Abstract:The deterioration of water quality from the intensive culture system and environmental pollution are common problems in the practical farming of Litopenaeus vannamei. The toxicity from deteriorating water, such as ammonia accumulation, has lethal effect on shrimp and can increase the susceptibility to pathogens, which might cause frequent high mortality in the period of early stage of L. vannamei cultured in the farms. The high heritability of acute ammonia stress in L. vannamei (0.575) indicated its ammonia tolerance could be improved largely by selection. However, the genetic gain of the disease resistance is very slow in shrimp because of the low heritability of disease resistance traits and low selection intensity by the limitation of pathogen infection test. To explore and improve the ability of disease resistance, we performed the WSSV infection test with ammonia-sensitive population (SP) and ammonia-tolerant population (TP) under ammonia stress (10 mg/L), named SPAV and TPAV, the normal ammonia level (<0.01 mg/L), named SPV and TPV, and no ammonia stress without WSSV infection as the control group (<0.01 mg/L), named SPC and TPC. The results showed that SP started to die at 5 h after WSSV infection, which was significantly earlier than that of TP (about 16 h). The mortality rate of SPAV increased rapidly after 60 h and reached 100% at 137 h, which was significantly higher than SPV (70.42%), TPAV (42.67%), and TPV (18.99%). The cumulative mortality rate of SPV was 73.67% at 144 h, which was significantly lower than SPAV, but significantly higher than TPAV (46.15%), TPV (18.99%) and SPC (34.79%). The cumulative mortality rate of TPAV at 144 h was significantly higher than TPV and TPC, but there was no significant difference between TPV and TPC. These results indicated that ammonia stress increases the susceptibility to pathogens, and that the population with high ammonia tolerance have high WSSV disease resistance, which will provide new ideas to improve the disease resistance and reduce mortality.
