The oceans of the world are home to animals that render themselves invisible with glowing eyeshadow.
They’re called glass squid and, as their name suggests, they are largely transparent. They’d be impossible to see in the darkness of the open ocean were it not for their eyes—the only obviously opaque parts of their bodies.
These animals live between 200 and 1000 metres below the ocean surface, where water is mostly dark. Still, some sunlight penetrates to these depths, and this light is hundreds of times brighter than anything reflected horizontally or upwards. As such, any predator looking upwards at a glass squid would see the squid’s eyes in dark silhouette against a relatively light background.
To hide itself, a glass squid uses a trick that’s common among many oceanic animals: counter-illumination. Two organs under its eyes, known as photophores, give off a dim light, which perfectly matches the weak light coming from the surface. Their glow cancels out the squid’s silhouette so that, from below, instead of just being mostly invisible, it is completely invisible.
It helps that the glass squid’s eyes stick out from the side of its head, and are controlled by powerful muscles. No matter where the squid’s body is pointing, its gyroscopic eyes always stay in the same position, with the light-producing photophore beneath them.
But that still leaves a significant problem. Without any guidance, light would leave the photophore in every direction, making the squid hard to see from directly below, but very conspicuous from other angles. Its glowing invisibility cloak would also be a beacon, were it not for yet another cunning anatomical feature.
Amanda Holt and Alison Sweeney from the University of Pennsylvania have now reported in the Journal of the Royal Society Interface that a glass squid’s photophore consists of long, skinny cells that are shaped like hockey sticks—they run parallel to the eye, and then take a sharp downward turn. The walls of these cells are lined with reflective proteins that turn them into living optic fibres. They channel the photophore’s light along their length and then downwards, into the ocean’s depths.
“They’re a way of building a literal pipe for light,” says Holt.
But wait, there’s more!
When the duo first saw the fibres, they “thought it was going to be straightforward and boring,” says Sweeney. “Oh, there are little fibres. That’s cute. We’ll describe how they work and move on.” But when they looked more closely, they noticed that the fibres are really leaky. That is, they’re not perfectly reflective. A little light always pours out along their length.
They don’t have to be like that. A few easy structural changes would turn them into perfect light guides. Instead, “they’re really inefficient,” says Sweeney. “We struggled with that for a while, before realising: Oh, that’s part of the point.”
In the deep ocean, most light comes from directly above, but a small fraction still travels at oblique angles. So the squid’s counter-illuminating light also needs to work in many directions. That’s why the photophore fibres are leaky. They’re like the diffusers that you can stick on a camera to spread the light from a flash over a large area.
Holt confirmed this by creating simulating of the fibres and calculating how much light they send sideways and downwards. She also calculated the light levels in the squid’s mid-water habitat and, again, calculated the amount of light travelling sideways and downwards. The two ratios matched.
“I remember sitting in my office comparing the two, and my jaw dropped,” says Sweeney. “I thought there must have been a mistake and we couldn’t possibly have been that lucky the first time round. But we were.”
So the glass squid’s photophores are omnidirectional invisibility cloaks. They obscure the animal’s eyes by perfectly matching the light coming in from every direction (at least, in the lower half).
The squid shows how imperfections can actually be a good thing—a lesson that, according to Sweeney, engineers should pay attention to. “Over and over in biology, we see that evolution harnesses disorder in very clever ways to make better devices than what you’d get with highly ordered structures,” she says. “Thinking [about this] will help engineers to leverage the disorder in their systems rather than trying to get rid of it.”