LOCOMOTOR-ACTIVITY RHYTHMS IN THE HAGFISH UNDER EXPANDED OR CONTRACTED LIGHT-DARK CYCLES

This study reports on the locomotor activity rhythms of Paramyxine ata mi exposed to expanded and contracted photoperiodic regimes (T in hour s) divided into light and dark periods of equal duration. Nocturnal ac tivity was entrained by the range of 23.5 h less-than-or-equal-to T le ss-than-or-equal-to 25.2 h on average. Above and under this range, the activity signals entered the light period with pronounced individual variability: complete or partial dispersion in the light period and oc currence of active phases in both light and dark periods with relative ly constant period length of activity (PL). In the latter case, PL ave raged 22.9 h for contracted regimes, and 24.2 h for expanded regimes, while it averaged 23.9 h under constant T = 24. The circadian period ( tau) of fish which were shifted to continuous darkness (DD) after enga gement in contracted regimes and expanded regimes averaged 23 h and 25 h, respectively. The similarity between tau and PL values suggests th at the period lengths of activity rhythms are strictly regulated to ar ound 24 h by the endogenous time control system, while the relative co ordination of the activity rhythms to external light cues is still ret ained within a small variation of T from 24 h.