, and in military prototypes, producing light to match their backgrounds in both brightness and wavelength.
Marine animals of the
mesopelagic (mid-water) zone tend to appear dark against the bright water surface when seen from below. They can camouflage themselves, often from predators but also from their prey, by producing light with bioluminescentphotophores on their downward-facing surfaces, reducing the contrast of their silhouettes against the background. The light may be produced by the animals themselves, or by symbioticbacteria, often Aliivibrio fischeri
.
Counter-illumination differs from
silvering
. All three methods make animals in open water resemble their environment.
Counter-illumination relies on organs that produce light, photophores. These are roughly spherical structures that appear as luminous 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]
Sagittal section of the large eye-like light-producing organ of Hawaiian bobtail squid, Euprymna scolopes. The organ houses symbiotic Aliivibrio fischeri
bacteria.
In the
iris, consisting of branches (diverticula) of its ink sac; and below that is a lens. Both the reflector and the lens are derived from mesoderm. 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]
Matching light intensity and wavelength
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.[6][7]
eyeflash squid (Abralia veranyi) a species which daily 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 required emission 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 perhaps A. 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 the midshipman fishPorichthys notatus.[6][9]
Photophores on a nocturnal midshipman fish, whose bioluminescence halves its rate of predation[6]
Hiding from predators
Reducing the silhouette is primarily an
Etmopterus granulosus, are bioluminescent, most likely for camouflage from predators that attack from beneath.[13]
Hiding from prey
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 deepwater
lantern sharks may use the light for signalling as well as for camouflage.[15]
Defeating counter-illumination camouflage
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 lanternfish Ceratoscopelus 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 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). All the same, the counter-illumination camouflage of these species is extremely effective, radically reducing their detectability.[2][a]
Canada's National Research Council from 1941 onwards, and then by the Royal 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, HMS Largs 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]
Grumman TBM Avenger raised the average brightness of the plane from a dark shape to the same as the sky.[b]
The Canadian ship concept was trialled in American aircraft including
TBM Avengers in the Yehudi 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 surfaced submarine to within 30 seconds from arrival before being seen, to enable the aircraft to drop its depth 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 a crosswind, 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 an
F-4 Phantom was fitted with camouflaging lights in the "Compass Ghost" project.[18]
Notes
^The pattern of photophores may, in addition to matching background brightness, also serve to break up the animals' silhouettes, just as spots and stripes of coloured paint do in disruptive coloration, but in the absence of experimental evidence it is uncertain how useful this is: it would only help when the sea surface background was uneven.[2]
^The effect may be seen by standing back a little from the image and half-closing the eyes. The upper image becomes indistinct where the lower image remains as a dark shape.
^ abBush, Vannevar; Conant, James; et al. (1946). "Camouflage of Sea-Search Aircraft"(PDF). Visibility Studies and Some Applications in the Field of Camouflage. Office of Scientific Research and Development, National Defence Research Committee. pp. 225–240. Archived from the original(PDF) on October 23, 2013. Retrieved February 12, 2013.
^ abDann, Rich (2011). "Yehudi Lights"(PDF). Centennial of Naval Aviation. 3 (3): 15. Archived from the original(PDF) on 2011-10-07. Retrieved 2017-02-19. the prototype Grumman XFF-1 .. was fitted with lights as an active camouflage method .. Counter-illumination was tested again in 1973, using a U.S. Air Force F-4C Phantom II with lights, under the name COMPASS GHOST
The National Archives, Kew: Admiralty. {{cite book}}: |work= ignored (help
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