Fig. 1

Behavioural arena. a Schematic representation of the behavioural arena. Visual stimuli were attached to a Plexiglas sheet (indicated by hatching) using Velcro, with the bottom of the stimulus on the floor of the arena. The sheet was lowered into the arena and fastened by clamps. The wall of the arena was white. For the experiments with black-and-white patterns, a mid-intensity grey cloth was attached to the inside wall of the arena. b Five different stimuli were presented to the animals: three black-and-white stimuli, a black circular stimulus and a grey circular stimulus. For each black-and-white stimulus the area of black was equal to the area of white. For all similar sized circular, or rectangular, stimuli the area of black was the equal. c The arena during an experiment. Recordings were made with a camera floating on the surface of the water. Abbreviations: c, camera; cl, clamps for attaching the Plexiglas sheet; m, middle of the arena; s, stimulus; sf, starfish. Example tracks for: d black circles (angular height 37°) on a white background and e the control experiment with only the Plexiglas sheet on a white background. The stimulus is located at 0°