Why do fish species that live in dark caves have poorly developed eyes?

Question

I realize they don't need eyes in the dark (so don't say that as your answer). My question is, "How did their smaller eyes develop?"

Guys, you aren't answering my question. Obviously it's evolution, and obviously they don't need eyes. My question is what evolutionary mechanism or process causes eye structures to diminish over millions of generations of living in the dark?

Guys.. No offense, but these answers are lame.

Answer 1

As others have pointed out, the evolutionary force known as natural selection cannot act on any organ, structure or character of any kind that is not "exposed" to it, that is, that does not have an adaptive value. Thus, larger eyes could be selected for if enhanced vision was needed, but the disappearance or regression of an organ does not happen just because "it's not needed". Evolution is not directional, it's not going "toward" anything.
Natural mutations can cause an organ to develop less, and if the organ has adaptive value, then the individuals with these mutations will be selected against, because although growing this part is costly (in terms of development time, growth, materials, energy) the advantages of having it outweigh the costs.
Now suppose this mutation causes the eyes to be less developed, and this does not imply a loss of fitness (as would be the case of eyes in completely light-less environments). In that case, this mutation is an advantage, because the individual will "save" the cost of building this complex organ that has no positive adaptive value anymore. Advantageous traits can be acted upon by natural selection, and losing the eyes would be an advantage. This is known as the "adaptation hypothesis".
The "neutral mutation hypothesis" suggests, more simply, that random mutations of the genes affecting eye development accumulate in absence of natural selection, either for or against.

The way in which mutations affect body parts is complex; a single mutation could result in drastic changes, or many mutations could interact to produce slow changes. All these may be possible and given what is now known about developmental genetics (evo-devo), it may be possible to reconstruct the genetic evolution or history of an animal by examining its genome.

Here's an interesting scientific article about the evolution of this trait in fish:
http://intl-icb.oxfordjournals.org/cgi/reprint/43/4/531

Answer 2

I would theorize that it all started with some fish that obtained poorly developed eyes by a mutation, this happens in almost all creatures, some are born with poor vision, I am sure you have heard of blind kittens and puppies.

Lets say this said mutation occurred in a fish population that lived in a bright environment, the blind (or poor eye-sighted fish) would've been severely handicapped for it’s environment, it just could not have survived there. It is possible that they hid in dark areas, under the rocks for protection, here they were not preyed upon and thus it become their home and they lived in the dark they survived and reproduced to pass on the mutation (genes). Overtime more genetic mutations lead to these fish being different enough to become their own species.

Remember, mutation creates variation and then the type that is most suitable for an environment survives, not the other way around! (i.e fish didn't go live in the dark and decide to become blind).

Answer 3

My son has quite a severe astigmatism...which means his eye is more oval than round, and also sits at an odd angle. Because of this, the eye doctor prescribed him with glasses that would bend the light into his eyes and explained that because his eye is situated differently and is an odd shape, the eye cannot receive the right amount of light without wearing the glasses. He said that with time, because the optic nerve kept giving out the signal to take a 'photo' but did not get a response (due to lack of light) to send a message to the brain that he would eventually be blinded, as the optic nerve would 'give up' trying to receive a signal it wasn't getting. So, that being the case, I would presume the same to be with a fish that lives in darkness. As its eyes were not developed to receive signals on low amounts to no light, then the optic nerve gives up, and thus the fish is blinded. I would therefore, also assume (though you will need to check further resources to know for sure), that if that same fish were born in a lighted area, that same type of fish probably would see fine. Either way, eyesight is down to the signals being sent via the optic nerve to the brain. If there is no light there is no image for the optic nerve to send to the brain, and with no signal, no sight as the optic nerve gives up trying to send and receive signals.

Answer 4

Not all have poorly developed eyes. Many have extremly good eye sight, b/c they have evolved to see certain illuminesent- lights, prey, and predators. If you took a fish that is used to dark water and put them in well lit water, they would virtually be blind, b/c they aren't built to see so much light, and they can't adapt to something like that very well. And even if they did they would probably die before that could happen from either lack of food, or the higher percentage of oxygen on the water will.

Answer 5

The key here is to think of this fish's evolution in turn of energetics. I have no clue what fish you are referring to (and it would make sense for you to be more specific), but a cave environment is probably resource-limited. The reason that these fish would evolve progressively smaller eyes is that a) vision would be useless, and b) there is an energetic cost to moving and providing blood to eyes. Assuming the survival of these fish was limited by food and energy, then fish born with smaller eyes would have more energy freed up for the other aspects of their survival. Eh?

Answer 6

all of the answers are not fully correct. somewhat recently, 'natural history' magazine ran an article on blind cavefish. in it, the cavefish were discovered to have an abnormally large sense organ in the front portion of their skull. this organ was large enough to actually fill some of the space in the skull where the eyes used to be.

since the cavefish were living in a place devoid of light, there was no need for eyes. the eyes were neither selected for or against since their presence or absence had no impact on fitness, so eyed and eyeless fish (a mutation that arose at some point which would normally be lethal in above ground lakes and ponds) were present. over time, a larger sensing organ in their skulls was selected for (aiding in the process of finding food). the eyeless fish had more space available in their skulls since the eyes were absent and therefore became more prevalent through selection of the fittest.

Answer 7

well according to an evolutionary theory, over time organisms adapt to their environment. If large eyes are not necessary, they're going to get smaller. It's like a human's appendix and wisdom teeth, they no longer serve a purpose, and over time, according to evolution, those will probably become smaller and smaller until we don't have them at all.

Answer 8

because having eyes is more energy than its worth in dark places like caves. organisms that use less energy can use more energy to gather food, reproduce, etc. so having no eyes in dark situations means that the organism with no eyes will pass on its genes to the next generation and that will eventually become the dominant gene in the pool, if you'll pardon the fish pun.

Answer 9

when animals live in areas with minimal light, they have poorly developed eyes cos its useless to them to have good eyesight.
i assume ur 'smaller eyes' refer to other senses. lately , its found that some deep water fishes use other senses to move around like electric pulses etc...
and i believe that all these are part of nature evolution, creatures adapt to environment, just like how human evolved to be walking upright.

Answer 10

every organ has a cost

if it does not deliver a return it is retired

thats evolution

every advantage in life counts - even if it is the advantage of not doing something