Counter-illumination

Counter-illumination is a method of active camouflage seen in marine animals such as firefly squid and midshipman fish, and in military prototypes, producing light to match their backgrounds in both brightness and wavelength.

They can camouflage themselves, often from predators but also from their prey, by producing light with bioluminescent photophores on their downward-facing surfaces, reducing the contrast of their silhouettes against the background.

Countershading fails when the light falling on the animal's underside is too weak to make it appear roughly as bright as the background.

This commonly occurs when the background is the relatively bright ocean surface, and the animal is swimming in the mesopelagic depths of the sea.

[6] At night, nocturnal organisms match both the wavelength and the light intensity of their bioluminescence to that of the down-welling moonlight and direct it downward as they swim, to help them remain unnoticed by any observers below.

In cold water at 11 Celsius, the squid's photophores produced a simple (unimodal) spectrum with its peak at 490 nanometres (blue-green).

In warmer water at 24 Celsius, the squid added a weaker emission (forming a shoulder on the side of the main peak) at around 440 nanometres (blue), from the same group of photophores.

The reduction of the silhouette from highly directional down-welling light is important, since there is no refuge in the open water, and predation occurs from below.

Some shark species, such as the deepwater velvet belly lanternshark (Etmopterus spinax), use counter-illumination to remain hidden from their prey.

[14] Other well-studied examples include the cookiecutter shark (Isistius brasiliensis), the marine hatchetfish, and the Hawaiian bobtail squid.

Much the same applies also to Abralia veranyi, but it was largely given away by its unlit fins and tentacles, which appear dark against the background from as far away as 8 metres (8.7 yd).

[16][19] In 1916 the American artist Mary Taylor Brush experimented with camouflage on a Morane-Borel monoplane using light bulbs around the aircraft, and filed a 1917 patent that claimed she was "able to produce a machine which is practically invisible when in the air".

[20] The Canadian ship concept was trialled in American aircraft including B-24 Liberators and TBM Avengers in the Yehudi lights project, starting in 1943, using forward-pointing lamps automatically adjusted to match the brightness of the sky.

Principle of the counter-illumination camouflage of the firefly squid, Watasenia scintillans . When seen from below by a predator, the animal's light helps to match its brightness and colour to the sea surface above.
Counter-illuminating photophores illuminating the underside of the hatchetfish Argyropelecus olfersii
Photophores on a lanternfish , the most common deep sea fish worldwide
Sagittal section of the large eye-like light-producing organ of Hawaiian bobtail squid, Euprymna scolopes . The organ houses symbiotic Aliivibrio fischeri bacteria.
Spectrum of visible light showing colours at different wavelengths , in nanometres
Diagram of a small type of photophore in the skin of a cephalopod , Abralia trigonura , in vertical section
Photophores on a nocturnal midshipman fish , whose bioluminescence halves its rate of predation [ 6 ]
Diffused lighting camouflage prototype, not quite complete and set to maximum brightness, installed on HMS Largs in 1942
Mary Taylor Brush 's 1917 patent application for camouflaging a Morane-Borel monoplane using light bulbs
Forward-pointing Yehudi lights on Grumman TBM Avenger raised the average brightness of the plane from a dark shape to the same as the sky. [ b ]