Fish switch their lights on to hunt by night

A school of flashlight fish on a reef flat during a moonless night. 

The light produced by flashlight fish at night can be seen up to 100ft away. Now researchers have studied the purpose of these flashing signals, which are produced by glowing bacteria in special organs below their eyes.
 

Flashlight fish, also referred to as lantern-eye fish, live in swarms of up to 50 animals that hunt plankton on coral reef roofs at night. During the day, the fish withdraw to a depth of up to 400 metres, where they presumably rest in underwater caves. They have light organs below their eyes which can be rotated backwards and which contain bioluminescent symbiotic bacteria. By rotating their light organs, the fish generate flash patterns in different frequencies. To date, researchers could only speculate about the light organs' function.

“Deploying methods of behavioural biology, we have now successfully demonstrated that Anomalops katoptron uses its light organs while foraging, and it adjusts the flash frequency to the context in which it operates,” explain Prof Dr Stefan Herlitze and Dr Jens Hellinger of Ruhr-University Bochum.

They observed that the fish blink 90 times per minute on average during the night. Once the researchers add zooplankton to the water, the animals promptly reduce their flash frequency and emit an almost continuous glow while feeding. Non-glowing control animals of the same species hardly ever caught any prey. They didn’t exhibit any changes to blink frequency in the presence of zooplankton, either. The blink frequency in non-glowing fish can be determined by monitoring light organ rotation.

“The results clearly indicate that Anomalops katoptron relies on its light organs while foraging for food,” conclude Stefan Herlitze and Jens Hellinger.

Moreover, the researchers have demonstrated that the animals’ blink frequency is also regulated by the lighting conditions in their environment. During the day, the animals rest in the reef aquarium in a dimly lit cave, keeping their light organs closed for the most part, interrupted only by brief blinks. During their active phase at night, on the other hand, the fish exhibit a high blink frequency. The precise function of that blink pattern is not yet fully understood. Additional analyses will reveal whether the fish use blink patterns to communicate or whether they confuse predators by blinking in a swarm.

In addition to the lighting conditions in its surroundings, the presence of food also determines the bioluminescence of the fish. A loss of luminescence is caused by food shortage.

The researchers published their report in Plos One.