There was a highly significant effect of the illumination regime on the moray eel's locomotor activity (paired t-test: t = − 4.460, P = 0.001, N = 12). The moray eel attacked the octopuses 18 ± 4 (ME ± SD) times per day in the darkness versus 9 ± 5 times in light. These attacks did not show a significant decrease for the duration of the experiment