ENTRAINABILITY OF LOCOMOTOR-ACTIVITY RHYTHM TO THE REVERSED LIGHT-DARK CYCLE IN THE HAGFISH

The hagfish Paramyxine atami shows a clear nocturnal rhythmicity of locomotor activity. Entrainability of locomotor activity rhythms in the hagfish were tested under reversed light-dark cycles with light intensities of less than 1.0 1x. The shifting of activity to the newly established dark period became defective and unstable with decreased light intensity in the light period. Entrainability to the LD cycle was still observed at the intensity of 2.53 x 10(-3) 1x. These findings for P. atami are compared to a closely related species E. burgeri. The functional threshold of photo-receptors in P. atami seems to be lower than that in E. burgeri. Likewise, there is a difference in the vertical range of their habitats: E. burgeri and P. atami inhabit, respectively, depths of 10-100 m and 50-500 m. It is suggested that the activity rhythms in P. atami can be regulated by weaker light intensities than those in E. burgeri and that this relates to the difference in their natural habitats.