Pattern formation and stochastic motion of the zooplankton Daphnia in a light field

We extend our study of single and collective motions of the zooplankton Daphnia both in experiment and computer simulation. Daphnia, as well as many other prey animals, can be observed to swarm under certain circumstances as a protective behavior against predators. Daphnia swarms can be induced by an optical marker such as a vertical shaft of light, to which they are attracted. For low Daphnia densities we observe that individual Daphnia develop a circular motion around the optical marker, whereas for high densities we can reproducibly induce swarming Daphnia to carry out a vortex motion. To learn more about this circular pattern and the associated spontaneous symmetry breaking, the motion of single Daphnia, as well as swarms, is characterized with respect to the light shaft. A stochastic model based on experimental data is compared with the observed Daphnia behavior as well as with existing models for single agents and agent swarms performing a circular motion to reveal the essential ingredients for vortex motion to occur. (C) 2003 Elsevier Science B.V. All rights reserved.