Circadian rhythms in locomotor activity of the hagfish, Eptatretus burgeri. 5. The effect of light pulses on the free-running rhythm

This study examined the effect of light pulses on the free-running rhythm of locomotor activity of the hagfish, Eptatretus burgeri, reared for prolonged periods under constant darkness. The free-running period ( tau ) of circadian locomotor activity of 55 hagfishes followed a normal distribution with mean 24.1 hr (min.: 22.8 hr; max.: 25.1 hr). After establishing the tau in constant darkness for each specimen, we examined the effect of single light pulses (intensity; 670 lx at the bottom of the aquarium, duration; 15 min) on the free-running rhythm. Light pulses in the early subjective night lengthened tau and caused a slight phase delay, whereas those in the late subjective night shortened tau and provoked a slight phase advance. Thus, the hagfish showed a distinct "period response" to short light pulse, but the "phase response" was not so clear as other organisms.