EVALUATION OF MICRODIETS AND FROZEN COPEPODS ON DIGESTIVE ENZYME ACTIVITIES, INTESTINAL AND LIVER MICROSTRUCTURES OF LARGE YELLOW CROAKER (PSEUDOSCIAENA CROCEA R.) LARVAE

A feeding trial was conducted to evaluate effects of three newly developed microdiets (MDs: Diet 1-Diet 3), a mixed diet (Diet 4: Diet 3 combined with frozen copepods) and frozen copepods (Diet 5, control) on digestive enzymatic activities, intestine and liver microstructures of large yellow croaker (Pseudosciaena crocea R.) larvae, which had an initial mean body weight of (13.80 0.40) mg from 25 to 60 days after hatching (DAH). The results showed that trypsin activities exhibited no significant difference between fish fed with the MDs at 35 and 60 DAH, although those at 25 DAH showed lower trypsin activities, and there was no significant difference between fish fed with mixed diet and frozen copepods. No significant differences were observed between fish fed with the experimental diets except frozen copepods, the latter led to higher amylase activity at 35 DAH than at 25 and 60 DAH. As fish grew older, aminopeptidase N and alkaline phosphatase activities increased in fish fed with the experimental diets except for Diet 4 and Diet 5. No lipid droplets were observed in anterior-mediate intestinal mucosa in 35-day-old fish that were fed with the experimental diets with the exception of the Diet 4. Fish fed with the MDs presented deeper intestinal mucosa folds and supranuclear vacuoles in the rectum, while rectal folds from fish fed with the frozen copepods were nearly flat and lacked supranuclear vacuoles. Fish fed with the MDs displayed numerous lipid vacuoles in hepatocytes with nuclear migration. Liver from fish fed with the mixed diet had numerous lipid vacuoles in hepatocytes with nuclear migration at 35 DAH but such vacuoles decreased at 60 DAH. Liver from fish fed with the frozen copepods resulted in some intercellular spaces, collapsed cytoplasm and pycnotic nuclei of hepatocytes that were similar to starvation condition. These results indicated that suitable MDs could improve the development of digestive system of large yellow croaker larvae. Diet 1 can be used as basal formula for further research on nutrition and MD for large yellow croaker larvae because it was superior to Diets 2 and 3.