Abstract
This study reports on the locomotor activity rhythms of Paramyxine atami exposed to expanded and contracted photoperiodic regimes (T in hours) divided into light and dark periods of equal duration. Nocturnal activity was entrained by the range of 23.5h_??_T_??_25.2h 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 occurrence of active phases in both light and dark periods with relatively constant period length of activity (PL). In the latter case, PL averaged 22.9h for contracted regimes, and 24.2h for expanded regimes, while it averaged 23.9h under constant T=24. The circadian period (τ) of fish which were shifted to continuous darkness (DD) after engagement in contracted regimes and expanded regimes averaged 23h and 25h, respectively. The similarity between τ and PL values suggests that the period lengths of activity rhythms are strictly regulated to around 24h by the endogenous time control system, while the relative coordination of the activity rhythms to external light cues is still retained within a small variation of T from 24h.
