Scallops as an emerging platform toward functional reproductive endocrinology

Scallops have become one of the few molluscan systems in which reproductive endocrinology can be examined beyond descriptive molecular surveys. This review summarizes a series of studies that have progressively positioned scallops as an emerging experimental platform for advancing functional approaches in comparative reproductive endocrinology. Early work identified GnRH/CRZ-superfamily peptides and their receptors in scallops, showing that these ancient neuropeptide-GPCR modules are present in bivalves and may have multifunctional roles, but should not be interpreted as direct counterparts of the vertebrate hypothalamic-pituitary-gonadal axis. Molecular analyses of sterol-related and nuclear-receptor-related pathways, together with sex-identification markers, further suggest that scallops provide a useful system for examining candidate mechanisms associated with gametogenesis, sex differentiation, and the stable maintenance of sexual phenotypes, without assuming vertebrate-type sex steroid regulation. At the cellular level, studies of germ cells and gonadal somatic cells have revealed the importance of the gonadal niche, whereas scallop hemocytes have emerged as experimentally accessible primary cells for cellular-level functional assays, rather than as established reproductive endocrine target cells. A major technical breakthrough has been the discovery and application of a strong OsHV-1 promoter, which has enabled transient transgene expression in scallop cells. Combined with primary hemocyte culture, this system has supported subcellular localization assays, recombinant protein production, and functional assessment of genes of interest, thereby providing a complementary route to gene function analysis in bivalves where individual-based functional genetics remains difficult. Despite current limitations, including low transfection efficiency, interindividual variability, and the limited extrapolation of hemocyte-based phenotypes to gonadal or organism-level reproductive physiology, scallop cell-based platforms provide a promising foundation for hypothesis-driven comparative endocrinology. These advances illustrate how non-model molluscs can be developed from observational subjects into experimentally testable systems, offering new perspectives on endocrine evolution, reproductive regulation, and physiological integration in marine invertebrates.