Effects of dietary methionine on growth and body composition, indicators of digestion, protein metabolism and immunity, and resistance to heat stress of abalone Haliotis discus hannai

The present study was conducted to investigate the effects of dietary methionine on growth performance, protein metabolism, immunity and resistance to heat stress of abalone Haliotis discus hannai. Seven isonitrogenous (about 32% of crude protein) and isolipidic (about 3% of crude lipid) diets were designed with graded levels of methionine (0.46%, 0.77%, 0.97%, 1.19%, 1.41%, 1.71% and 2.07%, respectively). The experimental diets were named as M1 (control), M2, M3, M4, M5, M6 and M7, respectively. An 84-day feeding trial was conducted in sea cages (40 cm × 35 cm × 70 cm) for abalones (initial body weight: 4.10 ± 0.01 g), which were fed once every two days. The results showed that the biomass gain (BG) of abalone significantly increased (from 560.20 g to 643.83 g) with the increasing dietary methionine levels from 0.46% to 1.19%, and then decreased (from 643.83 g to 484.29 g) (P < 0.05). Based on the broken-line regression analysis of BG, the requirement of dietary methionine for abalone was determined to be 0.99% (dry matter of diet). The highest trypsin activity (862.10 U/mg prot.) and α-Amylase activity (0.28 U/mg prot.) in the intestine of abalone were observed in the M4 and M5 group, respectively. In the digestive gland, the gene expression of mtor, s6k and eif4e showed a significant quadratic trend with the increasing dietary methionine levels, and all of them reached their highest values in the M4 group. The highest mRNA level of gcn2 was observed in the M1 group. The glutathione content and total anti-oxidative capacity in cell-free hemolymph (CFH) were significantly increased as dietary methionine levels increasing from 0.46% to 1.19%, and then decreased (P < 0.05). The lowest gene expression level of keap1, nf-κb and tnf-α, and the highest gene expression level of nrf2 and iκbα in the digestive gland were found in the M4 group. After the heat stress test at the end of feeding trial, the minimum falling rate (19.44%) and the highest gene expression level of hsp70 and hsp90 in the digestive gland of abalone were found in the group with 1.19% of dietary methionine. In conclusion, appropriate dietary methionine (0.97%–1.19%) could improve the growth, protein synthesis, immunity and resistance to heat stress of abalone.