Abstract:Seriola fish are long-distance migratory oceanic species inhabiting temperate and subtropical waters worldwide. Nine species have been identified in the genus Seriola, of which three species, namely S. lalandi, S. dumerili, and S. quinqueradiata, are found in the coastal waters of China. Seriola fish are large and fast-growing, and their flesh is highly favored by international consumers owing to its excellent taste, nutritional quality, and economic value. Moreover, Seriola fish are promising candidates for different culture modes, including deep-sea cages and recirculatory aquaculture systems. Since 2017, China has witnessed a great breakthrough in the seedling production of S. lalandi. Subsequently, in 2020, we achieved a breakthrough in the seedling production of S. quinqueradiata. The Seriola fish farming industry was developed in Liaoning, Fujian, and Shandong provinces, with the annual farming yield reaching approximately 20 000 tons. At present, ice trash is the primary feed for Seriola. However, excess trash fish feeding can produce a severe environmental pressure on coastal waters, which is not beneficial for sustainable development of the Seriola fish farming industry. Thus, there is an urgent need to develop specific commercial feed to solve bottleneck problems, including culture inefficiency and environmental pressure. The key to developing a specific commercial feed is to understand the digestive system structure and nutritional physiological characteristics of fish. Therefore, the distribution patterns of digestive enzyme activities in Seriola species warrant immediate attention. In December 2020, 3-year-old S. dumerili, S. lalandi, and S. quinqueradiata were collected from a cage culture base in Ningde, Fujian Province. The experimental fish were fed ice trash twice daily. Fish were healthy and vigorous. The total length and body weight of four fish of each species were measured, and their visceral structures were observed and recorded. The average length and weight were respectively (62.380±0.805) cm and (3.306±0.208) kg in S. dumerili; respectively (65.400±0.351) cm and (2.906±0.082) kg in S. lalandi; and respectively (74.640±1.041) cm and (4.622±0.258) kg in S. quinqueradiata. The stomach, intestines (to remove intestinal contents), and liver were collected, and the intestine was divided into the foregut, midgut, and hindgut according to the physiological curvature. Enzyme activity kits were purchased from Nanjing Jiancheng Reagent Company. The activities and distribution of five enzymes (trypsin, lipase, amylase, alkaline phosphatase, and acid phosphatase) in the digestive system (stomach, pyloric caeca, foregut, midgut, hindgut, and liver) of the three Seriola species studied were compared. In all Seriola species, trypsin activity was mainly distributed in the pyloric caeca and liver, α-amylase activity was mainly distributed in the pyloric caeca and midgut, lipase activity was evenly distributed in all tissues, alkaline phosphatase activity was mainly distributed in the pyloric caeca and intestinal tract, and acid phosphatase activity was mainly distributed in the pyloric caeca. Compared with the other two species, S. dumerili exhibited significantly higher trypsin activity in the liver; significantly lower α-amylase activity in the stomach, pyloric caeca, foregut, and hindgut; significantly lower lipase activity in the stomach, pyloric caeca, foregut, hindgut, and liver; significantly lower alkaline phosphatase activity in the stomach, pyloric caeca, foregut, midgut, and hindgut; and significantly lower acid phosphatase activity in the pyloric caeca, foregut, and midgut. Moreover, compared with the other two species, S. lalandi exhibited significantly higher trypsin and α-amylase activities in the pyloric caeca and foregut; significantly higher lipase activity in the stomach, pyloric caeca, and midgut; significantly higher alkaline phosphatase activity in the pyloric caeca and hindgut; and significantly higher acid phosphatase activity in the pyloric caeca. Furthermore, compared with the other two species, S. quinqueradiata exhibited significantly higher α-amylase activity in the stomach, midgut, and hindgut; significantly higher lipase activity in the foregut, hindgut, and liver; significantly higher alkaline phosphatase activity in the foregut and liver; and significantly higher acid phosphatase activity in the hindgut. In conclusion, the results of the present study indicate that the activities of the five enzymes studied show similar distribution patterns in the digestive system of the three Seriola species, with the pyloric caeca being the primary target organ for digestive enzymes. Comparative analysis revealed that the activities of all digestive enzymes, except trypsin, were significantly lower in S. dumerili; the activities of digestive enzymes in the pyloric caeca and intestines were significantly higher in S. lalandi; and the activities digestive enzyme in the pyloric caeca of S. quinqueradiata were significantly lower. These results may provide a theoretical basis for revealing the digestive physiological characteristics of Seriola species and developing highly efficient specific compound feed suitable for their digestion and growth.
