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The largest eye in the world belongs to the giant squid, which has eyes up to 27 cm in diameter (some reports from the 1800s say 40 cm). Extinct ichthyosaurs also had very large eyes, 20-30 cm in diameter. How large can eyes realistically get? What evolutionary pressures could result in obscenely large eyes?

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  • $\begingroup$ Note icthyosaurs had eyes rivaling that of the largest squid, both use them for the same functions to see with high acuity in near darkness of the deep ocean. $\endgroup$
    – John
    Commented May 22, 2020 at 4:09

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Oh, those sexy eyes: You need to have some advantage to having eyes, or else they degenerate like blind fish. So why do animals get distorted into seemingly impractical forms?

  • The most obvious thought is that if eyes become a sexual display somehow, the might grow to the point of being dysfunctional. If the opposite sex likes it, it will select for it.
  • Big eyes imply dark places - either nocturnal animals or ones living in dark places like the deep ocean (those giant squid). This is a decent link to the whys and hows. https://animals.mom.me/nocturnal-animals-large-pupils-7471.html
  • Some special selective pressure and nurturing could result in a specialized species with really big eyes. If an animal (say, an owl) were domesticated and selected for large eyes because they can see ships at sea (for example) and alert their owners, those with the freaky birds can flee threats (or pursue victims, for pirates) even at night. People are people, and for something like this, big eyes could be selected for even if they don't actually result in better vision (appearance creates reality).
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    $\begingroup$ That first one is an interesting concept. How large would the eyes have to grow to become dysfunctional? $\endgroup$
    – Praearcturus
    Commented May 22, 2020 at 4:17
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    $\begingroup$ WRT your second point, note that the land animals with the largest eyes are the horse and the ostrich, neither of which are especially nocturnal. $\endgroup$
    – jamesqf
    Commented May 22, 2020 at 4:47
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    $\begingroup$ All that is needed for an eye to be dysfunctional is to be selecting for size rather than function. I'd guess the eye would stay at least somewhat functional if it's needed for survival, but if a male can get away with using other sensory systems to compensate, Who knows? I've never actually heard of animals where the eyes were specifically described as sexual displays, but I think of the peacock. Those are, of course, not REAL eyes, but still. PS I definitely know humans who consider eyes sexual displays. $\endgroup$
    – DWKraus
    Commented May 22, 2020 at 13:58
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    $\begingroup$ @jamesqf, I agree, but the ostrich and horse are relatively large compared to their eyes, and I was thinking more in proportion. Most nocturnal animals are smaller. I'm trying to come up with scenarios where the animals could potentially have almost cartoonish eyes. $\endgroup$
    – DWKraus
    Commented May 22, 2020 at 14:02
  • $\begingroup$ @DWKraus: Ah, relative size rather than absolute. Then (depending on how you actually define "eye") the record probably goes to the purple sea urchin, since the entire surface of their body is one big eye: livescience.com/5970-body-sea-urchin-big-eye.html $\endgroup$
    – jamesqf
    Commented May 22, 2020 at 18:58

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