Relationship between quality of pearl cultured in the triangle mussel Hyriopsis cumingii of different ages and its immune mechanism

Hyriopsis cumingii is endemic to China and is the most productive freshwater pearl mussel. The age-related changes in the density, phagocytic activity of hemocytes, and the antibacterial and lysozyme activities of plasma were studied respectively in the triangle mussel H. cumingii. The density of hemocytes increased with an increase in the age (mean density, 2.09 × 106 cells/mL). The hemocyte-mediated phagocytic activity in 4- and 5-year-old mussels was significantly higher than that in 2-year-old mussels. The plasma of 2–5-year-old triangle mussels has been found to possess significant antibacterial activity against Micrococcus lysodeikticus, Staphylococcus aureus, Escherichia coli, and Aeromonas hydrophila. Further, the antibacterial activity in 3–4-year-old mussels was much stronger than that in 2- or 5-year-old mussels. A significant enhancement in the plasma lysozyme activity in mussels was noted along with an increase in mussel age. In conclusion, the quality of the cultured pearl was related to the immunological activities of hemocytes or plasma in triangle mussels.