The Pacific oyster , Magallana gigas (formerly Crassostrea gigas), is one of the most cultured bivalves in Mexico. Nevertheless, the Pacific oyster production has a significant challenge: continuous seed production to meet the current demand. For seed production, the hatcheries frequently use broodstock from the farming zones exposed to natural conditions that cannot be controlled, like those related to the oysters’ reproductive cycle or pollutants and parasites as Polydora sp. This has resulted in a constant variation of oysters’ condition and unpredictable time required to produce mature organisms. For more accurate control of the factors involved in the reproductive cycle of oysters , the use of recirculating aquaculture systems (RAS) may be significant. Little information on the reproductive cycle on bivalves cultured in RAS has been registered, especially the consideration of other interactions as the CO2-carbonate system (pH, alkalinity, CO2, carbonates, bicarbonates, calcite, and aragonite) and the water quality (temperature, salinity, nitrogen compounds, and dissolved oxygen). Thus, this work aimed to characterize and evaluate the effect of the water quality and the CO2 -carbonate system and Polydora sp. on the reproductive cycle of M. gigas cultured in a RAS. The broodstock was conditioning in four different temperatures (18°C, 20°C, 22°C, and 24°C ) for ten weeks. After the ten weeks of conditioning, the water quality factors and the CO2 -carbonate system were within the ranges reported as favorable for bivalves. However, no mature organism was observed, and high variability in the condition of the oysters was detected (Fig. 1.) . The results suggest a negative effect of the Polydora sp. on the reproductive cycle of M. gigas cultured in a RAS (Fig. 2).