Abstract
The effects of temperature and daylength on the initiation of sexual maturation of yoshinobori, Rhinogobius brunneus (orange type), were examined. In the natural habitat, onset and further progress of vitellogenesis and spermatogenesis were not synchronous among indi-viduals. Precocious females started yolk accumulation in March and reached pre-spawning stage in late May. Spermatocytes were found in all male specimens in March, though meiotic figures could be seen as early as December in the testes of larger males. In late May, some larger precocious individuals reached the final stage of spermatogenesis. Many deposited egg masses and brood-caring males were observed in June. In experiments, combination of high tempera-ture and long day alone elicited the onset of vitellogenesis. In the case of the testes, however, high temperature-long day regime was not the triggering factor of meiosis but only an accelerator. All the reproductive process of yoshinobori, from beginning of vitellogenesis to spawning, proceeds quickly under high temperature-long day conditions. Minimum temperature and daylength for the onset of female maturation is probably around 9°C and around 12hours respectively.
