1968 年 15 巻 1 号 p. 11-27
The annual, land-locked salmonoid fish, the Koayu, Plecoglossus altivelis, reared in the outdoor hatchery ponds and exposed to short-(8 hours daily), long-photoperiodic regime (16 hours daily) and natural day-length (control) were examined to elucidate the activities of hypothalamo-hypophysial neurosecretory complex and a possible interrelationship between the hypothalamic nuclei and maturation of the fish. The period of experiment extended from July 2, 1966 to January 22, 1967. Five females and five males collected at monthly intervals were supplied to inspect the growth, maturity and histological feature of the neurosecretory system. High mortalities were encountered between the months September and November in the short photoperiodicity, October and December in the control, and December and January in the long photoperiodicity, respectively, in parallel with the highest level of gonadal maturation and of cyanophilic gonad stimulating cells. Untill each prespawning season, the perikarya and juxta-somal axons of the preopticonucleic cells heavy loaded with AF-stainable neurosecretory material have been observed in each experimental group.Early in each spawning season, fine droplets of material in smaller amount are scattered around the nuclei of the cells. After that, a notable depletion of the material from the perikarya and juxta-somal axons is detected. Recovery phase of the neurosecretion is seen in the female fish survived beyond each spawning time and tided over the winter. On the contrary, all of the males were deceased untill the next year. Changes in the amount of the storage material in the neurohypophysis appear to be parallel with those of neurons. During each breeding time, on the other hand, the highest activity is reached by the nucleus lateralis tuberis which were stained with acidic dyes. These results suggest that there is a role of the light rhythm upon the maturation of the Koayu via hypothalamic neurosecretory system, and the possibility of artificial control of the breeding time in the fish.
