Flashlight fish use their bioluminescent organs to school at night – and only a few need actively flash to maintain the group, according to a study published August 14, 2019 in the open-access journal
by David Gruber from the City University of New York, USA, and colleagues.
Over 25 percent of fish species exhibit schooling behavior (individuals synchronizing their behavior and orientation); however, most fish schools disperse at low light levels, and nighttime schooling has rarely been observed. Gruber and colleagues studied the nighttime schooling behavior of the flashlight fish Anomalops katoptron, which unusually can generate its own light via bioluminescent bacteria in specialized organs under its eyes.
During two research expeditions off a remote tropical island in the Solomon Islands in 2013 and 2016, the authors used low-light video to film large assemblages (hundreds to thousands of individuals) of flashlight 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–and only a few actively-flashing fish were needed to control the direction and movement of the school.
After tracking each flashing fish in the analyzed video clips and localizing 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 percent 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.
Although specific to flashlight fish, this research may provide insight into the behaviour of other species of bioluminescent fish, especially deep-sea fish living in a permanent low light environment.
“Over 25% of fish species exhibit collective schooling behavior, but schooling based on bioluminescent signaling has not previously been demonstrated,” adds 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.”
Gruber DF, Phillips BT, O’Brien R, Boominathan V, Veeraraghavan A, Vasan G, et al. (2019) Bioluminescent flashes drive nighttime schooling behavior and synchronized swimming dynamics in flashlight fish.
This study was funded by NOAA OER grant #NA160AR0110198 to DFG, BP, JSS and VAP, National Geographic Expedition Council #EC0780-16 to BP, NSF grants #DBI-1556213 to DFG and #DEB-1257555 to JSS, and The Ray and Barbara Dalio Family Foundation Explore21 AMNH expedition grant to JSS, DFG, and VAP. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
The authors have declared that no competing interests exist.
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