Objective This study aims to understand the growth, gonadal differentiation process and regulatory mechanism of juvenile Thamnaconus modestus under artificial breeding conditions.

Methods  After 10 days post-hatching, 30 fish were sampled every 5 days. The growth indices such as body mass, total length (TL), body length (BL), preanal length (PL), head length (HL), body height (BH), snout length (SL), and eye diameter (ED) were measured. Paraffin-embedded sections and hematoxylin-eosin staining were used to observe the development of gonads. At the same time, the samples were homogenized, and the changes in the content of 17β-estradiol (E2) and testosterone (T) were determined.

Results There were no significant differences in the growth indices such as body mass, TL, and BL of juvenile T. modestus among 10−20 days (P>0.05), while they increased significantly from 25 days (P<0.05). There was no significant difference in PL between 10 and 15 days (P>0.05), while it increased significantly from 20 days (P<0.05). There were no significant differences in HL, BH, SL and ED between 10 days and 20 days (P>0.05), but they began to increase significantly at 30 days (P<0.05). In addition, at 25−35 days, the undifferentiated gonads were observed to appear in pairs near the intestine and connect with the mesentery, and the blood vessels were observed in the gonads. At 60 days, oogonia or spermatogonia were observed in some juvenile T. modestus, indicating that the gonads had differentiated into females and males. In the process of sex transition to female, the E2 content was low, when the gonads were undifferentiated. However, the E2 content significantly increased after the finish of sex differentiation at 60 days (P<0.05). In the process of sex transition to male, the T content was lower when the gonads were undifferentiated and increased after the finish of sex differentiation at 60 days.

Conclusion These results suggested that the growth turning point of juvenile T. modestus may occur at 20−30 days, and the age of sex differentiation may be 35−60 days. Estrogen and testosterone may be the key factors that induced juvenile T. modestus to change their gender to female and male, respectively. This study clarifies the process of sex differentiation and the changes in hormone content during the differentiation process, which may provide the theoretical basis for the sex regulation and artificial reproduction in T. modestus.