Annual variation in gonadal parameters and the effects of estradiol (E2) on ovarian development in the thick-shelled mussel, Mytilus coruscus

The thick-shelled muscle, Mytilus coruscus , is an important species targeted in aquaculture that has significant market potential but faces many challenges related to breeding. Although it is known that the steroid hormone estradiol (E2) regulates reproductive processes in mollusks, its effects on M. coruscus are not well understood. In this study, individuals representing four stages of gonadal development were collected from the Shengsi Sea area and were subjected to dissection followed by tissue sectioning. In addition, flesh-to-shell ratio analysis was performed for the proliferation (September), growth (December), spawning (March), and resting (June) stages. In addition, the effects of five E2 concentrations (0, 10, 100, 500, and 1000 μg/L) on female gonadal development were examined, revealing that a concentration of 100 μg/L E2 significantly increased oocyte diameter, promoted ovulation, and reduced the malformation rate of D-larvae (9.4 %). Transcriptomic analysis following 100 μg/L E2 exposure identified 5447, 4548, 30, and 43 differentially expressed genes (DEGs) across four comparison groups. Among these, zinc finger protein 76, CD163 antigen, chloride channel (CLCC1), Notch 2, guanine nucleotide-binding protein G, and Hyaluronan binding protein 2 were significantly differentially expressed between control females ( CF ) and E2-induced females (EF). Additionally, 2287 DEGs were identified as E2-responsive in females. KEGG enrichment analysis of these DEGs revealed “RNA degradation,” “Autophagy–animal,” and “Basal transcription factors” as the most enriched pathways. Expression of the Foxl2 gene, known to be associated with ovarian development, was upregulated by E2 but undetectable in testes, suggesting that E2 may suppress its expression in males. These findings provide insights into the gonadal development of M. coruscus in the Shengsi Sea and clarify the role of E2 in ovarian regulation, offering a foundation for artificial breeding and further studies on the underlying molecular mechanisms. • The annual gonadal variations of M. coruscus from Shengsi Sea area have been characterized. • 100 μg/L E2 enhances oocyte diameter and ovulation quality . • 2287 DEGs were identified as female-responsive to E2 genes. • Foxl2 is linked to female gonad development but absent in males.