Open-Ocean Fish Achieve Camouflage With Rotating Crystals Embedded In Their Skin

Soldiers often cloak themselves in camouflage print to hide in open spaces, and a recent study in Science suggests that some fish accomplish an even more impressive trick to conceal themselves in the open ocean. These fish have nanoscopic crystals in their skin that help them blend in against backgrounds of polarized light, a type of light invisible to humans, but visible to other fish.

All light travels through the air as waves that vibrate in and out perpendicular to the direction they are moving, and visible light waves that we use to see the world vibrate in many different planes. However, polarized light waves are special—as they travel, they only vibrate in a single plane.

The lookdown fish, Selene vomer, is a silvery, flat-bodied fish inhabiting the open ocean.

Visible light waves become polarized when they hit the surface of the ocean—only waves oscillating in a single plane make it through—which means that while our eyes interpret the open ocean as a uniform blue expanse, this is not the case for marine species with specialized receptors that detect variations in polarized light. “It would probably look something like walking through a kaleidoscope,” said principal investigator Dr. Molly Cummings from the University of Texas.

She explains that open-ocean fish are highly exposed, and as the sun moves across the sky throughout the day, the main plane for polarized light waves tilts with it, creating a changing and varied polarized background that fish have to blend in against. This does not occur in shallow water or coastal habitats because coral reefs or sand disrupt light waves and prevent them from becoming polarized.

Dr. Cummings worked with lead author Dr. Parrish Brady to determine how open-ocean fish species camouflage themselves in this unprotected, chaotic environment. Their study in Curacao compared two open-ocean species to two coral reef species in the Carrangidae family of fish.

Dr. Brady built a video polarimeter, a specialized underwater camera that records information about the polarized light field. Each live fish was secured in a net at the center of an X-shaped platform suspended just beneath the surface in the open ocean. The platform rotated 360° every 3 minutes while the camera, attached to an arm on the platform, collected data on how much the fish contrasted with its polarized background at different angles.

They found that the open-ocean species hid themselves by minimizing contrast with their polarized background, while the coral reef species remained highly visible. The researchers interpreted this as evidence that polarized camouflage evolved after the species diverged from a common ancestor and moved to different habitats.

The researchers also looked closely at the skin of the open-water species, which contains tiny, flat crystals called platelets. “They’re what are responsible for the silvery sheen on the fish skin,” Dr. Brady said. “The color produced by these platelets is very much correlated with how they are arranged in the skin.”

In open-ocean fish, the elongated, six-sided platelets orient themselves parallel to the fish gills, and continually rotate 360° along their long axis to match the changing polarized background. The fish are particularly well camouflaged when viewed from directly in front or behind, which may help them evade predators or stealthily chase down prey.

Dr. Sonke Johnsen, who studies marine camouflage at Duke University, is not entirely convinced by the study, and notes that polarized vision is most common in crustaceans and cephalopods. “The fish they’re looking at are pretty big, powerful predators themselves in many cases, and they are preyed upon by other fish. Because of the way eyes are designed in vertebrates compared to invertebrates, polarization vision is vanishingly rare among fish.” He thinks more research is needed.

Dr. Cummings’ project was funded by a basic research grant from the Office of Naval Research, for finding biological phenomenon that may have military applications. While she is not at liberty to discuss links between her work and the Navy, it is easy to envision how the research could be used.

“Presumably other scientists could take our biological properties and turn that into a material feature that we could put on submarines or other things that we float in the ocean, to have them not be picked up by satellites that might use polarization,” she said. “Our research drives home the point that nature has solved a lot of problems for us, and we can really benefit from paying attention and understanding their intricate, complex solutions.”