The Pacific oyster (Magallana gigas) is the most cultured bivalve in the world. In Mexico, many mollusk hatcheries from Baja California, produce oyster seeds to supply producers. Broodstock conditioning is necessary to obtain gametes and produce seeds. However, most of the conditioning is performed in natural conditions. Consequently, the maturation of broodstock can be variable. Recently, in our laboratory, Recirculating Aquaculture System (RAS) with control of the CO2-carbonate system has been designed to enhance oyster broodstock maturation. However, we observed that oysters did not mature and had been infested by Polidorids (shell-boring polychaete parasites). Thus, this work aimed to evaluate the effect of Polidorids and the CO2-carbonate system on the relative gene expression of Pacific oyster broodstock conditioned in RAS.
The experiment was performed in triplicate using two types of systems. Control systems consisted of RAS without control of the CO2-carbonate system (RAS-C) and the experimental with control of the CO2-carbonate system (RAS-R). Oysters were conditioned at 12°C for 45 days. Then, the water temperature increased to 18°C (60 days). After that, a second period continues for 70 days in which water temperature changes from 18°C to 20, then to 22, ending at 24°C. Gonad and mantle edge tissue was obtained at 18°C and 24°C. The parasite prevalence was determined by visualizing mud blisters in the inner oyster shell. The relative gene expression was determined by qRT-PCR; reproduction genes in the gonad (ATPhe, Gprot, SP1b); innate immune (MAPK, PRPL1, TLR2), and biomineralization genes (Tyr, VpATP) in the mantle.
Our results showed that at 18°C, oysters kept in RAS-C with Polidorids repressed reproduction effort, evidenced by less gene expression of signal and proliferation of germ cells (ATPhe, SP1b) and receptor of GnRH (Gprot) (Fig. 1A). Also, Polidorids promote change in the energy balance from reproduction to immune and shell protection, because of increased gene expression of innate immune wide recognition (TLR2), inflammatory response (MAPK), production of the shell matrix protein Tyrosinase and active transport H+ (VpATP) in the mantle. At 24°C, oysters in RAS-C had higher expression of signal and proliferation of germ cells than RAS-R, regardless of the presence or absence of Polidorids (Fig. 1B). In contrast, oysters in RAS-R reduced the proliferation of germ cells and increased innate immune wide protection. In conclusion, Polidorids affected the reproduction performance of oysters conditioned in RAS. Nevertheless, oysters with Polidorids in RAS-C changed the energy balance at 18°C allowing them to reach the same reproduction performance as those without Polidorids at 24°C.