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I have a sci-fi-like universe and one of the species has long, vertical eyes.

A diagram of the eye I am attempting to describe.

But would this be anatomically possible without a sphere-like structure? I know that compound eyes are a thing, but how "flat" can they be in 3D? Like how anime girls have ginormous eyes that, anatomically speaking, would leave no room for brain. (Which isn’t an option for my species, as they are highly intelligent.)

How would I design their eyes?

Do I need to change the shape, create a custom structure, or search more for other existing types of eyes?

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    $\begingroup$ Not all eyes are spheres. Have you tried finding a book of comparative anatomy? For example, the eyes of eagles are definitely non-spherical, with the back side wide and flat, which allows for sharper image formation on their retinas. By contrast, the eyes of snakes are flatter in front, with their corneas very much flatter than in humans, and their eyes appear compressed front-to-back compared to human eyes. (Compound eyes are globular because each ommatidium has a very narrow field of view. If the ommatidia were arranged in a plane the compound eye could see only directly in front.) $\endgroup$
    – AlexP
    Sep 19, 2023 at 11:44
  • $\begingroup$ What do you need for their vision to be like, as good as people's now or more basic? $\endgroup$ Sep 19, 2023 at 11:45
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    $\begingroup$ A disadvantage of non-spherical eyes (as in raptors and snakes) is that they cannot move in the orbits, and the animal needs to turn its entire head to shift its view... $\endgroup$
    – AlexP
    Sep 19, 2023 at 11:46
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    $\begingroup$ Note also that, judging from your picture, it might not necessarily be the eye you're talking about. I'm jumping to conclusions, but perhaps your problem is solved with the eye lids being that long, wonky shape? That might be easier to rationalize, especially if the eye was a compound (a series of irises that had specific features, there's only so much space for rods and cones in one eye). But if their planet had massive amounts of (e.g.) quartz, that long lid could evolve to offer refined protection against glare. $\endgroup$
    – JBH
    Sep 19, 2023 at 13:41
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    $\begingroup$ they eyes you have drawn could still be spherical, the shaped of the external opening for the eye s not the same as the shape of the eye underneath. What you have drawn could be human sized eyes. $\endgroup$
    – John
    Sep 19, 2023 at 20:24

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Eyes do not have to be round, but you have to ask yourself what the "pointing" mechanism is. How does the eye shift where it's pointing?

For a human, our eyes can move around in the socket, and the socket moves around with the rest of the head. This flexibility allows us to concentrate our retinal capacity on a relatively small, high-density patch. The skull socket provides protection and image stability.

There are many species that can't rotate their eyes in a socket at all. There are a few strategies for compensating. Flexible necks/stalks and numerous eyes are the most common strategy.

Your creature looks like it has a vertical slit that the eye can travel along. Perhaps the top and bottom of the slit have different viewing angles, so the creature can pull its eyes to the top of the slit if it needs a wide viewing angle, and can pull them to the bottom of the slit if it wants binocular vision. This mechanism would basically be a completely internalized eye on a stalk, with the connective nerves pulling back like the tongue of a chameleon.

You would want to consider the mechanism by which this creature could look up and down, since this scheme doesn't provide that.

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Those are just markings, not actually long eyes

The tell-tail iris & pupil in the middle actually very much implies that your alien has round eyes that are much smaller than they appear. So one distinct possibility is that the long slits are not actually part of the eyes themselves, but a separate adaptation.

I can think of 3 likely reasons for an animal to evolve this particular feature:

  1. It is a glare reduction feature designed to reduce how much sun shines off of its face blurring it's vision like the markings on a cheetah or racoon. The slits appear red instead of black because the alien can not see in the red part of the light spectrum; so, to the alien, these slits are essentially black.

https://en.wikipedia.org/wiki/Raccoon#/media/File:Raccoon_(Procyon_lotor)_2.jpg

  1. It is a light or sound collection feature like the discs around an owl's eyes used to amplify one of its senses. For sound, it could have small tympanic membranes at the tops of these slits, and their slits basically function like ears. For light, it could be that these aliens only see in the Red/IR part of the spectrum; so, to them, this is a white reflective surface designs to help collect a wider area of light in low-light conditions.

https://upload.wikimedia.org/wikipedia/commons/0/0f/Portrait_of_owls.jpg

  1. It is a form of protective mimicry meant to make it look like it has huge eyes so that when a predator spots it while its hiding, the large eyes would make it think it is part of a much larger creature like the spots on an certain kinds of butterflies.

https://en.wikipedia.org/wiki/Owl_butterfly#/media/File:Caligo_martia_(ventre).jpg

Either way, it may be much simpler to explain these as an illusion than actual eyes.

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(...) I have found myself with an issue: would this be anatomically possible without a sphere-like structure?

Yes, it would. Land vertebrates have round eyes because that shape helps keeping the fluids inside with well distributed pressure. Notice that eyes are not perfect spheres - in humans, for reference, the thicker your glasses are, the more flattened your eyes are.

Fish eyes are flatter. Look at the eyes of soles and flatfish.

I know that compound eyes are a thing, but how “flat” can they be when seen in 3D?

As flat as you want. Also as weird as you want. Some trilobites had cylindrical eyes coming out vertically from their heads, and made of crystal (calcite) on top of that.


Looking at your creature in the drawing, what I see is a small, green, quasi-spherical, flattened eye on each side of the head. The red parts around it are two large caruncles, which are not part of the eye per se.

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You might want to consider something inspired by octopi -- ie., a mix of shallow eyes and light sensors embedded into the epidermis in clumps. The octopus structure technically allows for any kind of surface shape for the "eyes" (they can be pretty flat and still work). And extra light sensors in the epidermis then will make up the difference if the eyes themselves aren't powerful enough or otherwise sufficient

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The ability to move eyeballs does not come for free. It is anatomically much more complex than an eye that is fixed in one position, and it also requires more brain power to process visual information when there are additional factors complicating it. You eg. have to deal with changes in parallax (when you are looking sideways without turning your head, your eyes are effectively closed to each other than when looking forward), and you also have deal with both head position and eye position together.

This goes because there are two big benefits:

  1. ability to see both close and far away, and
  2. speed and accuracy.

1: The resolution in the eye is not typically uniform in creatures on our planet. Density of cells near macula ( the part of your eye that processes what you see directly in front of you (your central vision) is much greater than in areas elsewhere. Near the edges, resolution is quite low. This is not evident, though, because your brain masks it in the same way it hides the blind spot.

If your eyes can move, you only need one area with increased resolution. You can point your eyes easily towards where you need it. However, if you cannot move your eyes, you either need to have much larger area with increased resolution or live with not being able to see well both near and far, you have to choose one.

Note that looking both close and far ways is not covered only by adjusting the lens of they eye. We cross eyes when looking near because of this.

2: Eyeballs are lightweight and therefore much much easier and faster to turn than a head, and it is much easier to make it accurate. You can orient your eyeballs several times in the time it takes to turn your head towards an object of interest. The alternative solution would be wider field of view.

Your aliens lack both. They have vertical slits, and no moving eyeballs. They have a problem if they get surrounded: they are effectively half-blind because they cannot keep monitoring their environment as effectively as creatures with moving eyeballs or wide field of vision. They need to turn their head constantly, and turning head reveals in a loud manner where their blind spots are, and because turning the head is also slowish, the enemy can take advantage of this shortcoming.


I am not saying that your eye design is completely implausible, though. There are just some issue with that kind of eye, but you might be able to work around them. Also, you can address some of the functional shortcomings in their anatomy like people complain about their back (the spine works well in aquatic environment where it evolved, but not on land, and even less so in an upright position).

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a look at the eye of the Mantis shrimp. This is a compound eye, so it does not need a spherical overall shape. It is about as hardcore bonkers as the rest of this animal.

Birds such as owls have binocular vision, but their eyeballs are not round. This is more compact than a spherical eyes, but they cannot pivot the eyes in their orbit. Most of the optical surfaces are approximately spherical.

There are plenty of other weird eyes out there - these were just the first two that came to mind.

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