Evaluation of Genetic Parameters for Survival Traits of Litopenaeus vannamei Under Hypoxic Conditions

Litopenaeus vannamei, also known as the Pacific white shrimp or white foot shrimp, is a member of the Crustacea, Decapoda, Palaemonidae, and Litopenaeus families. This species is mainly distributed on the Mexican and Pacific coasts of South America. Owing to its excellent immune characteristics, L. vannamei is currently one of the most important economic shrimp species in China.In the aquaculture process and under natural environmental conditions, low dissolved oxygen (DO) condition or even hypoxia frequently occurs. Under high-density culture operations, acute hypoxia is one of the major factors affecting the survival rate of shrimp and the quality of aquaculture water, causing not only a large number of shrimp deaths in the short term but also water decay and fermentation, increased turbidity, and other phenomena. The genetic improvement of L. vannamei, as well as the screening and breeding of exceptionally tolerant species under low DO conditions, are crucial to the sustainable development of the shrimp industry. Global selective breeding efforts based on quantitative genetics to improve genetic progress in L. vannamei have been extensive, and the main targeted traits were growth, survival, disease resistance, and stress tolerance. Few studies have been conducted on the evaluation of genetic parameters and screening of strains for hypoxic tolerance traits in L. vannamei; in particular, the genetic parameters and cytological characteristics of key tissues of L. vannamei under short-term highly lethal DO levels (0.3–0.5 mg/L) have not been reported.In this study, we used two strains of L. vannamei, GK (a strain with high disease resistance) and K (a strain with fast growth characteristics), each with a total of 6 560 shrimp from 41 families, to count survival traits at highly lethal DO levels, analyze differences in hypoxia tolerance traits between strains and within families of the same strain, and evaluate genetic parameters. Individual gill, muscle, and hepatopancreas tissues from families with significantly different tolerance levels were histologically investigated and compared at the cellular level. The results showed that there was a significant difference (P < 0.05) in semi-lethal survival (SS50) between families within both the GK and K strains when overall semi-lethality was reached under hypoxic environmental conditions of 0.3 mg/L, with SS50 values of 49.30% for GK and 42.52% for K. The coefficients of variation for survival times of families within each strain were 60% and 45% for GK and K, respectively. Using the threshold trait animal model, the genetic parameters were estimated to be between 0.345±0.031 and 0.378±0.029 using the survival status of the individual at semi-lethal levels as the observed value (1 for survival and 0 for death), and the heritability was between 0.219±0.031 and 0.237±0.029 following transformation on a continuously varying scale of observed values, indicating a moderate level of heritability. Moreover, the gill, muscle, and hepatopancreas tissues of the hypoxia-tolerant and hypoxia-sensitive families in the GK strain exhibited varying degrees of damage and different degrees of variations in the physiological characteristics of resistance among different families. This may be related to the resistance tolerance strength of different GK families. In contrast to the gill, muscle, and hepatopancreas tissues of the control shrimp, the equivalent tissues of shrimp from tolerant and sensitive families showed different degrees of changes following stress. The gill lumen of the filaments of the experimental group from sensitive families increased, the number of blood cells increased, the structure of the epithelial layer was gradually destroyed or even disintegrated, the vacuolation of the hepatopancreas was severe, the lumen of the ducts became irregularly deformed, and the muscle tissue muscle bundle interval widened with a certain degree of deformation. These results suggested that there was abundant genetic variation in the tolerance traits of L. vannamei at highly lethal DO levels and that this species was amendable to selective breeding practices. This study provides a reference and basis for the selection and breeding of hypoxia-tolerant traits in L. vannamei.