Histological characterisation of gonadal sex differentiation in Malabar red snapper ( <scp> <i>Lutjanus malabaricus</i> </scp> ) for aquaculture advancement

The Malabar red snapper (Lutjanus malabaricus) is a high-value tropical marine species receiving growing attention for aquaculture development in Singapore and Southeast Asia. At present, seed production relies primarily on uncontrolled mass spawning in sea cages, a practice that lacks consistency, biosecurity and control of genetic contributions. A clear understanding of gonadal sex differentiation is essential for designing effective sex control strategies and establishing selective breeding programmes. However, the progression of gonadal development from initial differentiation through to sexual maturity in L. malabaricus remains poorly characterized. In this study, a comprehensive histological investigation of gonadal differentiation was conducted by tracking individuals from 1 to 445 days post-hatch (dph). Gonads first appeared as undifferentiated structures at 14 dph, located between the swim bladder and intestine. The onset of ovarian differentiation was observed at 77 dph, marked by ovarian cavity formation, while testicular differentiation commenced at 128 dph, indicated by the development of lobular testis structures. Oogenesis was initiated by 169 dph, marked by the first observation of oogonia, and progressed to the chromatin nucleolus oocyte stage by 249 dph. Spermatogenesis was first evident at 249 dph, based on the appearance of lobular organization and spermatogonia, and further progressed by 353 dph, when spermatocytes, spermatids and spermatozoa were observed. These observations establish the first developmental timeline of gonadal sex differentiation in L. malabaricus, identifying key morphological features that distinguish early-stage ovaries from testes. Importantly, our study also revealed a highly skewed sex ratio in a harvest-size cohort from a commercial farm, with females comprising over 85% of the population. Given the species is considered gonochoristic, this finding raises concerns about potential influences from environmental factors, hatchery practices or genetic bottlenecks, and highlights the need for further investigation under controlled conditions. Together, these findings provide essential biological benchmarks for identifying the timing of sex differentiation and suggest that the optimal windows for sex manipulation occur around 60 dph to promote testicular development and approximately 110 dph to promote ovarian development. This work lays the foundation for future sex control techniques and contributes valuable knowledge toward the development of selective breeding programmes and sustainable seed production in this emerging aquaculture species.