Fish flashing below
Bioluminescent bacteria under the eyes allows them to school at night.
Above is the aptly named flashlight fish (Anomalops katoptron), and below is how it goes about its nocturnal business.
Researchers from the City University of New York, US, have discovered that they use their own light to allow them to school at night, and only a few need to be actively flashing to maintain the group.
They generate the light via bioluminescent bacteria in specialised organs under their eyes.
"Over 25% of fish species exhibit collective schooling behaviour, but schooling based on bioluminescent signalling has not previously been demonstrated," says research leader David Gruber.
"Being in the middle of one of these bioluminescing schools was one of the most magical things I've ever experienced as a marine biologist. It was like an Avatar moment as we watched rivers of bioluminescing fish merge like a blue-brick road and flow down the reef."
During two research expeditions to the Solomon Islands, Gruber and colleagues used low-light video to film large assemblages – hundreds to thousands of individuals – of the fish at night.
The recording showed that once fish velocity increased past a certain level, a loose group would transition to a closely-knit school, even with relatively-few flashing fish present.
After tracking each flashing fish in the analysed video clips and localising the fish in every frame, the authors created a model to simulate the movement of flashlight fish schools based on individual fish actions (cohesion, separation, and alignment) as well as water friction as a resistant force.
The model showed that less than 5% of the schooling flashlight fish needed to be flashing in order for schooling to be maintained in dark conditions.
Since fish reveal their location when they flash, the authors suggest that this may be a predator-avoidance strategy. They speculate that in order to confuse predators, some fish might flash, then rapidly change direction before flashing again.
Their findings are published in the journal PLOS ONE.