Most deep sea possess bioluminescent organs that are normally dependent on bacteria, maintained in special cells, for the production of the light.
The bacterial light is usually produced as a result of an enzyme (often luciferase) mediated oxidation reaction in which a molecule (often luciferan) changes its shape and emits a single photon of light in the process. Luciferan is a large complex molecule that can later be returned to its original shape through a reduction reaction during which it gains an amount of energy equivalent to the single photon of light it emitted earlier. Because the essential reaction is an oxidation reaction the host organism, in this case the fish, can control the amount of light emitted by controlling the blood flow, and hence the oxygen supply, to the cells containing the bacteria.
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