Counter-illumination is a method ofactive camouflage seen inmarine animals such asfirefly squid andmidshipman fish, and in military prototypes, producing light to match their backgrounds in both brightness and wavelength.
Marine animals of themesopelagic (mid-water) zone tend to appear dark against the bright water surface when seen from below. They can camouflage themselves, oftenfrom predators but also from their prey, by producing light withbioluminescentphotophores on their downward-facing surfaces, reducing the contrast of theirsilhouettes against the background. The light may be produced by the animals themselves, or bysymbioticbacteria, oftenAliivibrio fischeri.
Counter-illumination differs fromcountershading, which uses only pigments such asmelanin to reduce the appearance of shadows. It is one of the dominant types ofaquatic camouflage, along with transparency andsilvering. All three methods make animals in open water resemble their environment.
Counter-illumination has not come into widespreadmilitary use, but during theSecond World War it was trialled inships in the Canadiandiffused lighting camouflage project, and inaircraft in the AmericanYehudi lights project.
In the sea, counter-illumination is one of three dominant methods ofunderwater camouflage, the other two being transparency and silvering.[1] Among marine animals, especiallycrustaceans,cephalopods, andfish, counter-illuminationcamouflage occurs wherebioluminescent light fromphotophores on anorganism's ventral surface is matched to the light radiating from the environment.[2] Thebioluminescence is used to obscure the organism's silhouette produced by the down-welling light. Counter-illumination differs fromcountershading, also used by many marine animals, which uses pigments to darken the upper side of the body while the underside is as light as possible with pigment, namely white. 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 themesopelagic depths of the sea. Counter-illumination goes further than countershading, actually brightening the underside of the body.[3][4]
Counter-illumination relies on organs that produce light, photophores. These are roughly spherical structures that appear asluminous spots on many marine animals, including fish and cephalopods. The organ can be simple, or as complex as the human eye, equipped with lenses, shutters, colour filters and reflectors.[5]
In theHawaiian bobtail squid (Euprymna scolopes) light is produced in a large and complex two-lobed light organ inside the squid's mantle cavity. At the top of the organ (dorsal side) is a reflector, directing the light downwards. Below this are containers (crypts) lined withepithelium containing light-producing symbiotic bacteria. Below those is a kind ofiris, consisting of branches (diverticula) of itsink sac; and below that is a lens. Both the reflector and the lens are derived frommesoderm. Light escapes from the organ downwards, some of it travelling directly, some coming off the reflector. Some 95% of the light-producing bacteria are voided at dawn every morning; the population in the light organ then builds up slowly during the day to a maximum of some 1012 bacteria by nightfall: this species hides in sand away from predators during the day, and does not attempt counter-illumination during daylight, which would in any case require much brighter light than its light organ output. The emitted light shines through the skin of the squid's underside. To reduce light production, the squid can change the shape of its iris; it can also adjust the strength of yellow filters on its underside, which presumably change the balance of wavelengths emitted. The light production is correlated with the intensity of down-welling light but about one third as bright; the squid is able to track repeated changes in brightness.[6]
At night, nocturnal organisms match both thewavelength and thelight 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.[6][7]
In theeyeflash squid (Abralia veranyi) a species whichdaily migrates between the surface and deep waters, a study showed that the light produced is bluer in cold waters and greener in warmer waters, temperature serving as a guide to the requiredemission spectrum. The animal has more than 550 photophores on its underside, consisting of rows of four to six large photophores running across the body, and many smaller photophores scattered over the surface. 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. Other groups remained unilluminated: other species, and perhapsA. veranyi from its other groups of photophores, can produce a third spectral component when needed. Another squid,Abralia trigonura, is able to produce three spectral components: at 440 and at 536 nanometres (green), appearing at 25 Celsius, apparently from the same photophores; and at 470–480 nanometres (blue-green), easily the strongest component at 6 Celsius, apparently from a different group of photophores. Many species can in addition vary the light they emit by passing it through a choice of colour filters.[8]
Counterillumination camouflage halved predation among individuals employing it compared to those not employing it in themidshipman fishPorichthys notatus.[6][9]
The bioluminescence used for counter-illumination can be eitherautogenic (produced by the animal itself, as inpelagic cephalopods such asVampyroteuthis,Stauroteuthis, and pelagic octopuses in theBolitaenidae[10]) or bacteriogenic (produced bybacterialsymbionts). The luminescent bacterium is oftenAliivibrio fischeri, as for example in the Hawaiian bobtail squid.[6]
Reducing the silhouette is primarily ananti-predator defence for mesopelagic (mid-water) organisms. The reduction of the silhouette from highly directional down-welling light is important, since there is no refuge in the open water, andpredation occurs from below.[3][11][12] Many mesopelagic cephalopods such as thefirefly squid (Watasenia scintillans),decapod crustaceans, and deep ocean fishes use counter-illumination; it works best for them when ambient light levels are low, leaving the diffuse down-welling light from above as the only light source.[6][3] Some deep water sharks, includingDalatias licha,Etmopterus lucifer, andEtmopterus granulosus, are bioluminescent, most likely for camouflage from predators that attack from beneath.[13]
Besides its effectiveness as a predator avoidance mechanism, counter-illumination also serves as an essential tool to predators themselves. Some shark species, such as the deepwatervelvet belly lanternshark (Etmopterus spinax), use counter-illumination to remain hidden from their prey.[14] Other well-studied examples include thecookiecutter shark (Isistius brasiliensis), themarine hatchetfish, and the Hawaiian bobtail squid.[6] More than 10% of shark species may be bioluminescent, though some such aslantern sharks may use the light forsignalling as well as for camouflage.[15]
An animal camouflaged by counter-illumination is not completely invisible. A predator could resolve individual photophores on a camouflaged prey's underside, given sufficiently acute vision, or it could detect the remaining difference in brightness between the prey and the background. Predators with a visual acuity of 0.11 degrees (of arc) would be able to detect individual photophores of the Madeira lanternfishCeratoscopelus maderensis at up to 2 metres (2.2 yd), and they would be able to see the general layout of the photophore clusters with poorer visual acuity. Much the same applies also toAbralia 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). All the same, the counter-illumination camouflage of these species is extremely effective, radically reducing their detectability.[2][a]
Active camouflage in the form of counter-illumination has rarely been used for military purposes, but it has been prototyped inship andaircraft camouflage from the Second World War onwards.[16][17][18]
Diffused lighting camouflage, in whichvisible light is projected on to the sides of ships to match the faint glow of the night sky, was trialled byCanada's National Research Council from 1941 onwards, and then by theRoyal Navy, during the Second World War. Some 60 light projectors were mounted all around the hull and on the ships' superstructure such as the bridge and funnels. On average, the system reduced the distance at which a ship could be seen from a surfaced submarine by 25% using binoculars, or by 33% using the naked eye. The camouflage worked best on clear moonless nights: on such a night in January 1942,HMSLargs was not seen until it closed to 2,250 yards (2,060 m) when counter-illuminated, but was visible at 5,250 yards (4,800 m) unlighted, a 57% reduction in range.[16][19]
In 1916 the American artistMary Taylor Brush experimented with camouflage on aMorane-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". The concept was not developed further during theFirst World War.[20]
The Canadian ship concept was trialled in American aircraft includingB-24 Liberators andTBM Avengers in theYehudi lights project, starting in 1943, using forward-pointing lamps automatically adjusted to match the brightness of the sky. The goal was to enable a radar-equipped, sea-search aircraft to approach a surfacedsubmarine to within 30 seconds from arrival before being seen, to enable the aircraft to drop itsdepth charges before the submarine could dive. There was insufficient electrical power available to illuminate the entire surface of the aircraft, and outboard lamps in the manner of diffused lighting camouflage would have interfered with the airflow over the aircraft's surface, so a system of forward-pointing lamps was chosen. These had a beam with a radius of 3 degrees, so pilots had to fly with the aircraft's nose pointed directly at the enemy. In acrosswind, this required a curving approach path, rather than a straight-line path with the nose pointed upwind. In trials in 1945, a counter-illuminated Avenger was not seen until 3,000 yards (2.7 km) from its target, compared to 12 miles (19 km) for an uncamouflaged aircraft.[17]
The idea was revisited in 1973 when anF-4 Phantom was fitted with camouflaging lights in the "Compass Ghost" project.[18]
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