Histopathological Alterations in the Digestive System of Larval and Juvenile Japanese Flounder Paralichthys olivaceus Reared on Four Feeding Levels

The pathological effects of different feeding levels on the organ systems of larval and juvenile Japanese flounder Paralichthys olivaceus (also known as olive flounder) were studied sequentially from 3 to 45 d posthatch. The flounder were fed enriched live foods—the rotifer Brachionus plicatilis and brine shrimp in the genus Artemia–at four feeding levels (L1–L4). The respective food densities at L1, L3, and L4 were 0.5, 2.5, and 5.0 times that at L2. Level L2 is the one conventionally adopted by hatcheries. At that level, the initial density of rotifers is 0.5 individuals/mL of culture water; the density gradually increases to approximately 3.5 individuals/mL after metamorphosis. The initial density of brine shrimp at L2 is 0.1 individuals/mL, which gradually increases to 3 individuals/mL. Light and electron microscopy showed no evidence of viral, bacterial, parasitic, or mycotic invasions in developing Japanese flounder. However, pathological lesions were observed (including adhesions among the visceral organs themselves and between those organs and the peritoneal wall), along with blebbing and necrosis of gastric mucosal cells, vacuolar degeneration of gastric glands, and necrosis of enterocytes and the intestinal wall. In addition, atrophy and necrosis of pancreatic acinar cells, reduction of glycogen in hepatocytes, and hepatocytic necrosis were clearly found to increase in severity as the feeding level increased. There was considerable agreement between intestinal damage and mortalities at different feeding levels. No major changes were detected in the skin, gills, heart, kidney, spleen, trunk muscles, or central nervous system. These results suggest that lesions in the digestive organs are major causes of the higher mortality observed at higher feeding levels.