He explained that a large bean-shaped organ under each eye of the fish is filled with bioluminescent bacteria. It is a complete mystery as to how the fish evolved the ability to harbor such useful bacteria.
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When the fish blink, they "rotate the light organ backwards," Hellinger said, preventing each colony of bacteria from releasing visible light. When the light is on, he explains, "it is projected from the bean-shaped light organs under the eyes and not from the eyes."
Fascinated by the ability, Hellinger and his team studied the fish on moonless nights in waters off the Banda Islands of Indonesia. The fish are popular with aquarium enthusiasts, so the researchers also obtained a school of them from a commercial wholesaler and studied the fish back at their lab.
They found that during the darkness of night, the flashlight fish blink very frequently, at 90 blinks per minute. This causes their light to go on and off for an equal amount of time.