Induces oocyte maturation and ovulation in vitro in ovarian fragments and induces spawning behavior and gamete release in vivo (PubMed:19470645). Probably mediates its effects by binding to a G-protein coupled receptor located in follicle cell membranes (PubMed:21295575, PubMed:21967225, PubMed:24929230). Following receptor binding, stimulates GTP-dependent adenylyl cyclase activity in follicle cell membranes of fully grown ovaries at stage V but not in young ovaries at stage IV, leading to the production of cAMP (PubMed:21967225, PubMed:22134181). This stimulates the production of the maturation-inducing hormone, 1-methyladenine, in stage V but not stage IV ovaries (PubMed:19470645, PubMed:21295575, PubMed:22134181). The lack of activity in stage IV ovaries may be due to the very low levels of the G-protein alpha s subunit which is not expressed at high levels in follicle cells until stage V (PubMed:24929230). Heterodimer of a B chain and an A chain linked by two disulfide bonds. Expressed at high levels in the radial nerves (PubMed:19470645, PubMed:26166482). In the radial nerves, detected in the peripheral area proximal to the tube feet but not at the side of the hemal sinus (PubMed:19470645). Expressed at low levels in cardiac stomach and pyloric caeca with trace amounts in the ovary, testis and tube feet (PubMed:26166482). Belongs to the insulin family.