Name: 動物界の「目」全種類を解説 | WIRED Japan
Uploaded: 2025-05-23T08:08:57.000Z
Duration: 36 min 25 s
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Did you know that ambush predators tend to have vertical-slip pupils to better pinpoint their prey, while grazers tend to have horizontal pupils to better scan the horizon for incoming movement?

We created this tree of life that focuses on the evolution of animal eyes, and we're going to walk through it with Professor Lars Schmitz.

I study vision and the evolution of eyes, and I just love everything about eyes.

We'll start at the base of the tree with an early branching fork of animals called cnidaria.

The box jellyfish actually have 24 different eyes in clusters of six eyes each.

One pair of these eyes in each of these clusters are not unlike our own eyes.

We have the aperture, the pupil that lets the light in, behind which we have the optical system.

So what we nicely see here is the curved corneal, the optical apparatus in front of the eye, and the second optical element, the lens.

Those together refract light onto the retina and forms an image.

To stay with a camera analog, this would be where the film would be, or the receptors.

Cubizones are really interesting in that they don't have a centralized nervous system.

They have a nerve net, which allows them to process visual stimuli, but they don't have a real brain.

So it's actually really puzzling how they can actually interpret those visual signals.

From here, the tree of life has a major divergent path.

We'll follow first the site of the protostomes, which includes insects and many deep sea animals.

They actually have dozens of eyes around their mantle.

Every single blue dot you see in this image, these little blueberries, those are eye structures.

This ring of blueberry eyes is actually useful for them as a defensive mechanism and a little bit of a navigation help.

The way the scallops form images in their little blueberry eyes is very different from what we do.

It has a mirror that's sitting behind the retina, and that mirror is used to focus the light back onto the retina.

So it's a really complicated mechanism, but it works well enough to get you an image of the scenery.

The resolution is really not that good, but scallops can actively move.

So they can visually navigate to an extent.

And also when they see something approaching, they can shut down and close down and protect themselves.

Let's look at a few examples of eyes from some cephalopods.

The pinhole eye of the nautilus works just like the pinhole camera and has a very small opening through which light enters, and then an inverted image is formed on the retina.

So let's take a look at the human eye model here.

Light's coming in from right here, and then the image, as crude as it is, falls directly onto the retina.

The smaller the pupil, the better the resolution of the resulting image.

But the issue there is if you make the pupil really small, the image will be dimmer and dimmer the smaller the pupil is.

They do have a central nervous system to process visual information.

Actually a little bit of a mystery what exactly they're using that eye for.

The image will be a little bit dim, so it's kind of limited in what it can see with that eye.

Octopuses have a chambered eye type, and in difference to the nautilus, this one has a lens.

And what we have in common here is this deep eye cup.

A pupil that's really well defined, that's the only part where light can enter the eye and nowhere else.

If light from all directions could enter the eye, we wouldn't know where it's coming from.

One really important difference between octopuses and our own eye.

The photoreceptors are actually pointing away from the light.

And the axons that transmit the signal approach the incoming light.

But in octopuses, the retina is the other direction with the photoreceptors pointing towards the incoming light and the axons going away.

The side effect of the inverted retina that we have, the so-called blind spot in the retina where we do not have any photoreceptors.

So in this model, we can see the optic nerve really clearly right here.

So this is where all the axons from the photoreceptors are bundled together and exit the eye and transmit the signal off to the brain.

But in this part right here, where the axons exit, that's where we cannot have any photoreceptors.

From my understanding, it's not really well known how exactly and what may explain that pattern.

There may be a small advantage of having the photoreceptors a little bit further back.

The resolution of the image may be a tiny little bit better.

Personally, I'm not sure if that's a huge reason that would provide additional evolutionary fitness.

The bottom line is our retina, even though it's inverted, it works well enough.

One thing that's really interesting about squid is their cool pupil shapes.

If we just look at squid, here are two examples.

On the left, we have this W shape in here.

The function of these pupil shapes is a little bit of a mystery.

And a really interesting hypothesis in this context is color vision.

Color vision in squid is not well supported behaviorally, but it would make sense given that they use colors for signaling, for example.

In humans, in our own color vision, we use three different opsin types, so three different photopigments to distinguish different colors.

Depending on the strength of the relative excitation of these pigments, we can infer what color wavelength we're actually looking at.

In squid, we don't have different opsin types.

The image would be essentially a grayscale image.

How can they see color if they only have one opsin type?

Many animals correct an optical problem, chromatic aberration.

Chromatic aberration results from the fact that shorter wavelength light is refracted more strongly than longer wavelength light.

If you have blue light versus red light, the blue light would be focused a little bit before the retina and the red light a little bit behind the retina.

Now this is something that most animals actually try to get rid of.

There's the hypothesis that these pupils actually accentuate the chromatic aberration.

And they can use that to tell what color light they're actually looking at.

This blur hypothesis is a bit controversial in our community.

Some people say, while theoretical, possibly the practical benefit isn't all that big.

The key will be to test how these animals perceive colors and if there's good behavioral evidence to find out.

As we head back through the tree of life, let's stop to talk about the flatworm.

This little fellow has two very conspicuous eyes right here.

These are very simple eyes, like little eye spots.

What we see here is eyes that are really not good from our perspective, but are beneficial to this aquatic flatworm.

Essentially, it would help them to avoid getting eaten.

These eyes are consisting of just two cell types.

They have photoreceptors in a shallow cup.

So light can only reach these eye cups from above and a little bit from the side.

But it cannot reach the eye cup from below because that's where the pigmented cells are.

This flatworm would be able to tell if it's in bright versus dark environments.

And it would know if the light is coming from directly above or perhaps a little bit from the side.

Now that seems like utterly useless to us.

But for this flatworm, it's good to know like it's bright, it's exposed, it's in the dark, it's perhaps a bit protected.

Now let's cover the other fork of the protostomes, which include insects, spiders, and some other arthropods.

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動物界の「目」全種類を解説 | WIRED Japan

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