Ammonia,toxic to aquaculture organisms,represents a potential problem in aquaculture systems,and the situation is exacerbated in closed and intensive shrimp farming operations,expecially for Litopenaeus vannamei.Assessing the potential for the genetic improvement of resistance to ammonia in L.vannamei requires knowledge of the genetic parameters of this trait.The heritability of resistance to ammonia was estimated using two descriptors in the present study:the survival time(ST) and the survival status at half lethal time(SS_(50)) for each individual under high ammonia challenge.The heritability of ST and SS_(50) were low(0.154 4±0.044 6 and 0.147 5±0.040 0,respectively),but they were both significantly different from zero(P<0.01).Moreover,these two estimates were basically the same and showed no significant differences from each other(P>0.05),suggesting that ST and SS_(50) could be used as suitable indicators for resistance to ammonia.There were also positive phenotypic and genetic correlation between resistance to ammonia and body weight,which means that resistance to ammonia can be enhanced by the improvement of husbandry practices that increase the body weight.The results from the present study suggest that the selection for higher body weight does not have any negative consequences for resistance to ammonia.In addition to quantitative genetics,tools from molecular genetics can be applied to selective breeding programs to improve the efficiency of selection for traits with low heritability.
