The extent of reduced fecundity in triploid shellfish exhibits significant variation and is primarily correlated with the alteration of physiological and metabolic responses. In this study, the reproductive performance of diploid and triploid Haliotis discus hannai was analyzed throughout the reproductive cycle, focusing on gonadal development, steroid hormone levels, and corresponding changes in the foot muscle-soft tissue index (FMSI). A targeted division criterion for assessing gonadal development in triploid abalone was established. The results demonstrated that the ability to produce functional gametes was maintained in triploid females, while in triploid males, spermatogenesis was interrupted at the spermatocyte stage. More specifically, triploid ovarian development was characterized by the appearance of massive oogonia in the follicles, whereas apoptotic or autophagic cells were observed in the developing triploid testis. The estradiol and testosterone levels in triploid gonads dropped significantly at the stagnant stages; hence, insufficient steroid levels may be related to reproductive impairment.
Additionally, there was a significant decrease in gonadal fullness in triploids during the maturation season, as indicated by the gonad bulk index (GBIn ). The relationships between GBIn and FMSI, as well as their impact on gender, further suggest that gonadal development negatively affects FMSI, particularly since female triploids exhibit decreased gonadal fullness and increased FMSI. These findings have clarified the disruption of gonadal development and associated alterations in meat yield and physiological responses in triploid H. discus hannai, and thus are relevant to the development of breeding strategies for triploid abalone.