Researchers discover a new camouflage method for ocean-dwelling fish


Fish can hide in the ocean by altering the way light reflects off their skin, according to researchers at The University of Texas at Austin.

This discovery could aid development of new camouflage materials for use in the ocean, overturning 40 years of knowledge about fish camouflage.
Researchers found that lookdown fish camouflage themselves by manipulating polarised light hitting their skin. Laboratory studies showed this camouflage outperforms the "mirror" strategy, which was previously thought to be the best in fish camouflage, by up to 80 per cent.
The study, published in the Proceedings of the National Academy of Sciences, was funded by the U.S. Navy, with an interest in developing better ocean camouflage technology and detection for similar strategies.
Molly Cummings, associate professor of integrative biology in the College of Natural Sciences, said: "The open ocean represents a challenging environment for camouflage.
"There are no objects to hide behind in three-dimensional space, so organisms have to find a way to blend in to the waters itself."
It has been assumed that the best camouflage technique for open ocean fish is to reflect sunlight like a mirror. The lookdown, as well as many other fish, has reflective skin elements.
This method works well for some aspects of light, however is not optimal when light is polarised.
Physicist Parrish Brady, a postdoctoral associate in Cummings' lab, sayd: "In the polarised light field, there is a lot of structure in the open ocean.
"Humans can't see it, but more than 60 different species of fish have some degree of polarisation sensitivity.
"They can perceive the structure in the light."
Cummings and Brady caught lookdowns because they are good camouflagers. They simulated the passing sun and used a custom polarimeter to measure how the fish reflected the polarised light.
Their study found that the lookdowns could alter their reflective properties in ways that were near to the theoretical optimum.
Cummings said: "When we mimicked the light field when the sun is overhead, as it would be at noon, the fish just bounced back that light field. It acted like a mirror.
"Then we mimicked the light field when it's more complex, and the lookdown altered the properties of the polarised light it was reflecting so that it would be a better blend into its specific background at different times of day."
The lookdown's skin selectively reduces the degree of polarisation and transforms the angle of polarisation of the reflected light depending on the conditions.
Cummings is unsure if this is a passive process or if the lookdowns alter their bodies' position relative to the sun, or by neurologically increasing certain processes.
The researcher's results will be particularly relevant for the U.S. Navy whose camouflage is not as good as that of the lookdowns.
Cummings said: "I am always excited when evolution is one step ahead of humans.
"There is this problem out there - how to blend in to this environment, and though we haven't quite solved it yet, an animal has.