Effects of ammonia nitrogen on early growth and survival of the ‘Zebra 2’ strain of the manila clam ( Ruditapes philippinarum )

Abstract

The ‘Zebra 2’ strain of the Manila clam (Ruditapes philippinarum) is a newly bred for aquaculture in the low temperature Bohai and Yellow seas in northern China. The goal of this study was to determine the tolerance of ‘Zebra 2’ clams at early developmental stages to total ammonia nitrogen (TAN) under acute stress and continuous subacute stress levels. ‘Zebra 2’ were cultured under TAN stress at water salinity of 27–28, temperature of 21–28°C and pH of 8.0–8.2. Survival rate and production of clams from the fertilized egg to juvenile stages were measured. The acute stress test revealed that the median effective concentrations of TAN for hatchability and deformity of fertilized eggs at 24 h were 7.08 and 15.12 mg/L, with the unionized ammonia (UIA) at 0.22 and 0.36 mg/L respectively. The 50% lethal concentrations for D-shaped larvae (shell length 101.50 μm), umbo larvae (179.67 μm) and juveniles (610.50 μm) at 96 h were 3.28, 48.90 and 57.57 mg/L, with UIA at 0.10, 2.08 and 2.99 mg/L respectively. These results suggested that the ‘Zebra 2’ clams hatching and D-shaped larvae stages were most sensitive to TAN and that the tolerance significantly increased after the umbo larval stage. In the continuous subacute stress test, ‘Zebra 2’ juveniles were observed under all the treatments after 53 days of exposure to TAN stress at 5.45 mg/L (0.23 mg/L UIA). The maximal acceptable toxicant concentration of UIA used to determine the mortality rate, yield and relative yield was ranged from 0.12 to 0.23 mg/L. These findings suggested that maintaining TAN <0.22 mg/L and UIA ≤0.021 mg/L at seawater pH ≤8.5, salinity ≥25 and water temperature ≤25°C would improve larval rearing of ‘Zebra 2’ clams.