Flashlight Fish Blinks Bioluminescent Light on and off to Stun Prey

The fish projects its unusually bright light from bean-shaped organs filled with glowing bacteria under its eyes.

The splitfin flashlight fish lives up to its name, as researchers have just determined it blinks its eyes to switch a self-generated light on and off.

The light controlled by the fish (Anomalops katoptron), is so bright that it can illuminate and stun prey.

The findings, published in the journal PLOS ONE, help explain at least one purpose of the fish's light, which has long puzzled scientists, not to mention scuba divers and snorkelers who have been startled by the sight of the fish in its "on" mode. The light can be viewed from up to 100 feet away.

"I have observed Anomalops several times in the field and a thousand times in the lab and the luminescent light is definitely bright and impressive," Jens Hellinger, chair of the Department of Zoology and Neurobiology at Ruhr-University Bochum in Germany, told Seeker.

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.

When the fish detected plankton, which they love to eat, they stayed in the "on" mode longer, blinking five times less frequently than when in the absence of prey. This strongly suggests that the fish are using the light to detect and probably startle prey.

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While the fish are popular in many aquariums worldwide, little is still known about how they evolved and what other problems they might solve with their flashlight skills. For example, the fish could be communicating with each other using a Morse code-like system that no human has yet been able to decode.

For now, Hellinger said, "Our study will provide new insights into the role of luminescence in tropical reef coral systems."

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