Surprise Finding: Zebrafish Break Out Into 'Panic Wave' Mosh Pits to Social Distance

Social distancing is nothing new in the time of COVID. But new research of the well-studied zebrafish (Danio rerio) has captured a previously undocumented behavior of the animal that takes such measures to the extreme — and the end result resembles the frenzy of a circle pit typically associated with heavy metal concert-goers.

In the journal Frontiers in Physics, a student-faculty team of biologists and mathematicians at New Jersey Institute of Technology have described a collective social-avoidance behavior in larval zebrafish that occurs when their small, enclosed habitat reaches a critical mass. The fish spontaneously burst into a frenetic vortex, or “panic wave”, along the rim of their petri dish confines.

While the zebrafish has been a staple in lab research for nearly 50 years as a model to study human development and disease, researchers say this newly observed avoidance behavior, or escape response, makes the species potentially relevant as a model system for exploring the physics of social interactions as well.

“It really was a revelation. ... I have been observing larval zebrafish swimming individually for over 15 years, and this observation was a total surprise,” said Kristen Severi, assistant professor of biology at NJIT and corresponding author of the study. “We expected the fish to space out evenly to avoid each other when we placed them inside a petri dish, but through their physical interaction with the walls and fluid, and their social interaction with each other, what we instead observed was this panic wave of circular swimming along the walls. At that point, we knew we had something to report.”

“Coordinated collective motion is observed at all scales: human crowds, bird flocks, fish schools, bacteria colonies and even the movements of cell-constituents,” added Enkeleida Lushi, professor at NJIT’s Department of Mathematics and co-author of the study. “We wanted to test if this holds true for larval zebrafish, and indeed, circulation emerged as a panic wave — one fish would move, then others nearby got triggered and started to move too. While circulation was perhaps predictable from a theoretical standpoint, the panic-wave form of it was just not expected at all.”

According to the study, larval zebrafish typically prefer a socially distanced area of approximately 50 square millimeters, and to maintain it, will initiate escape responses in the form of short-duration/high-velocity “beat-and-glide” movements directed away from contact with neighboring fish or other anxiety-inducing stimuli.

But to learn what would happen if the fish were deprived of their prefered personal space, the team gradually added zebrafish to small circular petri dishes, and used high-speed cameras and tracking software to capture the individual trajectories of larvae over the course of 30 seconds (or 6,000 frames). In all, the team observed experimental trial groups ranging from 5 to 130 zebrafish as they interacted inside the dish.

When 130 fish enter the 5.4cm-diameter arena, the pandemonium starts.

Video Credit: NJIT

“To the untrained eye in real time, it is hard to make out any pattern when you see a panic wave propagate, but high frame rate video allows one to observe the phenomenon relatively clearly,” said Haider Zaki ’21 (December), an undergraduate biology student involved in the lab’s computational tracking and analysis. “My reaction initially was ‘wow’, because the duration and definition of the movement were undeniable, and not something I had ever expected to see from larvae that are only 5-6 days old. 

“The comparison that comes to mind would be someone yelling ‘fire’ in a movie theater, except that the theater was round … once the wave is started, it propagates and cycles through the involved larvae.”

The team now want to further investigate what triggers the panic wave in the fish, including neural circuits and brain activity that may underly the phenomenon. The team also says future studies could investigate ways of controlling collective motion and preventing panic waves by making use of a given confinement’s geometry. 

“Each individual zebrafish adheres to some guidelines of locomotion which determine their next moves based on factors such as vision, fluid flow near their body and tactile input. When all the individuals follow the same scheme, collective motion can emerge,” explained Zaki. “At this stage, we cannot confidently say exactly what is at play, but we are working towards these explanations now.”

“We have many potential avenues to explore how this behavior is generated collectively, from modeling the interactions to using machine learning-based tracking, to get a better handle on the movements,” added Severi. “Since our lab primary focuses on the neural circuits which control swimming, we want to ask questions about what the fish are sensing that could be triggering this, how their internal state impacts their response and where this is being processed in the brain.”