- [Speaker] This is a hogfish.

And watch closely, it's gonna do something pretty magical.

- They would go from a stark white over sand to this striped pattern when they were approached, when they felt like they were in danger, maybe when they saw others in their own species.

And so we know that they had this color changeability - [Speaker] Watch again.

Much like a chameleon, the reef-dwelling hogfish can toggle its skin color between white, brown and a stripe pattern in seconds.

What scientists didn't know is that a hogfish is a kind of zombie fish.

It can change color even after it's dead.

- We harvested this fish.

I captured one fish and I'd put it on the deck of my boat.

And later when I returned to the boat, I picked up the fish and the side that had been facing the deck, changed color white and took on a pattern that matched the texture of the deck.

It actually color changed, mimicked the environment that was beneath it, and I immediately took a picture.

So I have this old picture of me just standing on the boat, ropes in the way, of this unbelievable color change, an animal matching a boat deck.

At the time, it made me think something extraordinary was happening about how the animals see the world.

We knew that they could change color, but what we didn't know was that they could do it postmortem right after they had died.

And then to change color to match something so artificial.

[light music] - [Speaker] Hogfish utilize specialized cells called chromatophores, which each contain red, yellow, or black pigments to change color.

The fish changes color by shifting how the pigments are organized in the cell.

But to understand how hogfish pull off the after death color switch, the team first discovered that opsin, the same light-detecting proteins found in human retinas, are found in the hogfish's skin.

- [Lori] The very same proteins that let you see blue light with your retina is the same protein, this opsin, that is in the skin of the hogfish, all over the hogfish.

So we knew that a light-detecting protein was in the skin.

- [Speaker] But that discovery only deepened the mystery.

What were the opsins doing and why see through the skin when hogfish have a perfectly good set of eyes?

- Well, we found... We used this amazing method where you can make proteins of interest glow.

So we took skin of hogfish, we cross-sectioned it, cut it in thin sections.

We applied this marker to make our opsin glow so we could find where it was.

And surprisingly, instead of right on the surface of the skin where maybe they'd be looking at the surrounding environment, we found this opsin buried beneath their color change cells, almost as if they had a camera pointed at the TV screen of their color change.

[light music] - [Speaker] The opsins are those glowing cells, but the electron microscope revealed the opsins don't have a view of the world at all.

The proteins are packed into specialized cells under the chromatophores, the pigment cells that allow the hogfish to change color.

- This is a cross-section through the skin.

If light is passing like from the outside, this is the outermost layer of the skin.

So this is a bunch of epithelial cells.

And then we get to the chromatophore layer.

So you see this black pigment, that's a chromatophore, and actually these smaller globs here, these are yellow or red chromatophores, so they're filled with yellow and red pigment.

And then the green labeling is right underneath these chromatophores.

- [Speaker] Here's the key.

While the pigments in the chromatophores change the outward-facing color of the hogfish, the pigments also filter how much light reaches the opposite.

- So the light is coming in from this direction.

This would be the scale chromatophore and then photoreceptor.

So it is capturing light in a similar way to the cones and rods of your eye, but it's not using that for image formation.

It might be, it's using that for the feedback systems.

- [Speaker] In effect, the opsin is able to see what color the fish is from the inside.

- Basically, when the animals have a dark pattern over their body, it's... the pigment is dispersed in these color chain cells.

It blocks the blue light needed by the opsin to be activated, shuts them off.

But when the animals turn white or take on this lighter pattern of the body, the pigment of these cells gets out of the way.

The blue light can pass by and turn the cells on.

In this way, we had this whole shift of perspective.

Maybe the animals aren't using this skin vision to watch their surroundings, but rather to watch themselves to know what color they are if they're performing color change correctly.

- [Speaker] The working hypothesis then is that the hogfish doesn't use its skin vision to watch its surroundings.

It's using it to watch itself, a way to double-check what's happened.

- For a fish that has to color change for survival, perhaps to camouflage in its environment, it needs to be completely sure that it is color changed correctly.

[light music] The eyes, of course.

You have beautiful detecting system on the face, the eyes image of the surrounding environment, telling the animal, "You need to color change.

You need to accomplish white.

You need to accomplish the stripe."

But the skin vision is watching itself to make sure it did that command correctly.

Why is that so cool is not just an answer for hogfish, but virtually every color changing animals from octopuses to reptiles, amphibians, and fishes.

We just keep making these discoveries that they can detect light with the skin.

Look how whimsical it is.

Look how mysterious it is.

And this is something worth exploring.

[light music continues]