On the relationship between gonadal development and response time to photostimulation of sticklebacks living under natural conditions and under constant short-day conditions for long periods of time

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Abstract
In the threespine stickleback, Gasterosteus aculeatus L., gonadal development proceeds in two phases, each occurring in a different season. Phase 1 (spermatogenesis, endogenous vitellogenesis) begins in late summer and is completed during winter. Phase 2 (Leydig cell activation, exogenous vitellogenesis) does not start before late February and is completed in April; the breeding season lasts from April to August. These different developmental phases are also characterized by differences in response times to the stimulating influence of a long photoperiod, 16L:8D at 20 °C. As long as phase 1 was not complete, the response times (number of days until complete sexual maturity is attained) were longer than 34 ± 6 days in males and 33 ± 5 days in females. From the time phase 2 was initiated, the response times became progressively shorter. When sticklebacks were raised from the egg stage, or from within 3 months after hatching in an outdoor pond, under constant short-day conditions (8L:16D at 20 °C), only phase 1 was completed, whereas no development in phase 2 took place; the response times of these fish were comparable to those given above. Only small numbers of these short-day animals attained maturity spontaneously under the experimental photoperiod. The fish matured in June and July of the year following the one in which they hatched in June. These results suggest an endogenous annual reproductive rhythm.

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