PHOTOSENSITIVITY OF THE HAGFISH REPRESENTED BY ENTRAINABILITY OF LOCOMOTOR-ACTIVITY RHYTHM TO THE REVERSED LIGHT-DARK CYCLE

Locomotor activity rhythms in the hagfish Eptatretus burgeri were reco rded by use of an electronic photo-switch system. The activity in the hagfish occurred in the dark period of a 12L: 12D cycle even if the cy cle was reversed. Entrainability of the activity rhythm to the reverse d LD cycle was also measured under dim light intensities of the light period. Defective phase-formations were produced when the light intens ity fell as low as 5.96 x 10(-3) lx. The activity phase was no longer entrained by the LD cycle in the intensity of 2.53 x 10(-3) lx. It is estimated that the light intensities used in the experiments occur at depths of 100-150 m of Sagami Bay, which the hagfish inhabits in summe r. Thus, an external LD cycle, even with such dim light, might regulat e locomotor activity in the hagfish under natural conditions.