Effects of dietary EPA/DHA ratio on the growth performance, intestinal microbiota, immunity and resistance to heat stress in abalone Haliotis discus hannai Ino

The effects of different dietary EPA/DHA ratios (2.19, 1.59, 0.98, 0.55 and 0.38, respectively) on the growth performance, intestinal microbiota, immunity and resistance to heat stress in abalone Haliotis discus hannai Ino was evaluated after a 93-day feeding trial. Five isonitrogenous and isolipic (about 33% crude protein and 3% crude lipid) practical diets were formulated to fed abalones (initial body weight: 25.82 +/- 0.51 g; initial mean shell length: 59.57 +/- 0.40 mm). Results showed that 0.98-2.19 dietary EPA/DHA ratio significantly increased abalone's weight gain rate (WGR) and daily increment in shell length (DISL). As for the intestine enzyme activity, trypsin activity was significantly higher when 0.55 EPA/DHA ratio was applied to the diet. The lipase activity in the groups with 0.98 and 0.55 dietary EPA/DHA ratio was significantly higher than those in other groups. And 1.59 dietary EPA/DHA ratio significantly increased alpha-amylase activity. In regard to the intestinal microbiota, the 0.98 dietary EPA/DHA ratio significantly increased the diversity indices and number of OTUs in the intestine, followed by the 2.19 dietary EPA/DHA ratio. At the phyla level, Firmicutes, Proteobacteria, Bacterioidota and Actinobacteria were dominant in all groups. The Mycoplasma was the most prevalent genus among all the groups. The highest abundance of Clostridium_sensu_stricto_11, Mycobacterium and Bacteroides belonged to the group with the 0.38 dietary EPA/DHA ratio. The abundance of Bacilli, Terasakiellacease and Ahrensia in 0.38 group were higher than that in the group with 0.98 and 2.19 dietary EPA/DHA ratio. Moreover, 1.59 dietary EPA/DHA ratio significantly reduced the falling rate of abalone after heat stress. And the hsp70 and hsp90 mRNA expression levels in the digestive gland were up-regulated significantly in the group with the 2.19 dietary EPA/DHA ratio. The 0.98 dietary EPA/DHA ratio could increase the content of malondialdehyde in cell-free hemolymph and the expressions of tnfa and nrf2 in digestive gland. In conclusion, the optimal dietary EPA/DHA ratio for abalone was estimated to be 1.63 using a second-order quadratic regression based on WGR. The 1.59-2.19 dietary EPA/DHA ratio could increase the immunity and resistance of abalone against heat stress.