Do vertebrate-type steroids play a role in bivalve reproduction? A case study across two reproductive cycles of Magallana gigas

In bivalves, reproduction is regulated by a complex interaction between endogenous and environmental factors, including temperature and food availability. However, hormonal control of the reproductive cycle in bivalves is poorly understood. There is a general belief that vertebrate-type steroids (testosterone, progesterone and 17β-oestradiol) are involved in the reproductive cycle of bivalves, even though the necessary enzymes and receptors for such steroids are apparently lacking in the molluscan genome. To investigate the potential role of vertebrate-type steroids in reproduction, the current study characterized two consecutive reproductive cycles of the Pacific oyster ( Magallana gigas ), integrating biochemical composition, reproductive indices, environmental parameters, and steroid levels. Gonadal index (GI), gonadosomatic index, and condition index exhibited clear seasonal fluctuations. Protein, glycogen, and total lipid displayed distinct seasonal patterns, with protein exhibiting structural functions, whereas glycogen and total lipid acted as energy reserves supporting gametogenesis. Both progesterone (0.69-142.33 ng g -1 ) and testosterone progesterone (2.55-262 ng g -1 ) levels fluctuated during sampling; progesterone showed correlations with GI, temperature, and protein content, whereas testosterone was mainly associated with environmental factors. These patterns suggest that progesterone may reflect gonadal tissue development and energetic investment rather than acting as a classical sex steroid. The absence of sex-specific steroid patterns, together with the lack of classical steroid receptors in the molluscan genome, supports the hypothesis of an environmental origin and/or an eco-physiological rather than an endocrine role for vertebrate-type steroids in oysters. These findings demonstrate that hormonal studies in bivalves should integrate biochemical, reproductive, and environmental data to understand reproductive strategies.