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Counter-illumination
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<p>Counter-illumination is a method of <a href="/facts/Active_camouflage/3kz0cMg4">active camouflage</a> seen in <a href="/facts/Marine_animal/TF5MGzFb">marine animals</a> such as <a href="/facts/Firefly_squid/mwXHpaUD">firefly squid</a> and <a href="/facts/Midshipman_fish/dwYLA91d">midshipman fish</a>, and in military prototypes, producing light to match their backgrounds in both brightness and wavelength. </p><p>Marine animals of the <a href="/facts/Mesopelagic/C9y5cjVA">mesopelagic</a> (mid-water) zone tend to appear dark against the bright water surface when seen from below. They can camouflage themselves, often <a href="/facts/Anti-predator_adaptation/nI5VgWco">from predators</a> but also from their prey, by producing light with <a href="/facts/Bioluminescence/IlzgyV4E">bioluminescent</a> <a href="/facts/Photophore/qtB8x9DJ">photophores</a> on their downward-facing surfaces, reducing the contrast of their <a href="/facts/Silhouette/h08xsCFs">silhouettes</a> against the background. The light may be produced by the animals themselves, or by <a href="/facts/Mutualism_(biology)/29hCDDS2">symbiotic</a> <a href="/facts/Bacteria/wC8xSCsU">bacteria</a>, often <i><a href="/facts/Aliivibrio_fischeri/tFMLns6E">Aliivibrio fischeri</a></i>. </p><p>Counter-illumination differs from <a href="/facts/Countershading/oEWFkYsE">countershading</a>, which uses only pigments such as <a href="/facts/Melanin/QtGsXAm4">melanin</a> to reduce the appearance of shadows. It is one of the dominant types of <a href="/facts/Aquatic_camouflage/gtRuxFMs">aquatic camouflage</a>, along with transparency and <a href="/facts/Silvering_(camouflage)/GSjGqPuR">silvering</a>. All three methods make animals in open water resemble their environment. </p><p>Counter-illumination has not come into widespread <a href="/facts/Military_camouflage/93IgXGIa">military use</a>, but during the <a href="/facts/Second_World_War/2efV5L1U">Second World War</a> it was trialled in <a href="/facts/Ship_camouflage/IUE4qHeN">ships</a> in the Canadian <a href="/facts/Diffused_lighting_camouflage/U0uMYsDc">diffused lighting camouflage</a> project, and in <a href="/facts/Aircraft_camouflage/Ws26QLBI">aircraft</a> in the American <a href="/facts/Yehudi_lights/n0vz4t25">Yehudi lights</a> project. </p>