Juvenile Intersexuality in a Cyprinid Fish, the Sumatra Barb, Barbus tetrazona tetrazona

The process of sex differentiation and development of the gonad in the Sumatra barb, Barbus tetrazona tetrazona, reared in laboratory conditions was studied histologically in the present study. Gonads of all juvenile fish were initially provided with ovarian structures, which were characterized by the development of germ cells into meiotic prophase and auxocyte stages and by the formation of ovarian cavity, during the period from 20 to 25 days after hatching. The ovary continued to grow steadily in part of the fish on successive days, whereas in other fish it remained in ill-developed states in which sparsely distributed oocytes, a relatively large number of gonial germ cells and compact stromal tissue with increased somatic cells were notable. In gonads of the latter type, testicular development began with clustering of gonial germ cells to form primordial seminal lobules, which indicated definitely the differentiation of spermatogonia, and subsequent initiation of spermatogenesis in some of them. Concomitant degenerative changes of oocytes accompanied in some cases by marked infiltration of somatic cells over the stromal tissue were also prominent in these gonads. Intersexual gonads with developing male germ cells and degenerating oocytes thus occurred apparently within 45 and 50 days after hatching. The trans­ formation of gonads from ovarian to testicular nature occurring in future males seemed to be completed rapidly, since no ovarian characteristics remained in the testes of the fish examined later than 60 days after hatching, though residual ovarian cavities were occasionally retained in the testis of adult males. Thus true gonochorism with a nearly 1:1 sex ratio was established following the intersexual stage in the Sumatra barb.