of the article I quoted was talking about several isolated populations. If one were to breed all possible combinations of them, the isolated populations farthest apart would be most likely to produce sighted offspring. Seems to me, if they’re isolated, it shouldn’t matter if they’re in different caves or on different planets. ---
**Author:** ![Darwins\_Finch](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darwins_finch/32/20304_2.png) [@Darwins\_Finch](https://boards.straightdope.com/u/Darwins_Finch)\ **Post date:** [January 10, 2008, 9:52pm UTC](https://boards.straightdope.com/t/why-are-cave-dwelling-fish-blind/426627/51 "2008-01-10T21:52:46Z")
[QUOTE=groman] I apologize if this issue has already been addressed but could it be that perhaps fish individuals that had vision defects or blindness managed to reproduce only by seeking out the relative security of a dark cave – an even playing field with prey and predators? [/quote] While it’s not outside the realm of possibility, there’s no evidence that this would have been the case. As noted above, the apparatus remains in place to develop functional eyes; that apparatus is simply “overshadowed”, so to speak, by the development of the surrounding tissues. Further, certain structures (_e.g._, the lens) are re-absorbed after they have developed. All of which points to something more than mere “loss of eyesight as an adaptation unto itself”. I would expect to see fish with assorted eye deficiencies (which, assuming they affect only the eyes, would no longer be disadvantageous) in various populations, if your hypothesis were true. Instead, we see various stages of loss of sight, but no particular increase in the frequencies of deleterious (in the surface world), eye-affecting mutations. ---
**Author:** ![BlueKangaroo](https://avatars.discourse-cdn.com/v4/letter/b/3d9bf3/32.png) [@BlueKangaroo](https://boards.straightdope.com/u/BlueKangaroo)\ **Post date:** [January 11, 2008, 12:47am UTC](https://boards.straightdope.com/t/why-are-cave-dwelling-fish-blind/426627/52 "2008-01-11T00:47:30Z")
[QUOTE=Santo Rugger] I understand what you’re saying, but I understood the part of the article I quoted was talking about several isolated populations. If one were to breed all possible combinations of them, the isolated populations farthest apart would be most likely to produce sighted offspring. Seems to me, if they’re isolated, it shouldn’t matter if they’re in different caves or on different planets. [/QUOTE] A couple of profs at my old university were studying this very topic. I don’t understand a lot of it, but this I can explain to a degree. Just because you and I think a cave is not related to another cave in a region doesn’t mean they are actually unconnected. So they may not be isolated populations, or not completely isolated. Furthermore, it depends on how the cave system was formed. Was this all once a giant sea, or is it a result of a bunch of different systems? Start out with the same local adaptations, and you are probably more likely to see similar losses in populations once they become isolated. So the farther apart you are geographically, the less likely you are to not truly be isolated or to have started from the same “base stock”. I suspect, but do not know, those are the reasons for distance mattering. Caves are really confusing, and cave critters even more so. I hope this helps some. [Previous page](https://boards.straightdope.com/t/why-are-cave-dwelling-fish-blind/426627.md?page=2)