Effects of restricted photoperiod and melatonin administration of gonadal weight in the Japanese killifish.

a percentage of total body weight) is marked in fish maintained under continuous darkness or natural daylight (a short photoperiod) conditions during the period from October to December, but the index increases after pinealectomy (Urasaki, 1972). In Oryzias it is obvious that the reduced light period is effective in lowering gonadal weight, but the mechanism is still unknown. However, it is postulated that the pineal gland plays some role in the mechanism. The present investigation was carried out to study the mode of action of the pineal in inhibiting gonadal development in the fish. Adult specimens, 360-450 mg in body weight, of Oryzias latipes were used in the experiment. On 12 November (Expt 1) and on 20 January (Expt 2), removal of both eyes or pinealectomy was performed and intact fish served as controls. On the same day, the fish were divided into three lots of equal number and exposed to different light conditions (16 h light: 8 h darkness; 8 h light: 16 h darkness; and natural day¬ light) for 46 days. In Expt 2 the fish were further treated by daily i.p. injection of 2 μ\ of isotonic saline (pH 7-3) or 2 μ of saline containing 0-04 μg melatonin (Sigma Chemical Co.) for 10 days up to termination of the experiment. In total, Expt 1 contained 9 groups and Expt 2 18 groups each comprising 15 males and 25 females. At the end of the experiments (28 December in Expt 1 and 6 March in Expt 2) all of the fish were killed and the whole body and the gonads were weighed immediately. Gonosomatic indices of males and females in all groups were calculated and analysed statistically by the one-way layout method. At the beginning of the experiment,