It’s important to note that most people are unfamiliar with bristlemouth fish, despite the fact that they may be among the most common vertebrates on Earth. They live in the mesopelagic zone, which is the middle layer of the ocean between 200 and 1,000 meters deep. Although their exact numbers are unknown, oceanographers regularly describe them as being massive. They have rows of light-producing organs called photophores running along their undersides, and they are small, black, and delicate. It has taken some time for scientists investigating specimens from U.S. West Coast Pacific seas to adequately characterize what they have discovered inside the skin enclosing those photophores: a crystal architecture that bends, reroutes, and recycles light instead of just reflecting it.
Guanine, one of the four nucleobases that carry genetic information, is the molecule that makes up the crystals. It is also present in DNA and RNA. Large amounts of guanine are produced by living things during regular cellular functions, and some have developed strategies to repurpose it into structural materials. Guanine platelets placed in flat stacks produce structural color thru thin-film interference, giving some fish scales their iridescent sheen. The crystals’ form and arrangement around the photophores are different in Sigmops gracilis and similar bristlemouths. Here, the crystals are needle-shaped and arranged in configurations that work more like prisms than flat platelets that serve as mirrors.
Once you know what it’s doing, the optical physics of this is very elegant. When managing diffuse internal emission in numerous directions, a flat mirror surface is inefficient because it reflects light at a predictable angle. Depending on the angle of incidence, light is bent by a structure that resembles a prism and scattered along several routes. This means that in the case of a photophore inserted in tissue, photons that would normally flow inward or sideways—basically wasted—are instead caught by the crystal array and redirected. Utilizing photons that the animal’s biology had already expended energy on creating, the crystal structure serves as a light recycling mechanism.
The two main reasons this is important for fish are metabolic economy and stealth, both of which are major issues in the mesopelagic. These fish employ counterillumination as a tactic to evade detection from below. When hunting upward in the gloomy twilight zone, predators search for shadows, which are black forms that obstruct the surface’s faint downwelling light. The fish successfully erase their silhouette by producing their own light from the ventral surface and adjusting its intensity to match the surrounding downwelling light. By providing the fish with more exact control over the distribution of the photophore light—a controlled diffusion that smoothes the match to the ambient light field rather than a single directed beam—the crystal system aids in the fine-tuning of this camouflage.
Although distinct, the energy argument is connected. The ecosystem in the deep water is not abundant. The restricted food supply that filters down from above in the form of sinking organic matter limits the metabolic resources accessible to mesopelagic fish. Survival is extended by every metabolic process that can be improved. For every unit of chemical energy used in bioluminescence, the guanine crystal system produces more photons. This efficiency advantage has obvious evolutionary importance when multiplied by the massive population sizes of bristlemouths and accumulated over the course of the fish’s lifetime.
The biophotonics literature discusses engineering applications that are real and not exaggerated. How to effectively manage light in small, enclosed places while avoiding losses from internal scattering and absorption is an issue that optical engineers deal with on a daily basis. The crystal architecture in Sigmops gracilis is a working example of this problem. Variations of this issue are faced by photonic microdevices, optical fiber systems, and medical imaging electronics. When an evolutionary solution has been refined over millions of years under tremendous material constraints, it usually merits close inspection. The system functions even tho the fish didn’t create it, and functional systems in harsh settings frequently have design elements that weren’t immediately apparent.
