They Communicate by Flashing Blue Lights On and Off. They’re Fish.


Marine biologist David Gruber was filming fluorescent coral in a shallow reef in the Solomon Islands when another diver gestured for him to look at something. Suddenly, Gruber saw “thousands of blue blinking bioluminescent lights.” The lights undulated into circles and shapes, flowing down the reef “like a blue bioluminescent brick road,” he recalled in a video interview with National Geographic.

They were flashlight fish, in what Gruber believes is the largest aggregation of the species Anomalops katoptron ever seen. But it wasn’t just the sheer spectacle that intrigued him. What was the purpose of this underwater light show, and what’s the mechanism behind it?

It turns out these lighting displays allow the fish to communicate as they form schools and change direction. The lights are made possible by a symbiotic relationship with bioluminescent bacteria that grow under their eyes. Gruber, a professor at Baruch College and The Graduate Center, described the findings in an article in the journal PLOS ONE co-authored with colleagues from the University of Rhode Island, Rice University, Yale University School of Medicine, and the American Museum of Natural History.

Flashlight fish. The white area beneath the eye indicates where the bioluminescent organ is located. Photo: John Sparks and Robert Schelly

Most fish that swim in schools during the day disperse at twilight and in the dark. But flashlight fish — which are a few inches long — use their blinking lights to synchronize schooling at night. They not only blink when they’re feeding, but they also blink to signal a change in direction for the school.

What’s more, flashes from fewer than 5% of fish in a school of hundreds or thousands can move the entire group. “If a small cohort of fish become motivated to go in some direction (e.g. pursuing prey or evading a predator), the rest of the fish will follow, causing a rapid and coordinated change in the overall direction of the school,” the authors wrote. These “blink-and-run” behaviors may confuse and distract predators, while also serving as a warning to the group.

The study was “the first to demonstrate that schooling in fishes can be facilitated by bioluminescent flashes in the absence of ambient light. The research raises the possibility that fish schooling may occur in the deep sea, where it was previously assumed to be too dark for fish to coordinate their movements,” according to the American Museum of Natural History.

But it’s not just the research findings that made the expedition so meaningful, Gruber told the AMNH. “Being in the middle of one of these bioluminescing schools,” he said, “was one of the most magical things I’ve ever experienced as a marine biologist.”