Relationship between seawater temperature, brain GnRH-like peptide expression, and gonadal development in wild bigfin reef squid (Sepioteuthis lessoniana)

Abstract Background Cephalopods are widely distributed in oceans worldwide and are important fishery resources. Most species have a lifespan of approximately one year and die after reproduction. In cephalopods, gonadal development may be influenced by seawater temperature; however, the endocrine mechanisms underlying reproductive maturity remain unclear. In recent years, gonadotropin-releasing hormone (GnRH)-like peptide has been identified in invertebrates, including cephalopods, as a possible endocrine regulator, similar to their role in vertebrates. Nevertheless, knowledge of its specific functions in cephalopod reproduction remains limited. This study aimed to clarify the involvement of the endogenous peptide in gonadal development in cephalopods in the bigfin reef squid ( Sepioteuthis lessoniana ). We performed histological observations of gonadal maturation and analyzed brain expression levels and localization of the peptide throughout sexual maturation. Additionally, we examined the relationship between annual gonadal maturation and the seawater temperature cycle. Results We identified coding sequences for peptides with conserved functional regions similar to those of other mollusks. Quantitative analysis revealed that brain expression significantly increased during the spermatid stage of testicular development, whereas no association with ovarian development was observed. Immunoreactivity was primarily localized in the optic lobe and around the optic gland, a central site of reproductive regulation in cephalopods. Although ovarian development progressed with increasing seawater temperature, testicular development showed no clear association with the temperature cycle. Conclusions These findings suggest that GnRH-like peptides may contribute to early testicular development in S. lessoniana through optic gland signaling or direct neural pathways. In contrast, ovarian maturation appears to be strongly influenced by seawater temperature. This study provides foundational insight into the reproductive physiology of cephalopods and highlights regulatory mechanisms governing male and female gonadal development.