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I'm designing a sentient, aquatic creature, and trying to decide how many eyes it should have.

This creatures biological strategy is one of redundancy - it has multiple mouths, multiple tentacles, multiple...well, lots of multiples of lots of things. I was thinking of giving it multiple relatively primitive eye-patches scattered across its 'head', but I also want it to have something close to human-level vision. I'm afraid I don't quite follow the quality differences between compound eyes vs complex eyes, so:

Is it plausible for a creature with many small eyes to have vision as good as a human? If not, how good would each individual eye have to be to roughly match human sight?

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  • $\begingroup$ An aquatic creature with many eyes but no head. $\endgroup$ – AlexP Dec 5 '16 at 20:28
  • $\begingroup$ @AlexP Indeed, something like the scallop is what I had in mind when I started developing this idea. Trouble is, as the article said, "These eyes are not exactly competing with eagle eyes for visual acuity." I'm trying to figure out how well creatures with this sort of setup can see. $\endgroup$ – Werrf Dec 5 '16 at 20:30
  • $\begingroup$ This may be a red herring. The nature of the medium makes great visual acuity impossible -- water is not isotropic and not particulary transparent. Look at the details captured in underwater movies compared to movies shot out of the water. $\endgroup$ – AlexP Dec 5 '16 at 21:09
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@MichaelKjorling gives a good answer that should be expanded upon.

I'm guessing that you creature is an invertebrate since you mention it has "multiple tentacles" and imply that its 'head' is not a tradition head. It may be helpful to think about the evolutionary advantages for an animal with multiple methods of sight (and why we evolved sight in the first place). Why does an eagle need such great visual acuity? To see its prey from a great distance. Did you know birds can't turn their eyes? Its easiest to see where you're going with biological "blinders" built in. Why do fish have an eye on both sides of their head (yeah, yeah...except the flounder)? To see predators better. Did you know deep sea fish tend to see more blue/green while fish in lakes are rivers see more red? And sometimes you don't need any visual acuity at all. Sometimes you just need to know if something is between you and the light source, like an octopus that can sense light/dark with its skin. The eyes of dragonflies are linked directly to their wings to allow them to track prey without using any brain power. But in my opinion, the king-daddy of ocular evolution has to be the Mantis Shrimp. It has 12 different types of photo receptors (compared to our 3).

So thinking of your creature, perhaps it only has two "good" Mantis Shrimp-like eyes, but a multitude of other "eyes" that see varying degrees of detail or color. Its skin (which could sort of count as an eye) senses only light or dark so it knows when it is in hiding. Scattered around its body are numerous simple eyes that can sort of see. They're better than just sensing light/dark, but they lack a lens or mechanism for focusing so their main purpose is to grab the attention of better eyes when they detect something. The main eyes (2, 3, 5 whatever) would be like those of the Mantis Shrimp. These eyes would have numerous types of photoreceptors to see a broad spectrum of light as well as its polarization. Throw a couple on some retractable eye stalks since they will be the ones that use the most brain power and have the highest evolutionary cost (and therefore in need of the most protection), and you have a believable creature.


Edit: Expanding on the idea of brainpower and evolutionary costs. Have you seen any of the YouTube videos where kids flip a bottle of water and get it to land correctly (yeah, I don't get it either :/ )? What you don't see is the huge number of failed attempts they make before finally getting it. Nature does the same. There isn't any intelligence involved in evolution. Evolution is a series of mutations. Some mutations help the organism survive, and some don't. This is natural selection at work. "Oh, that extra rib prevented you from jumping out of the way and didn't stop the hawk from eating you once he caught you? Good thing it took those genes out of the gene pool."

The evolution of vision really exploded during the Cambrian period. Not coincidentally, body coloration (as camouflage, as a warning that "Hey, I'm poisonous!", or as a reproduction method "Hey, look at me! I'm sexy!") also exploded during that time. But this came at a cost. Senses take energy. A study of blind Mexican cave fish found that they use 5 to 15% less of their metabolism by not having sight. And why have a circadian rhythm when there's no day/night cycle? That's another big energy savings. Want to eat on cold days? That will cost ~50% of your metabolic energy. Chimps have something like twice as many glucose transporters in their muscles than we do which is why they are pound for pound stronger than a human (sorry, I can't find a link to back that up...just pulling it from a memory of an article I hope was factual). Somewhere on our evolutionary path, we "transferred" some of our glucose transporters from our muscles to our brains. Our ability to "out smart" other animals costs us about 20% of our caloric intake. Every biological activity has a cost.

If you want to have lots and lots of biological activity (eg, multiple eyes, mouths, limbs, etc.), you either need to eat A LOT, find energy dense food, or cut energy consumption in other areas. This known as the Expensive Tissue Hypothesis. In a nutshell, the greater the evolutionary cost, the more energy it consumes. And if it consumes a lot of energy, it must be pretty vital to the survival of the organism. If it isn't vital to the survival of the animal, it will eventually go away. Things that are vital for survival get protected: vital organs have a rib cage, the brain has a skull, eyes have lashes or eyelids or nictitating membranes, etc.

And finally coming back around to your creature. The simple light/dark sense of the skin doesn't require much on the order of brain power or metabolic energy (at least, I'm presuming it doesn't since this is the earliest "eye" in evolutionary terms). The simple eyes that can make out vague shapes can be wired directly to the mantis-eye stalks (thereby saving brain power) so that when they see "something", the eye stalks automatically swivel to gather details. This isn't very far fetched. Horseshoe crabs have two simple eyes on their belly so they can see predators above them when they are swimming (because they swim upside down). In terms of your creature, I'm imaging a species of octopi that figures out how to cook food over thermal vents which in turns leads to even larger brains.

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  • $\begingroup$ Nice you mentioned brainpower. If you could elaborate on that a bit more? +1 anyway. $\endgroup$ – Mołot Dec 6 '16 at 7:41
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    $\begingroup$ From your comment and excellent detail, I'm imagining the creature as having a number of small, weak eyes distributed around the 'head' that can detect gross changes in the environment (movement, sudden change of light level or colour), interspersed with three or four eye-clusters that can resolve more detail, and a single powerful eye, probably on a stalk, that can see clear detail; if the 'cheap' eyes detect something, the body can turn to focus on it with a cluster; if it still seems potentially dangerous or interesting, the Big Eye can be brought to bear. Reasonable? $\endgroup$ – Werrf Dec 6 '16 at 16:57
  • $\begingroup$ That seems very reasonable. $\endgroup$ – Tim Dec 6 '16 at 18:13
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I don't think there is any inherent reason why a large number of "eyes" would necessarily have to imply the downsides of compound eyes.

For an extreme example, you could consider each cone or rod in the human eye to be a separate organ. (Stay with me!) In the case of humans, they all share only two openings for the light to pass through, but there's little reason why it has to be that way.

Your creature could have a large area covered with rods and/or cones, and a large number of small openings to let light in and focus it on the rods and/or cones. This would resemble a grille more than it would resemble human eyes, but the basic biology could still be very similar.

The obvious downside seems to me to be that the creature would be highly vulnerable to physical trauma to that part of its body, much more so than creatures that only have a pair of eyes. This is because its skull would be comparatively weak in that area, as it cannot be particularly thick without restricting the creature to a particular field of vision. An adaptation in the direction of reinforcing the skull could visually resemble something like a VISOR, only integrated into the skull instead of outside of it.

I'm at a loss to explain how something like that would evolve in a natural setting, but...

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Why has this creature so many redundancies? That should only evolve through harsh life circumstances, many enemies (own species, predators if the creature is prey or dangerous prey if the creature is a predator). if you can answer why the creature has redundacies, you can answer the "eye question"

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  • $\begingroup$ Um...no, I can't. The "eye question" was simply "can multiple primitive eyes generate images as detailed as more 'advanced' eyes", not "why does it have a lot of eyes". $\endgroup$ – Werrf Dec 7 '16 at 12:44
  • $\begingroup$ oh, ok. Then maybe you could read about fly eyes. As far as I remember, they are very capable altogether. $\endgroup$ – Julian Egner Dec 8 '16 at 19:20

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