Counter-illumination

Principle of the counterillumination 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-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.

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 through bioluminescence on their lower surfaces, reducing the contrast of their silhouettes against the background. Counter-illumination differs from countershading, which uses only pigments such as melanin to reduce the appearance of shadows. It is one of the dominant types of aquatic camouflage, along with transparency and silvering. All three methods make animals in open water resemble their environment.

Counter-illumination has not so far come into widespread military use, but during the Second World War it was trialled in ships in the Canadian Diffused lighting camouflage project and in the American Yehudi lights project.

In marine animals

Photophores (light-producing organs) on a nocturnal midshipman fish

Among marine animals, counter-illumination camouflage occurs where bioluminescent light from an organism's ventral surface is matched to the light radiating from the environment. The bioluminescence is used to obscure the organism's silhouette produced by the down-welling light. Counter-illumination differs from countershading, also used by many marine animals, which uses pigments to darken the upper side of the body. Counter-illumination goes further than countershading, actually brightening the under side of the body.[1][2]

The bioluminescence used for counter-illumination can be either autogenic (produced by the animal itself, as in pelagic cephalopods such as Vampyroteuthis, Stauroteuthis, and the Bolitaenidae[3]) or bacteriogenic (produced by bacterial symbionts). The luminescent bacterium is often Aliivibrio fischeri.[4]

Reducing the silhouette is primarily an anti-predator defence for mesopelagic (mid-water) organisms. The reduction of the silhouette from highly directional down-welling light is important as there is no refuge in the mesopelagic, and predation occurs from below.[1][5][6] Many mesopelagic cephalopods such as the firefly squid, 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.[4][1] At night, nocturnal organisms match the wavelength and 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.[7] This strategy halved predation among individuals employing it compared to those not employing it in midshipman fish.[4][8]

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 velvet belly lanternshark, use counter-illumination to remain hidden from their prey.[9] as does the Hawaiian bobtail squid.[4] Other well-studied examples include the cookiecutter shark, the marine hatchetfish, and the Hawaiian bobtail squid.[4] More than 10% of shark species may be bioluminescent, though some such as lantern sharks may use the light for signalling as well as for camouflage.[10]

Military prototypes

Active camouflage in the form of counter-illumination has rarely been used for military purposes, but it has been prototyped in ships and aircraft from the Second World War onwards.[11][12][13]

For ships

Diffused lighting camouflage prototype, not quite complete and set to maximum brightness, installed on HMS Largs in 1942

"Diffused lighting camouflage", in which visible light was projected on to the sides of ships to match the faint glow of the night sky, was trialled by Canada's National Research Council and then by the Royal Navy during World War II. 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.[11][14]

For aircraft

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.

The Canadian concept was trialled in American aircraft including B-24 Liberators and TBM Avengers in the Yehudi lights project, 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 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 (2,700 m) from its target, compared to 12 miles (19 km) for an uncamouflaged aircraft.[12]

The idea was revisited in 1973 when an F4 Phantom was fitted with camouflaging lights in the "Compass Ghost" project.[13]

References

  1. 1 2 3 Young, R.E,.; Roper, C.F.E. (1977). "Intensity Regulation of Bioluminescence during Countershading in Living midwater animals". Science. 191 (4231): 1046–1048.
  2. ↑ Rowland, Hannah M. (2009). "Abbott Thayer to the present day: what have we learned about the function of countershading?". Philosophical Transactions of the Royal Society B. 364: 519–527. JSTOR 40485817. PMC 2674085 Freely accessible. PMID 19000972. doi:10.1098/rstb.2008.0261.
  3. ↑ Lindgren, Annie R.; Pankey, Molly S.; Hochberg, Frederick G.; Oakley, Todd H. (2012). "A multi-gene phylogeny of Cephalopoda supports convergent morphological evolution in association with multiple habitat shifts in the marine environment". BMC Evolutionary Biology. 12: 129. doi:10.1186/1471-2148-12-129.
  4. 1 2 3 4 5 Jones, B. W.; Nishiguchi, M. K. (2004). "Counterillumination in the hawaiian bobtail squid, Euprymna scolopes Berry (Mollusca : Cephalopoda)" (PDF). Marine Biology (journal). 144 (6): 1151–1155. doi:10.1007/s00227-003-1285-3.
  5. ↑ Young. R.E, Roper. C.F.E. 1976. Bioluminescent countershading in Midwater Animals from living Squid. Science, New Series. Vol 191,4231: 1046-1048.
  6. ↑ "Science & Nature - Sea Life - Ocean info - Counter-illumination". BBC. 2004-03-11. Retrieved 2012-10-03.
  7. ↑ Guerrero-Ferreira, R. C.; Nishiguchi, M. K. (2009). "Ultrastructure of light organs of loliginid squids and their bacterial symbionts: a novel model system for the study of marine symbioses". Vie et Milieu. 59 (3–4): 307–313. ISSN 0240-8759.
  8. ↑ Harper, R.; Case, J. (1999). "Disruptive counterillumination and its anti-predatory value in the plainfish midshipman Porichthys notatus". Marine Biology. 134 (3): 529–540. doi:10.1007/s002270050568.
  9. ↑ Claes, Julien M.; Aksnes, Dag L.; Mallefet, Jérôme (2010). "Phantom hunter of the fjords: camouflage by counterillumination in a shark (Etmopterus spinax)" (PDF). Journal of Experimental Marine Biology and Ecology. 388 (1–2): 28–32. doi:10.1016/j.jembe.2010.03.009.
  10. ↑ Davies, Ella (26 April 2012). "Tiny sharks provide glowing clue". BBC. Retrieved 12 February 2013.
  11. 1 2 "Diffused Lighting and its use in the Chaleur Bay". Naval Museum of Quebec. Royal Canadian Navy. Archived from the original on 22 May 2013. Retrieved 3 February 2013.
  12. 1 2 Bush,, 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. Retrieved February 12, 2013.
  13. 1 2 Dann, Rich (2011). "Yehudi Lights" (PDF). Centennial of Naval Aviation. 3 (3): 15. 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
  14. ↑ Admiralty (1942). Trial Report D.L. 126: DL Trials on HMS Largs in Clyde Approaches. ADM/116/5026 Diffused Lighting. The National Archives, Kew: Admiralty.
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