Cf. Randall et al., PHOTOPERIOD-INDUCED PHASE-SHIFTS OF THE ENDOGENOUS CLOCK CONTROLLING REPRODUCTION IN THE RAINBOW-TROUT - A CIRCANNUAL PHASE-RESPONSE CURVE, Journal of Reproduction and Fertility, 112(2), 1998, pp. 399-405
Different groups of winter-spawning female rainbow trout that had been
maintained under seasonally changing daylength and temperature were e
xposed to 2 months of continuous light at different times of the year.
The same photoperiod produced advances in the time of spawning of up
to 232 days and delays of up to 80 days, depending upon the timing of
exposure in relation to the phase of the reproductive cycle. The propo
rtion of fish spawning in each group varied from 18% to 100%, again de
pendent on the timing of exposure to continuous light. The photoperiod
-induced changes in spawning time can be interpreted as phase-dependen
t phase-shifts of an endogenous circannual clock controlling maturatio
n. It is proposed that long days, occurring earlier or later than they
would under a natural photoperiod, were perceived as indications that
the clock was running slow or fast, thus initiating corrective forwar
d adjustments (advance phase-shifts) or backward adjustments (delay ph
ase-shifts), respectively. Collectively, these responses can be descri
bed in the form of a circannual phase-response curve.