page_title: Underwater camouflage
section_title: Counter-illumination
hierarchical_section_title: Underwater camouflage / Methods / Counter-illumination
caption_attribution_description: English: Principle of the counterillumination camouflage of the firefly squid, Watasenia scintillans. When seen from below by a predator, the bioluminescence helps to match the squid's brightness and colour to the sea surface above.
context_page_description: Underwater camouflage is the set of methods of achieving crypsis—avoidance of observation—that allows otherwise visible aquatic organisms to remain unnoticed by other organisms such as predators or prey.
  Camouflage in large bodies of water differs markedly from camouflage on land. The environment is essentially the same on all sides. Light always falls from above, and there is generally no variable background to compare with trees and bushes. Three main camouflage methods predominate in water: transparency, reflection, and counter-illumination. Transparency and reflectivity are most important in the top 100 metres of the ocean; counter-illumination is the main method from 100 metres down to 1000 metres; while camouflage becomes less important in the dark waters below 1000 metres.
  Camouflage in relatively shallow waters is more like terrestrial camouflage, where additional methods are used by many animals.
context_section_description: Counter-illumination through bioluminescence on the underside (ventral region) of the body is found in many species that live in the open ocean down to about 1000 metres. The generated light increases an animal's brightness when seen from below to match the brightness of the ocean surface; it is an effective form of active camouflage. It is notably used by some species of squid, such as the midwater squid, Abralia veranyi. These have light-producing organs (photophores) scattered all over their undersides, creating a sparkling glow that prevents the animal from appearing as a dark shape when seen from below. Counter-illumination camouflage is the likely function of the bioluminescence of many marine organisms, though light is also produced to attract or to detect prey and for signalling.
caption: Principle of squid counter-illumination
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