Some marine fish glow in the dark without making any light themselves. They host bacteria that do the work, and some species can even switch the glow off at will.
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In the flashlight fish Anomalops katoptron, the light comes from organs located beneath the eyes, where luminous bacteria live. These microbes are not casual guests: they occupy densely packed tubules that open to the outside.
The fish tilts its light organ downward, which hides the glow. By alternating these shutdowns, it produces precise blinking patterns, much like a torch being switched on and off. A laboratory observation suggests the blinking becomes more frequent under stress, but it rests on a small number of individuals.
Other species follow a different path. In young larvae of Nuchequula nuchalis, the light organ forms before bacteria colonise it, acquired from the surrounding water rather than passed directly from the mother.
