The two moray eels and the two triggerfish were each exchanged in the experiment, so that each predator had to stay in the experimental tank for a maximum of 3 weeks, to prevent excess stress. During the experiment, the tank was visually isolated and exposed to a 12:12 h light–dark cycle (LD), including 60 min of simulated dawn and dusk by progressive dimming of the light. Fluorescent artificial light was used and 350 lx light intensity at the water surface was provided during the light phase (L). At night, the light intensity was maintained below 1 lx (D). A red-light filter (Schott Glass filter, wavelength > 650 nm) was used to aid filming at night. All sessions were recorded with a digital video camera. Recording was done for one second every 30 s, using the interval recording program.