Fish’s quirky behaviour helps them estimate swimming direction, speed

Fish glance down when swimming because the stable riverbed underneath them provides more trustworthy information about their swimming direction and speed, according to new simulations.
The study, published in the journal Current Biology, is the first to combine simulations of the zebrafish’s brain, native environment, and spatially-varying swimming behaviour into one computational model. By analysing this model, the researchers concluded that this quirk — looking down while swimming forward — is an adaptive behaviour that evolved to help the fish self-stabilize, as when swimming against a current.
As water moves, fish are constantly trying to self-stabilize in order to stay in place rather than getting swept away in a moving stream. Focusing on other fish, plants, or debris might give the fish the false impression that it’s moving. However, the stable riverbed below them gives fish more reliable information about their swimming direction and speed.
“It’s similar to sitting on a train car that isn’t moving. If the train next to yours starts to pull away from the station, it can trick you into thinking you are moving too,” said Northwestern’s Emma Alexander, who led the study. “The visual cue from the other train is so strong that it overrides the fact that all of your other senses are telling you that you are sitting still. That’s exactly the same phenomenon that we are studying in fish. There are many misleading motion cues above them, but the most abundant and reliable signals are from the bottom of the river.” Alexander is an assistant professor of computer science in Northwestern’s McCormick School of Engineering, where she runs the Bio Inspired Vision Lab.
To conduct the research, Alexander and her collaborators focused on zebrafish, a well-studied model organism. But, although many laboratories have tanks full of zebrafish, the team wanted to focus on the fish’s native environment in India.
“It was recently discovered that fish respond to motion below them more strongly than motion above them. We wanted to dig into that mystery and understand why,” Alexander explained. “Many zebrafish that we study grow up in lab tanks, but their native habitats shaped the evolution of their brains and behaviors, so we needed to go back to the source to investigate the context for where the organism developed.