A photosensitve daily rhythm in the female medaka, Oryzias latipes

Variations of light and temperature regimes and resulting effects on reproductive parameters in medaka (Oryzias latipes)

In seasonally breeding fish species, altered fecundity, fertility, and spawning interval are associated with changes in environmental cues such as temperature and photoperiod. To determine quantitative impact of these cues on a suite of reproductive endpoints, groups of medaka (Oryzias latipes; two breeding pairs per group) were subjected to varying photoperiod and temperature regimes. Embryo production ceased after photoperiod reduction from 16L:8D to 8L:16D (at 25 degrees C). A severe decline in production was observed after a temperature decrease of 10 degrees C (25 degrees C to 15 degrees C [16L:8D]). Under reduced photoperiod, histologic analysis showed no mature ova and moderate oocyte atresia in all individuals. However, reduced temperature (15 degrees C) produced only mild oocyte atresia and fewer mature ova. Under both reduced photoperiod and reduced temperature regimes, mature spermatozoa were observed. Offspring viability, along with spawning interval, were not affected by photoperiod reduction. Temperature change had no effect on offspring viability but caused an increase in spawning interval. A shortened photoperiod profoundly affected medaka reproduction, whereas decreased temperature reduced, but did not arrest, fertility; reduced photoperiod decreased fecundity. These findings have important implications for culture of medaka as well as use of this teleost model for reproductive toxicology studies.

Smoltification induced by a 'skeleton' photoperiod in underyearling coho salmon (Oncorhynchus kisutch)

Underyearling coho salmon fry were subjected to three initial photoperiod treatments (6L∶18D, 10L∶14D, 14L∶10D) for two months and subsequently to three final treatments (16L∶8D, 9L∶6D∶1L∶8D, 10L∶14D) in a factorial design. Growth rates and seawater adaptability were monitored regularly. The groups that were exposed initially to 6L∶18D or 10L∶14D and then to 16L∶8D grew faster and had lower plasma sodium ion levels after seawater challenge tests than any of the other groups. Fish which were initially exposed to 6 L or 10 L daylength and then to a 9L∶6D∶1L∶8D skeleton photoperiod, showed a slightly lower growth rate and seawater adaptability than those given the corresponding complete 16L∶8D photoperiod. However fish maintained on skeleton photoperiods had significantly greater growth rates and seawater adaptability than those kept on the 10L∶14D photoperiod. This indicates that it is not the accumulated number of hours of exposure to light that initiates smolting, but rather the time during the day when light is experienced. Fish exposed initially to 14L∶10D showed little or no response to subsequent changes in photoperiod, suggesting that responsiveness to inductive photoperiods depends on the initial photoperiod treatment.

Effects of the light-dark cycle and scheduled feeding on behavioral and reproductive rhythms of the cyprinodont fish, Medaka, Oryzias latipes

Medaka were maintained on a 16:8 light-dark cycle and fed once daily on one of 5 different feeding schedules. The daily rhythm of agonistic behavior rapidly entrained to the scheduled feeding time and maintained this entrainment during a 3-day starvation period. In contrast the daily rhythms of egg laying and courtship stayed entrained to the L:D cycle regardless of the feeding schedule. Thus, temporal integration of this fish with its daily environment can involve multistimuli which concurrently and differentially entrain externally expressed circadian systems.

Constant photoperiod regimes and the entrainment of the annual cycle of reproduction in the female rainbow trout (Salmo gairdneri)

The effect of constant photoperiod regimes on the timing of maturation and the associated changes in serum oestradiol-17 beta (E2), testosterone (T), and total calcium (Ca2+, as an index of vitellogenin) were investigated in virgin female rainbow trout (Salmo gairdneri). From mid-January four groups of fish were maintained on constant long days (18L:6D) and one group on constant short days (6L:18D; group A). On May 8 the photoperiod of three of the groups on long days was abruptly reduced from 18L:6D to 6L:18D (group B), to 10L:14D (group C), or to 14L:10D (group D); Group E remained on 18L:6D throughout the experiment. The reduction in photoperiod per se appeared more important than the magnitude of the reduction in the advancement of the time of spawning with the first fish in groups B, C, and D spawning on September 3, 13, and 24, respectively; spawning of the remaining fish in each group was spread over 6-week periods. In contrast, spawning in group E began on October 24 and was spread over a period of 12 weeks. Spawning in group A was delayed until the following March. Although a long photoperiod earlier in the year was clearly necessary for the advancement of spawning, as evidenced by the delay in maturation under constant short days, no clear increases in E2, T, and Ca2+ were detected until after the reduction in photoperiod. Thereafter, the sequence of changes in serum E2, T, and Ca2+ levels was similar in all groups although the timings of these alterations were modified in relation to the time of spawning. Thus, peak levels of the three serum parameters were attained earliest in the year by fish in group B followed by those in groups C, D, E, and A, respectively. Collectively, these data support the hypothesis that photoperiod entrains an endogenous rhythm of maturation rather than having a direct, driving influence on the control of reproduction.

Photoperiodic mechanisms and rhythms of reproduction in the female rainbow trout

The present work investigates the importance of circadian and circannual rhythms in the photoperiodic control of reproduction in the rainbow trout. Maintenance of groups of 20-30 female trout under continuous light (LL), constant long (18L:6D) or short (6L:18D) days and conditions of constant temperature (8.5-9.0°C) and feeding rates (0.5% body weight, day(-1)), starting in February, produced markedly different spawning periodicities during the first and subsequent years of treatment. At the end of the first year, spawning was advanced by up to 2 months in the fish under LL and 18L:6D and delayed by up to 5 months in those under 6L:18D when compared with the December spawning of control fish under ambient light. Continued exposure of the fish to the same photoperiod regimes produced cycles of spawning and peak levels of testosterone, oestradiol-17β and calcium (as an index of vitellogenin) at intervals of 150-170 days under LL and 18L:6D and 320-420 days under 6L:18D. The functional importance of these spawning cycles and their relationship to circannual rhythms and the control of reproduction is discussed.Exposure of fish to skeleton (6L:4D:2L:12D, 6L:6D:2L:10D and 6L:8D:2L:8D) and resonance (6L:42D, 6L:48D and 6L:54D) procedures produced ranges of spawning times up to two months in advance of control fish. Results with the resonance regimes, where fish received only a half the light-dark cycles and a quarter of the total daylight hours of those on ambient light cycles show that trout do not measure daylength and time by counting daily cycles or by hour-glass mechanisms. Collectively, these data support the proposition that rhythmic processes of photosensitivity are involved in the modulation of reproductive development in the rainbow trout and that circadian and circannual rhythms cooperate in the timing and entrainment of this cycle.