What are these bizarre headlights under the eyes for A study shows how flashlight fish use their biological luminosity to coordinate schooling behavior. In the event of a threat, they blink more frequently with their light organs by means of a kind of blinking. This “Morse code” causes the schooling fish to move closer together, based on experiments in water tanks and observations during dives in the habitat of the flashlight fish.
Bioluminescence is the technical term: some living things can generate light through chemical processes. The best-known example is the firefly, but the phenomenon can also be found in many other organisms – from unicellular organisms to vertebrates. Some fish species also belong to the bioluminescent community. But actually they don’t produce the charisma themselves – they use symbiotic bacteria instead. These bioluminescent microbes sit in certain organs of the fish and make them glow. This also applies to the flashlight fish (Anomalops katoptron) that occur in the tropical waters of the Indo-Pacific.
Like a Milky Way in the water
The fish, which are up to 35 centimeters long, have a luminous organ under their eyes that is filled with bioluminescent bacteria. This organ can also be closed so that it looks like the animals are blinking. The flashlight fish hide during the day in caves, crevices or in dark water up to a depth of 400 meters. “In the dark nights, however, up to a thousand individuals migrate in a swarm into the plankton-rich surface water,” says Peter Jägers from the Ruhr University in Bochum. “It is a surreal experience to see the swarms – like a Milky Way in the water,” says the researcher, who was able to observe the fish himself in nature on a diving expedition in the Indo-Pacific.
For some time now, Jägers and his colleagues have been researching the bioluminescence system of flashlight fish in captive specimens at the Ruhr University in Bochum. In order to understand the function of the flashing patterns, they examined the behavior of some test animals in a special water tank as part of their current study. It contained digitally controllable fish dummies that could imitate the animals’ light signals. During the experiments, the scientists recorded the movements of individual animals in response to the artificial flashes with infrared cameras.
Fast flashing with attraction
The results of the investigation showed that if the researchers placed a single dummy flash light in the middle of the tank, the faster the light blinked, the closer the fish would be. In a further experiment, they then positioned 13 lights around the tank, which lit up one after the other at different time intervals. “We found that the flashlight fish were highly motivated to orient themselves to the light stimuli,” reports Jägers. From these results, the researchers concluded that faster blinking is a signal for the animals to join their conspecifics more closely – to form a more compact swarm. This could serve to better protect yourself from predators in the group.
The team was then able to confirm this interpretation of the results during night dives in the natural habitat of the flashlight fish. The scientists waited in the dark until a swarm of animals approached and transformed the underwater world into a “starry sky”. With a weak red light, the Bochum team then triggered fear reactions in the fish and at the same time recorded their blinking patterns with special cameras. The evaluations then showed: Stress is actually associated with an increased blinking frequency.
“We assume that the increased blinking frequency is the signal to orientate oneself closer to the other group members under stress,” says Jägers. “In our study, we were able to demonstrate for the first time a precise relationship between visually communicated signals under restricted lighting conditions, such as those that prevail at night or in the deep sea, and the schooling of fish. We hope that these findings can also be helpful in future studies of, for example, the largely unexplored fish of the deep sea, ”says the scientist.
Source: Ruhr University Bochum, specialist article: Scientific Reports, 2021, DOI: 10.1038 / s41598-021-85770-w
Video: A school of flashlight fish in the Indo-Pacific. Credit: Ruhr University Bochum