The midwater region of the ocean is the world’s largest habitat by volume, making up 99 percent of Earth’s livable space. It’s home to a myriad of bizarre marine occupants, many of which have evolved peculiar abilities that allow them to survive.

“They spend their whole lives sort of suspended in the middle,” says Alison Sweeney, an assistant professor in the Department of Physics & Astronomy in the School of Arts & Sciences. “Because they live in this three-dimensional void, they have to deal with being potentially visible from every angle. There’s literally nothing to hide behind, and so they end up hiding within the light itself. The deep sea is a really amazing place to look for cool biological optics because so much of the ecology is driven by light.”

Hatchetfish, so named because the shape of their bodies resembles the blade of a hatchet, are one of the “classic-example weirdo fish denizens of the midwater,” Sweeney says. Because many deep-sea creatures hunt by looking up and seeing shadows or silhouettes, the large flat bodies of hatchetfish keep them relatively well hidden. Their skin, she says, is somewhat metallic-looking, resembling the dull side of aluminum foil.

Hatchetfish also have a line of photophores on their bellies that produce light, or bioluminescence. This is useful for the fish when they are swimming in waters shallow enough for sunlight to dominate. By producing their own light with the same intensity as the faint sunlight coming from above, the hatchetfish make themselves invisible to predators.

But this counter-illumination technique doesn’t work in the deep sea where sunlight doesn’t reach. In this region, other predatory sea creatures have evolved to create light with their own bodies, which they can use as searchlights to hunt for prey.

Until recently, scientists believed that hatchetfish were able to hide in the void because their reflective scales allowed them to behave like mirrors: Light traveling toward the fish would bounce back at the same angle, matching the light coming from behind them and effectively cloaking the fish.

But the Penn researchers realized that acting like mirrors would actually make fish more vulnerable in the deep sea: Light would be sent back to the predator, signaling the fish’s location.

The researchers dug deeper into the hatchetfish’s mechanisms for camouflage to reveal that they’re really not like mirrors at all. Rather than bounce light directly back, they scatter it in a diffuse, non-mirror-like pattern that makes them much less visible to predators hunting with light.

The researchers also discovered that when they shined light directly onto the side of the fish, the structures they were studying actually piped the light through the fish’s body, funneling it downward through the photophores in their bellies, sort of like a “beam dump.”

Because the habitat of hatchetfish seems to include both shallow parts of the ocean, where they can be seen in the dim, downwelling sunlight, and the deeper parts, where they are most vulnerable to searchlights, this mechanism may assist in their camouflage in two ways.

In the top zone where solar light dominates, hatchetfish may direct some of the sideways sunlight down through their photophores, which may assist in their counter-illumination. In the deeper part of their range where they’re vulnerable to searchlights, dumping the light downward will throw predators off their trail.

By looking at the mechanisms by which biological materials control light, scientists may be inspired to use similar designs in technological applications.

“I think there’s a fundamental curiosity of basically just how sophisticated nature is in terms of photonics,” Sweeney says. “We want to know if we can we actually learn mechanisms from nature that we wouldn’t necessarily have gotten to through a top-down engineering approach. And the answer to that is, ‘Yes.’”