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Estrogen levels in the gonads of marine bivalves, the Pacific oyster Crassostrea gigasand scallop Patinopecten yessoensiswere determined by high performance liquid chromatography (HPLC) using an electrochemical detector. Estrone (E1), estradiol-17β(E2), and a small amount of estriol (E3) were identified in the ovary, while only E2was found in the testis. The level of E2in the ovary was consistently higher than E1and it increased with sexual maturation. These results indicate that E2may play a role in the reproductive events of the oyster and scallop. In vitroexperiments demonstrated the presence of 17β-hydroxysteroid dehydrogenase (17β-HSD) in the ovaries of both bivalves. The activity of 17β-HSD in the ovary was lower in the postspawning stage than in the early differentiating stage. The evidence for the presence of aromatase activity in the scallop ovary was obtained by 3H-water assay. The immunoreactivity against 3β-hydroxysteroid dehydrogenase (3β-HSD), P450 aromatase and E2was detected in the cells along the outside of germinal acini of the scallop ovary. It is concluded that estrogens can be synthesized in the gonad, that their levels vary with the reproductive cycle, and that they have a role in the development of gametes.

Serotonin (5-hydroxytryptamine, 5-HT) is a major neurotransmitter that triggers the spawning of bivalve molluscs and oocyte germinal vesicle breakdown (GVBD) of oocytes. The present study employed [³H] 5-HT binding assay to characterize a putative receptor on oocyte membranes prepared from scallops and oysters. Results demonstrated a profile of hormone control of 5-HT receptor formation during oocyte growth. The dissociation constants for 5-HT on the surfaces of scallop and oyster oocytes were estimated to be 2.64 μM and 2.01 μM, respectively, and the maximal binding capacity of the oocyte surface was 152 pmol/mg protein in scallops and 529 pmol/mg protein in oysters. The inhibition of [³H] 5-HT bound by various agonists and antagonists provided pharmacological evidence for a 5-HT receptor in each oocyte, a mixed profile of 5-HT1/5-HT2 in scallops and a single profile of 5-HT1 in oyster. Formation of 5-HT receptor on the surface of the scallop oocyte was induced by estradiol-17β(E2), which was suggested as a primary promoter of the effect of E2 on sensitivity to 5-HT, resulting in the induction of spawning. This formation occurred during oocyte growth controlled by E2. J. Exp. Zool. 281:124–131, 1998. © 1998 Wiley-Liss, Inc.