Sunlight has been reduced to something hardly worth calling light at all at a depth of around 600 meters below the ocean’s surface. It is a weak, diffuse blue wash that comes from above and is so dim that it would appear to human eyes to be almost completely dark. The mesopelagic zone, often known as the twilight zone, is one of the most unusual locations on Earth where life has established itself. There is a great deal of pressure. There’s not enough food. Nevertheless, millions of tiny fish move through this ecosystem on a daily basis, and some of them use their skin in ways that scientists took years to completely comprehend.
Science has long been aware of the hatchetfish, a tiny, laterally compressed animal with a deep keel-shaped body that resembles a hatchet blade. The reason it is so hard for predators to find in an area where the only light is the dim downwelling glow from high above has been more challenging to figure out. The fish’s ventral side contains bioluminescent organs, which are microscopic light-producing cells that produce a glow that corresponds to the brightness of light from above. They understood that much. The skin itself and what happens to light when it strikes those guanine crystal plates embedded just below the surface were more difficult to understand.
The hatchetfish uses guanine, a biological substance that crystallizes into thin, hexagon-shaped plates, in a precise geometric pattern over its silvery flanks. The plates function as reflectors, but not in the straightforward flat-mirror manner that most reflective surfaces do. Because of their orientation and layering, incoming light—the blue light that is streaming down from above—is scattered and redirected in a pattern that mimics the surrounding water’s ambient glow. The fish virtually vanishes into its own visual surroundings. The hatchetfish creates no shadow, contrast, or visible edge to spot from below, where a predator would be searching upward for the silhouette of something delicious. Optically, it appears as more water.
This is complemented by the belly photophores’ counterillumination. The lateral and dorsal surfaces are handled by the crystals, which passively diffuse the downwelling light. At any given depth, the bioluminescent organs actively fill in the ventral silhouette by matching the intensity of any light coming from above. Together, the two systems provide what is essentially full-body optical camouflage, which eliminates the visual information that would allow a predator to generate a search image in the first place rather than relying on matching a background pattern. It’s an uncommon tactic that developed under a very particular set of evolutionary circumstances.
In recent years, guanine crystal optics have drawn attention from the material science community. Engineers working on optical sensors and low-observability surface coatings find these biological plates’ ability to scatter and divert light without significant energy loss to be genuinely intriguing. In the past, biomimetic research based on deep-sea adaptations has yielded useful outcomes; for example, the design of drag-reduction surfaces has been affected by shark skin, and polarized light sensors have been informed by the eye structures of mantis shrimp. It remains to be seen if guanine crystal configurations result in something equally useful, although research interest is genuine and expanding.
