Light emitting organs on the underside of hatchet fish help camouflage them through bioluminescent counterillumination.

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References

“Even in ocean depths where sunlight barely penetrates, the faint
silhouette that a fish throws to predators beneath it in the water
column can make it an easy target. Accordingly, many fish, crustaceans
and squid have developed bioluminescent ‘counterillumination’ abilities.
Light-emitting organs called photophores line their undersides. These
creatures can adjust the light output of these organs to match the light
their eyes receive from above to help eliminate their shadows. The
hatchetfishis one such species equipped with an underbody
that lights up to camouflage it from hungry eyes below.” (Hadhazy 2009)

Web page
Shining Examples: 10 Bioluminescent Creatures that Glow in Surprising Ways Slide Show

Journal article
Propagation and Perception of Bioluminescence: Factors Affecting Counterillumination as a Cryptic StrategyBiological BulletinJanuary 23, 2007
Sonke Johnsen, Edith A. Widder, Curtis D. Mobley

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Living System/s

Organism
Marine HatchetfishesSternoptychidaeFamily

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