ANNUAL REPRODUCTIVE-CYCLE BASED ON HISTOLOGICAL-CHANGES IN THE OVARY OF THE FEMALE MOSQUITOFISH, GAMBUSIA-AFFINIS, IN CENTRAL JAPAN

To clarify the annual reproductive cycle of wild female mosquitofish, Gambusia affinis, in Mie Prefecture, central Japan, changes in ovarian histology were investigated. Female mosquitofish kept in aquaria unde r constant temperature (25 degrees C) and photoperiod (16L: 8D) condit ions produced successive broods at intervals of 22.1 +/- 0.46 days (n= 7). Between days 0-3 following parturition, females began active vitel logenesis. Between days 3-5, fully grown oocytes matured and were fert ilized and embryonic development began in the follicles. By day 10, ss fertilized eggs continued embryonic development, some oocytes at the oil-droplet stage had begun to accumulate yolk globules for the next g estation. Thus, vitellogenesis of the succeeding batch of oocytes over laps with gestation during reproduction in the mosquitofish. A rearing experiment showed the annual reproductive cycle of mosquitofish breed ing in Nagashima to be as follows. Although oocytes had not at that po int developed to the yolk globule stage, copulation occurred in Februa ry. Females began vitellogenesis in early May, the first pregnancy of the year commencing in mid-May. From mid-May to August, females repeat ed the gestation cycle (vitellogenesis, maturation, fertilization, pre gnancy and parturition) at around one month intervals. In September, o ocyte recruitment from the oil-droplet to the yolk globule stage cease d. After the final parturition, the ovaries contained only non-vitello genic oocytes. Spermatozoa in the ovarian cavity were scarce from Nove mber to January.