Confinement-driven state transition and bistability in schooling fish

TL;DR

This study shows how confinement density influences state transitions and bistability in fish schools, providing experimental insights into active matter models of confined, self-propelled agents.

Contribution

It demonstrates a continuous, confinement-driven state transition with bistability in fish schools, modeled effectively by a two-state Markov process, offering a benchmark for active matter theories.

Findings

Confinement density controls the transition between polarized and milling states.

Bistability allows the group to switch randomly between states.

A simple Markov model accurately describes the state transitions.

Abstract

We investigate the impact of confinement density (i.e the number of individuals in a group per unit area of available space) on transitions from polarized to milling state, using groups of rummy-nose tetra fish (\textit{Hemigrammus rhodostomus}) under controlled experimental conditions. We demonstrate a continuous state transition controlled by confinement density in a group of live animals. During this transition, the school exhibits a bistable state, wherein both polarization and milling states coexist, with the group randomly alternating between them. A simple two-state Markov process describes the observed transition remarkably well. The confinement density influences the statistics of this bistability, shaping the distribution of transition times between states. Our findings suggest that confinement plays a crucial role in state transitions for moving animal groups. More generally, they provide an experimental benchmark for active matter models of macroscopic, self-propelled, confined agents.