Open ocean fish display impressive biological cloaking

Scientists and predators alike have long been confounded by ocean fish that seem to disappear from sight even in the open waters. Since last week, however, the former have determined how the invisibility trick works. A study published in the journal Science last week reports that some fish have embedded platelets in their skin cells that enable them to reflect polarized light (that is, light waves in which the electric charges vibrate on a single plane as opposed to the multiple ones exhibited by unfiltered sunlight or a light bulb’s glow).

Unpolarized light can be thought of as vibrating in both vertical and horizontal directions — underwater, light is often polarized. While the human eye cannot perceive the full variation in polarized light, many fish can, and Molly Cummings, professor of integrative biology at the University of Texas, Austin, posits that this has been accompanied by the ability to conceal themselves in polarized light as well.

“We suggested they’ve probably evolved the means to hide in polarized light,” she told Phys.org. “If we can identify that process, then we can improve upon our own camouflage technology for that environment.”

One of the fish species — the aptly named lookdown — had demonstrated in a previous study the ability to reflect polarized light advantageously — this new study confirmed its ability to manipulate light for camouflage. Cummings and her colleagues developed a rotating platform to house a polarimeter built by the lead author, Parrish Brady. The platform held a fish on a mirror opposite the polarimeter, which recorded polarized light as it reflected off the fish.

Two species in particular, the lookdown and the bigeye scad, outperformed the mirror when it came to camouflage in polarized light. The team also discovered that both exhibited the best camouflage at their “chase angles,” or the directions from which a fish would be pursued by a predator, or would pursue its own quarry.

The researchers put the camouflage prowess down to the structure of the skin’s embedded platelets. The way these platelets reflected and scattered the polarized light was dependent on the light’s angle. The next step for the team is determining whether fish consciously take advantage of the reflectance — be it by altering their swimming angles or adjusting the platelets by some mechanism.

While it was long assumed that mirror-like reflectivity of silvery fish was the primary camouflage mechanism among open ocean fish species, this means proves to only be effective when the surroundings are uniformly colored (as they appear to human eyes). In reality, polarized light — common in the underwater environment of lookdowns and bigeye scads — is variable. The use of platelets seems to be a more sophisticated approach to invisibility there.

“I think it’s a great example of how human applications can take advantage of evolutionary solutions,” Cummings told Phys.org.