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 rhy thm of locomotor activity of the hagfish, Eptatretus burgeri, reared f or prolonged periods under constant darkness. The free-running period (tau) of circadian locomotor activity of 55 hagfishes followed a norma l 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 exami ned the effect of single light pulses (intensity; 670 1x 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 sligh t phase delay, whereas those in the late subjective night shortened ta u and provoked a slight phase advance. Thus, the hagfish showed a dist inct ''period response'' to short light pulse, but the ''phase respons e'' was not so clear as other organisms.