Enhanced ammonia tolerance at higher salinity reflects metabolic adjustments in Litopenaeus vannamei postlarvae

Abstract

Ammonia, a major by‐product of protein catabolism in shrimp, can rapidly accumulate in high‐density culture systems, particularly under limited water exchange. It is a common toxicant and a key limiting factor in intensive aquaculture, entering through the gills and causing metabolic disruption, reduced growth, and increased mortality. In this study, ammonia tolerance of Pacific white shrimp, Litopenaeus vannamei postlarvae (PL) (45 ± 02 mg) was investigated by a series of acute toxicity experiments at eight salinity levels (0, 1, 5, 10, 15, 20, 25, 30, and 35 ppt) at pH 8.0 and temperature of 28°C. Oxygen consumption rates (OCR) were also measured to determine the Routine metabolic rate (RMR) of the PLs at different salinity (1, 5, 15, 25, and 35 ppt) and total ammonia nitrogen (TAN) (0, 5, 10, 20, and 40 mg/L) combinations. The tolerance of the PLs to TAN and unionized ammonia (NH ₃ ) increased significantly with increasing salinity levels. The safe level of ammonia for the PLs was changed from 0.24 to 1.70 mg/L and 0.016 to 0.110 mg/L and 0.4, 1.0, and 1.6 mg/L for TAN and NH ₃ at the salinity levels mentioned above, respectively. At any, each salinity level, the RMR of the PLs increased with increasing TAN concentration ( p  < 0.01) and were 98.2, 115.1, 126.4, 134.9, and 136.2 μg O ₂ mg ⁻¹  h ⁻¹ at 1, 5, 15, 25, and 35 ppt, respectively, at 0 mg/L TAN concentration ( p  < 0.01). The results indicate that salinity, ammonia concentration, and exposure time interactively affect the survival and physiological responses of Litopenaeus vannamei PLs. Salinity was a key factor, with higher levels enhancing ammonia tolerance, while low salinity likely increased sensitivity due to osmotic stress and greater ammonia uptake.