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If you're hunting for food on videos of older women having sex on public transporta pitch-black night, it helps to bring a flashlight.

That's exactly how reef-dwelling Anomalops katoptronfish find and gobble up their planktonic prey, German scientists found.

Splitfin flashlight fish have bean-shaped organs beneath their eyes that host light-producing bacteria. The fish can turn this light on and off by blinking, sending Morse code-like signals into the deep abyss.

While many other ocean creatures are bioluminescent, relatively little is known about reason for this particular fish's blinking patterns.

The scientists, who published their findings this week in the journal PLOS ONE, sought to learn just how these fish use their "flashlights."

Jens Hellinger, a zoologist with Ruhr-University Bochum in Germany, and other researchers studied the school of flashlight fish in artificial coral reef tanks under various lab conditions.

In the darkness of night, the fish blinked very frequently, at about 90 blinks per minute, the team found. The fish's lights were on and off for roughly the same length of time.

But when the fish detected living planktonic prey at night, they kept their lights on longer. Fish in the experimental tank blinked five times less frequently than they did when there wasn't any prey around.

During the day, when fish retreated to a cave inside the tank, they blinked only about nine times per minute, with longer stretches of off-time for their flashlight organs.

All this suggests that splitfin flashlight fish use their bioluminescent light to detect prey during the night, Hellinger said in a news release. They also adjust their blinking frequency to fit their current situation.

However, the findings are still preliminary, the researchers noted.

Hellinger and his colleagues said additional field research is needed to see whether Anomalops katoptron display the same behavior outside the lab and in the wild.

In their natural environment, in the Banda Sea near Indonesia, the flashlight fish live in coral reefs. They're typically more active on moonless nights and spend their days tucked inside reef caves and crevices.