The Physiology Of Digestion In Different Developmental Stages Of Turbot

Scophthalmus maximus, is a species commonly exploited in extensive aquaculture in Europe. At present, this species is also in large scale for aquaculture in northern China. However, as a new species fcr aquaculture in our country, the technique of breeding and the manufacture of formulated diets are still the two major problems until now. The aim of this work is to supply references for the improvement of formulated diets through studying digestive physiology in different developmental stages of turbot.Firstly, Histological changes of the digestive system were studied in turbot from hatching until 60 days after hatching. During the period of endogenous nutrition, the lengthening of the digestive tube and yolk sac absorption are the most conspicuous elements of development. The most noticeable events occurring from the day 4 to day 15 were the transformation of the epithelia type, the differentiation of the swim bladder and the epithelial cells, gut segmentation, increasing in mucosal folding and the growth of liver and pancreas. From day 15 to the end of the metamorphosis, the juvenile period began with the formation of gastric glands and the onset of gastric digestion. Two pyloric caeca were clearly visible by day 20. About 60 days after hatching, the morphology and function of digestive system are closed to the adultturbot.On the basis of histological observation, the activities of some digestive enzymes and alkaline phosphatase were investigated during the different developmental stages of the turbot. Alkaline protease activity was identified after hatching. Acid protease was not present until the appearance of gastric gland. Three extreme low protease activities were observed during the period of the first feeding, the complete absorption of yolk sac and the transition from larva to juvenile. The noticeable amylase activity was present during the early stage of larvae and decreased significantly after 5 days old, from then on, it always kept a low relative activity. The activity of lipase was very low before the completion of metamorphosis. Activity of alkaline phosphatase maintained a lesser increasing state from day 1 to day 50 but thereafter revealed a sharp increase. At the end of the metamorphosis, the activities of protease, lipase and alkaline phosphatase increased significantly, which indicate the transition fromjuvenile to young and the improvement of the digestive system of turbot.After entering the young stage, the digestive system is almost completed, there are several biotic and abiotic factors influence the levels of digestive enzymes. The objective of the present study was to examine the separate and combined effects of temperature, pH and salinity on the activities of protease and amylase. The protease activity found in the stomach showed an optimal pH of 2.2, whereas the optimal pH was in the alkaline range (9.5-10.0) in the intestinal extracts. But in hepatopancreas, the optimal pH was inclined to low alkaline (8.5). The optimal reaction temperatures of protease were all above 40 C in stomach, intestine and hepatopancreas. By increasing the temperature, a shift of the optimal pH to 2.6 was determined in stomach. On the contrary, in intestine, the optimal pH decreased when temperature increased. The effect was clearer in intestine than in stomach, which responsed the combined effect of pH and temperature. The effect of NaCl concentration on protein digestion differed between hepatopancreas and other two organs, which is inhibitory in the former. The main protease for protein digestion in turbot seemed to be pepsin. Moreover, the maximum protease activity in different segments of intestine existed in hindgut. The amylase activity of stomach and intestine showed their maximum at the value close to neutral pH (7.5). The optimum pH in hepatopancreas was inclined to low alkaline (8.0). The highest hydrolysis activity was all recorded at 35 癈 in stomach, intestine and hepatopancreas. By increasing the temperature, no displacement of the optimal pH was detected in gut segments. High salinit...