THE EFFECT OF LIGHT-PULSES ON THE CIRCADIAN-RHYTHM IN LOCOMOTOR-ACTIVITY OF THE HAGFISH

Light pulses of 15 min to 6 h were given to the hagfish Paramyxine atami, individuals displaying a free-running rhythm in locomotor activity under continuous darkness. The fish sensitively responded to the pulses; the active phase was promptly delayed or advanced when pulses were respectively given at the beginning or at the end of the duration of locomotor activity (alpha). The responses to light pulses in P. atami were characterized both by an initial phase-shift and by the changing period length (tau) in the ensuing active phase. In contrast, a closely related hagfish species Eptatretus burgeri responded to the light pulses by taking another stable course of the ensuing active phase with relatively constant tau. These results suggest that the sensitivity to light pulses in P. atami is higher than that in E. burgeri.