Abstract. Triploidy was induced in the zebrafish, Brachydanio rerio (Hamilton), by varying all possible combinations of the time after fertilization (AF) (1‐3min after insemination), temperature (36‐42°C) and shock duration (1‐7min). A thermal shock of 41°C for 4min, 2.5min AF ensured 100% triploidy and maximum (51%) survival. Induction of triploidy was confirmed by measurement of erythrocyte nuclear volume and chromosome counting. There was no significant difference in the growth rate of triploid and diploid fishes. All surviving triploids developed into males, and produced a few spermatozoa unable to fertilize normal eggs. A study on thermal and other characteristics required to ensure 100% triploidy rate in fish indicates that these characteristics are species specific.

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The effect of progestogens on nuclear or germinal vesicle dissolution (GVD) was tested in goldfish (Carassius auratus) follicle‐enclosed oocytes in vitro. The results indicated the following hierarchy of GVD activity: 17α20β dihydroxy progesterone (DHP) > 17α hydroxy progesterone (HP) > progesterone (P). Further studies with DHP indicated that nuclear migration (GVM) preceded GVD, and that no significant GV size changes (ie, diameter in animal‐vegetal pole axis) occurred during GVM. The in situ GV visualization technique used (ie, acetic acid yolk clearing with concurrent nuclear precipitation) was verified using standard paraffin sectioning procedures. The two techniques yielded highly correlated results and indicated that GVM is initiated between 12–18 hr, after DHP addition at 15°C. The goldfish oocyte in vitro system as described here is a useful model for the study of hormonally induced GVM and GVD.

Triploidy was induced in the fighting fish, Betta splendens Regan, by varying all possible combinations of temperature (37-4l'C). time after insemination (2-3 min) and shock duration (2-4 min). Heat shock at 39°C for 3 min duration initiated 2-5 min after insemination gave high frequencies of triploids (867o) as assessed from chromosome number and red blood cell nuclear volume. There was no significant difference in the growth rate of triploid and diploid fish. Gonadal development in both sexes was retarded in triploids at 5 months of age. Eggs fertilized with milt from triploids developed to gastrulation. Beyond gastrulation there was increasing mortality associated with abnormalities and none of them hatched. The display frequencies of air gulping, erection of operculum and fins, striking and biting, and undulating movements were fewer in triptoids compared to diploids. It appears that triploids are less aggressive than diploids. The aggressive behaviour of fighting fish may be related Io their reproductive activity.

The effects of two synthetic androgens, 17a-methyltestosterone (5-50mg/kg food), 19-nor-ethynyltestosterone (2-50 mg/kg food) and two natural androgens, 11 ketotestosterone (10-60mg/kg food) and androstenedione (20-100mg/kg food) were investigated in the fighting fish, Betta splendens Regan. Androgens were administered for 40 days from the first day of feeding. Masculinization occurred in 100% of individuals fed 8, 15, 60, 90 mg/kg food 19-nor-ET, 17a-MT, 11-KT and AT respectively. Mortality due to treatment of natural steroids was significantly less than that with the synthetic steroids. Sex-reversed males were sexually functional and their genotype was identified by progeny testing. Sex-reversed males produced 100% female monosex when mated with normal females, indicating that the mechanism of sex determination in this fish is homogametic female and heterogametic male.