Ontogeny of the digestive tract in yellow catfish Pelteobagrus fulvidraco larvae

The study on histological and ultrastructural characteristics of the digestive tract of yellow catfish (Pelteobagrus fulvidraco) was carried out from hatching (0 day after hatching, DAH) until 35 DAH. Larvae for this study were maintained in the laboratory conditions (water temperature ranged from 23 °C to 25 °C). They were fed with zooplankton from 3 DAH to 17 DAH, with zoobenthos added from 10 DAH, and only zoobenthos from 18 DAH to 35 DAH. Development of the digestive tract in P. fulvidraco followed the general pattern described for other fish species with some peculiar findings. At hatching, it consisted of an undifferentiated straight tube laying over the yolk sac. The digestive tract was differentiated into buccopharynx, esophagus, primary stomach and intestine by 2 DAH. The liver and pancreas also appeared at this time. The intestine became differentiated into anterior and posterior regions separated by the intestine bend at 3 DAH. Gastric gland appeared in cardiac stomach at 3 DAH, the earliest appearance time among fishes studied to date. Oxynticopeptic cell contained pepsinogenic granules and abundant tubulovesicular systems at 3 DAH. As larvae grew, more pepsinogenic granules but less tubulovesicular systems were found in oxynticopeptic cell. The abundant visible tubulovesicular systems suggested that oxynticopeptic cell was still in rest phase with little hydrogen chloride (HCl) secreted at the first appearance time. The ultrastructure of oxynticopeptic cell indicated the asynchronous development of acid-secreting and pepsinogen-secreting function. The epithelial absorptive cell of the anterior and posterior intestinal segments showed electron-opaque lipid droplets and heavy pinocytosis, respectively at 3 DAH. Heavy pinocytosis could be observed in the posterior intestine until 25 DAH. Lipid vacuole accumulation appeared in liver at 13 DAH, the same time as the storage of abundant glycogen. These results suggested that the development of the digestive tract of P. fulvidraco larvae was functional rapidly, however it was still incomplete at 3 DAH. The functions of digestive tract and accessory glands were developed gradually until 25 DAH.