The beauty and wonder of a squid's eyeball


Look at this squid's eye. Just look at it. See anything eerily familiar?

Squid, along with the rest of the family Cephalopoda, haven't shared a common ancestor with us vertebrates in some 500 million years—long before the evolution of our camera-like eyes. And yet, there the cephalopods are, flagrantly swimming about with eyes that use a lens to project an image onto a retina. Call it Squid Eye for the Vertebrate Guy. So, how's it work?

Convergent evolution, my friends. Convergent evolution. We happened to hit on similar solutions to the same problem of sight, even though the eyes of vertebrates and cephalopods evolved separately, in very different ways, at different times. Today, we can see that legacy in cephalopod and vertebrate fetal development. With vertebrates, the eyes grow on stalks, reaching out from the brain. In cephalopods, the eyes start as a clumping of cells on the surface of the skin and reach backwards, into the head, to make brain contact. Similar destinations. Very different road maps.

This lovely illustration—featuring dissections of the head, funnel, mantle and eye of a Thaumatolampas diadema—comes from The Cephalopoda Part I: Oegopsida and Part II: Myopsida, Octopoda Atlas written in 1910 by zoologist Carl Chun following a German expedition to the Indian, Atlantic and Great Southern oceans.

You can see more of Chun's detailed, passionate illustrations at the BibliOdyssey blog.

Image: Some rights reserved by peacay


    1. #1 is right. Even if the eye doesn’t ‘prove’ evolution, it certainly proves that god is an idiot. Even the most incompetent video-camera designer wouldn’t put the wiring in front of the lens.

  1. Evolution of eyes is straightforward, even obvious, once you’re aware of how simple converging lenses actually work. A light-sensitive spot can detect crude blurry images if that spot is down within a crater. If the crater’s edges close to form a pinhole camera, the image becomes sharp (yet dimmer, of course.) A curved blob of transparent material in the crater will allow the small pinhole to become wider, yet the image remains sharp. That’s the pinhole–> lens transition.

    I’ve found that many people reject the above because they have misconceptions about cameras/eyes, and this because they were taught their misconceptions by children’s science books which explain lenses incorrectly. While some books do get it right, many do not. A lens is basically a pinhole; it forms a sort of “wide pinhole” camera when placed in front of a retina or a piece of film. But with lenses, the “pinhole” can be large, yet rays from tiny points on the object are still steered to tiny points on the retina. See article linked in next comment.

  2. For those of us who’ve always had trouble understanding the usual “evolution of the eye” explanation, take a look at my article below. Could your stumbling block be this bad ray diagram found widely in children’s science books?

    A widespread optics misconception

  3. DOH, forgot this one below. Lots of good diagrams showing how eyes can repeatedly evolve.

    Google images: “evolution of the eye”

    Here’s something to try next time you’re in a swimming pool. Most of the lens power of human eyes comes *not* from the small internal lens, but instead from the cornea (from the curved air/water interface.) So, if you dunk your head in water, you remove most of the lens focussing power, and convert your eyes *almost* into pinhole cameras. The underwater view is about the same view you’d get if you remained out in the air, but used a wickedly sharp melon-baller to scoop away your curved cornea (while carefully preserving your iris and pupil.) Heh.

  4. My favorite strangely-evolved eyeball is the jumping spider’s. These freaky bastards have lenses, so usually they just get lumped into that category. But the thing is, their lenses are fixed to their carapace, while their retinas are mounted on muscles and are the things that actually move around inside the head. Furthermore, their retinas are basically one-dimensional, just a strip of photoreceptors with no lateral resolution at all, so the spiders have to scan their targets like god damn insectoid cylons. Cephalopods ain’t got nothing on these guys’ eyes.

  5. Feynman famously mentioned the squid’s eye as an example of convergent evolution.

    Although, as I read over this now, I don’t see the passage I remembered. Can anyone help?

  6. So Cthulhu created squid in his own image, in the image of Cthulhu he created him; male and female he created them. — Genexiz 1:27

  7. I always thought that squid got their eyes from the bodies of drowned humans. This isn’t nearly as cool.

  8. I think it’s important to point out that in spite of the similarities, the exact structures are still different in significant ways even for the cephalopods that have the most human-like eyes. The cephalopod lacks a cornea (in fact, the nautilus eye doesn’t even have a lens…the retina is exposed to the environment), has a convex (everted) retina, and is sensitive to the polarization of light.

  9. Such a beautiful proof of evolution. No efficient god would do the same thing two ways. I heart squid. Although I steer clear of grumpy ol’ Humboldt.

  10. these cephalopods seem created in the image of something familiar…. Don’t those eyes look a bit like meatballs?

  11. @ beelzebuddy

    this quote from your link about jumping spiders will give me wonderful nightmares:

    “Because the retina is the darkest part of the eye and it moves around, you can sometimes look into the eye of a jumping spider and see it changing color. When it is darkest, you know the spider is looking straight at you, because then you are looking down into its retina. “

  12. The eyes of the colossal squid (Mesonychoteuthis hamiltoni) are the largest in the animal kingdom, they are the size of beach balls.

Comments are closed.