Counter-Illumination: Camouflage by Light

Ben Perez-Flesler STEM Jul 22, 2020

The deeper you dive into the ocean, the darker it gets. That’s why looking up to the sun from below makes everything seem bright, but looking down from above is far murkier. This makes blending in difficult, to say the least, because the background changes drastically depending on where you look. But as it turns out, a number of creatures have actually found a way around this.

By creating artificial light on the underside of their bodies, marine animals are effectively camouflaged from both top and bottom. A predator underneath them will see a white belly matching the brightness of the sky, while its darker back (or top) blends in with the depths below. Some can even adjust the level of light so they stay hidden after the sun sets!

A fish viewed from below, showing the difference made by illumination; image by the Smithsonian Institution

Those adaptations are an example of bioluminescence—where a living thing makes and emits its own light, instead of using natural light. Usually, it's done by a chemical reaction in special organs called photophores, like the ones that fireflies have. But some organisms rely on bacteria instead, which live inside them in a symbiotic relationship. One example is the tiny Hawaiian bobtail squid, providing the bacteria with food and a place to stay in exchange for keeping it safe from predators.

Despite weighing less than a hundredth of of an ounce at birth, Hawaiian bobtail squids begin searching for their microbial partners almost immediately. As thousands of different bacterial species try to swim through the squid’s mucus, only one can make it. Alberto fischeri, a bioluminescent bacterium, has evolved specifically to work with this squid. After just 10 hours of life, they’ll be together forever!

Adult Hawaiian bobtail squid; CC BY 4.0

More commonly, some species use pigments to permanently color their bellies lighter than their backs, giving virtually the same effect. Often called ‘countershading,' it’s seen in nearly every variety of shark and penguin, and a number of sea turtles and small fish. A species of catfish that swims upside-down (S. nigriventris) has the reverse shading too.

Interestingly enough, the discovery of counter-illumination hasn’t been useful to just ocean life. During World War II, several American aircraft tested forward-pointing lights that automatically detected and adjusted to the brightness of the sky, directly inspired by the aforementioned marine life. The trial proved successful, keeping planes hidden until they were within 3,000 yards of a ship (compared to 12 miles without ‘Yehudi lights’). While it quickly became obsolete after advancements in radar technology, it’s just one example of the countless things that we can learn from nature.

Featured image from the Smithsonian Institute

Sources:

https://ocean.si.edu/ocean-life/fish/fish-using-counterillumination

https://apps.dtic.mil/dtic/tr/fulltext/u2/221102.pdf

https://link.springer.com/article/10.1007%2Fs00227-003-1285-3

https://royalsocietypublishing.org/doi/10.1098/rstb.2008.0261

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