How Hatchetfish Camouflage in the Deep Sea

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

According to Alison Sweeney, an assistant professor of physics and astronomy, 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.”

Because many deep-sea creatures hunt by looking up and seeing shadows or silhouettes, hatchetfish’s large flat bodies keep them relatively well hidden. Their skin is somewhat metallic-looking, resembling the dull side of aluminum foil.

Hatchetfish also have a line of photophores on their belly that produce light, or bioluminescence. This is useful for when the fish 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 a mirror: light traveling towards the fish would bounce back at the same angle, matching the light coming from behind it and effectively cloaking the fish.

But Penn Arts and Sciences researchers realized that acting like a mirror 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, 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.

They also found 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 its belly like a “beam dump.”

In the shallow part of the ocean, hatchetfish may direct some of the sideways sunlight down through their photophores to assist in their counter-illumination. In the deeper part of the ocean, dumping the light downward will throw predators off their trail.

Sweeney says that one of the themes in her lab is to push physics by reaching a fuller understanding of what nature and evolution can do. 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.”