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Locomotor activity rhythms in the hagfish Paramyxine atami were studied using the same procedures as were employed in a prior study of Eptatretus burgeri. Swimming activity under 12L: 12D (7:00-19:00 light, 19:00-7:00 dark) appeared at the beginning of the dark period and at the onset of the light period. In E. burgeri such activity occurred regularly in the first 2/3 of the dark period. P. atami exhibited a labile free-running rhythm of swimming under continuous darkness. After reversal of the light-dark cycle (7:00-19:00 dark, 19:00-7:00 light), the average time necessary to shift to the new dark period was 3.7 days, in contrast to 7.7 days which E. burgeri required. The differences in the activity patterns of the two species may be ascribed to differences in their natural habitats: P. atami resides in deep waters, and cannot be so much influenced by sunlight as E. burgeri. It may be speculated that P. atami has a relatively "delicate" circadian oscillator, and the external light used in these experiments has a relatively strong effect in regulating locomotor activity, with the result that activity shifts in a short time after reversal of the light-dark cycle, and temporary activity is evoked at the onset of the light period.

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This study reports on the locomotor activity rhythms of Paramyxine atami exposed to expanded and contracted photoperiodic regimes (Tin hours) divided into light and dark periods of equal duration. Nocturnal activity was entrained by the range of 23.5 l i•…T•…25.2 h on average.