Comparative evaluation on the effects of dietary docosahexaenoic acid on growth performance, fatty acid profile and lipid metabolism in two sizes of abalone Haliotis discus hannai Ino

A 93-day feeding trial was conducted to comparatively evaluate the effects of graded levels of dietary docosahexaenoic acid (DHA) (0.06%, 0.46%, 0.99%, 1.57%, 1.95% and 2.25%) on growth performance, fatty acid profile and lipid metabolism in two different sizes of abalone Haliotis discus hannai (initial body weight: 2.83 +/- 0.03 g and 26.81 +/- 0.17 g, respectively). Results showed that 0.99%-1.95% of dietary DHA contents significantly increased the weight gain rate (WGR) in large size of abalone (P < 0.05), while the WGR in small size of abalone was significantly increased by the 0.46%-0.99% of dietary DHA contents (P < 0.05). There was no significant difference in the survival of abalone among all the six groups (P > 0.05). Based on the WGR, the optimal dietary DHA content for large and small size abalone were estimated to be 1.32% and 0.88%, respectively. Dietary DHA supplementation significantly increased the activities of trypsin and lipase in the intestine, the content of n-ary sumation n-3 polyunsaturated fatty acids (PUFA) and n-ary sumation n-3/n-6 PUFA ratio in digestive gland in both sizes of abalone (P < 0.05). The total glyceride concentration in the cell-free hemolymph in two sizes of abalone was decreased. In large size of abalone, 1.95%-2.25% of dietary DHA reduced the lipid content in digestive gland. Lipogenic genes in the digestive gland, including sterol regulatory element binding protein-1 (srebp-1) and diacylglycerol acyltransferase, were up-regulated in the 0.46%-1.57% DHA group (P > 0.05). The mRNA expression levels of hormone sensitive lipase (hsl) and carnitine palmitoyl transferase-1 (cpt-1) of lipolytic genes in the digestive gland were significantly increased in the 0.46% and 0.99% DHA groups, respectively (P < 0.05). In the small size of abalone, the lipid content in digestive gland and soft body decreased significantly in the 1.95% and 2.25% DHA groups, respectively (P < 0.05). Lipogenic genes in the digestive gland, including srebp-1 and acetyl-CoA carboxylase, were significantly down-regulated in the 0.99%-1.95% DHA group (P < 0.05). Lipolytic genes in the digestive gland, including hsl and cpt-1, were not significantly different from the control group (P > 0.05). In conclusion, based on the WGR, the dietary DHA requirements for large and small size of abalone were estimated to be 1.32% and 0.88%, respectively. Dietary DHA reduced lipid deposition in both sizes of abalone by promoting lipid decomposition and inhibiting lipid synthesis.