ABSTRACT
The gastrointestinal tract consists of the pharynx, esophagus, stomach, and intestine that ends at the anus. The differences in the structures among fish gastrointestinal tracts are related to feeding habits, food, age, body shape, and weight. The esophagus of many teleosts is divided into anterior and posterior parts However, taste buds are present in the esophagus of some species. The stomach of fish possesses different configurations according to the feeding habits, while juveniles and adults of a variety of fishes are stomachless. The stomach of catfish is divided into three regions according to structure: cardiac, fundic, and pyloric regions. The cardiac mucosa is characterized by a large number of well-defined long folds, occupy by closely packed simple branched tubular gastric glands. The fundic region is characterized by a great number of simple branched tubular gastric glands composed of oxyntico-peptic cells that contain a tubulovesicular network. The pyloric region is characterized by the absence of gastric glands. The absence of the true stomach in carp is replaced by a simple dilatation at the anterior part of the intestine called the intestinal bulb. The mucosa of the bulb shows numerous folds, arranged in zigzagging-like patterns. The surface epithelium consists of simple columnar, enteroendocrine, and goblet cells (GCs). No multicellular glands of gastric or intestinal-type are found in their lamina propria. The intestine of fish is divided into three portions according to the thickness of the wall, length of mucosal folds, and thickness of muscularis; anterior, middle (posterior), and rectum. The surface epithelium of the three regions consists of enterocytes covered with microvilli with numerous GCs.
The gastrointestinal tract consists of the pharynx (ph), esophagus, stomach, and intestine that ends at the anus. Food enters a short, often greatly distensible esophagus leading to a thick-walled stomach. The histological structure of the gastrointestinal tract of numerous fish species generally consists of tunica mucosa, submucosa, muscularis, 138and serosa (SE). The differences in histological structures among fish gastrointestinal tracts are related to feeding habits, food, age, body shape, and weight.
The gastrointestinal tract of carnivorous fish as Nile catfish occupies a small part of the abdominal cavity, and it consists of the oesophagus, stomach, and intestine. Its mean length is 38.20 ± 3.90 cm. While the gastrointestinal tract of herbivorous fish, such as grass carp is coiling repeatedly and occupies a greater part of the abdominal cavity, and it consists of the oesophagus, intestinal bulb, and intestine. The gastrointestinal tract of grass carp shows a highly significant increase in length compared with that of catfish and its mean length is 71.50 ± 4.80 cm. The relative gut length (RGL) (%) that represents the length of the digestive tract to standard body length is short in catfish, approximately 1.07%, while that of grass carp is 1.92% (Figures 8.1 and 8.2).
