野生メダカの生殖年周期〔英文〕

概要

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The wild type medaka Oryzias latipes were collected from irrigation canals near Ushikunuma pond (Ibaraki Pref.) almost every month for a year and a half. Seasonal changes of body length distribution, gonadosomatic index and gonadal histology were examined in both sexes. The surface water temperatures, probably the main factor for the maturation of the medaka, were measured at noon of sampling days and compared with those of the circulating outdoor tank. The underyearling fish were distinguished from the yearling fish by body length. In July and August the ovaries of some large underyearling females contained yolk globule stage oocytes, but all the fish had regressed ovaries in September. Yolk vesicle formation proceeded from autumn through winter and the spawning period began in mid-April after yolk globule accumula-tion since early April. Underyearling males in June already had matured testes and spermato-genesis was observable until October. After inactivity during winter spermatogenesis resumed in early spring to restore its full activity in mid-April. After July the yearling fish of both sexes were hardly collected and the spawning in August was mainly conducted by the underyearling fish. The surface water temperatures of the capture sites at about noon were usually higher than the maximum temperature of the circulating outdoor tank of the day.