The proverbial sailor who went to sea and saw nothing but the deep blue sea may in fact have been fooled by the amazing camouflage capabilities of open-ocean fish. The secret to this amazing disappearing trick has now be revealed in a new paper in the journal Science, which lifts the lid on how these elusive aquatic creatures disguise their presence.
To the human eye, the uniform blueness of the deep appears completely featureless, yet in reality the polarization of the submarine light is highly variable and in a state of constant flux. This means that animals capable of detecting variations in polarization perceive a scene that is anything but constant. As a result, simply appearing blue is not enough to camouflage oneself in open-water environments.
Polarization occurs when light waves oscillate (basically bob up and down) in the same direction. As water particles reflect and refract light, angles of polarization tend to vary wildly in the ocean. In order to deal with this, a number of open-water fish species have developed specialized platelets – which are components of blood cells – in their skin that reflect polarized light in order to make the fish blend in with its background.
Perhaps the most impressive thing about this adaptive feature is the way in which these platelets are designed to reflect polarized light more efficiently when the fish is viewed from “chase angles.” These refer to positions that a predator would approach prey from. For instance, the researchers found that the fish exhibited “exceptional camouflage” when viewed from a perpendicular perspective.
To conduct the experiment, marine scientists used a polarimeter to measure the reflection of polarized light produced by two open-ocean species of fish from the Carangidae family, comparing this to other fish from reef and coastal habitats. Specimens were held in place against a mirror, and recordings were taken from a number of angles.
Not only did the open-ocean species display greater polarization than the other fish, but they were also more reflective of polarized light than the mirror, thereby discrediting previous theories which held that silvery, mirror-like scales provided the most effective camouflage mechanism for these species. The fact that reef fish did not exhibit this adaptation is explained by the fact that the ocean floor has a depolarizing effect on aquatic light.
The study authors conclude that this technique represents a superior camouflage modality than many modern devices such as invisibility cloaks. As such, they suggest future attempts to improve this technology “should turn to natural systems for new materials and the means to use them effectively.”
The research was supported by the U.S. Navy, which hopes to be able to harness the findings in order to develop better aquatic camouflage techniques. Should this come to fruition, it would not be the first time that the military has turned to nature to improve its aquatic machinery. For instance, shark skin, which is covered in tiny tooth-like scales that prevent barnacles from attaching, has provided the inspiration for the latest coating for the hulls of Navy ships.