Morphological, Histological and Histochemical Adaptations in Digestive Tubular Part, Concerning the Feeding Strategy of Skipjack Tuna (Katsuwonus pelamis) Inhabiting Abu Galum, Aqaba Gulf, Red Sea, Egypt

This work aimed to describe the morphological, histological, and histochemical adaptations in the tubular part of the digestive tube of skipjack tuna (Katsuwonus pelamis) according to its feeding and dietary preferences. Fish samples were gathered from Abu Galum, Aqaba Gulf, the Red Sea, Egypt, in May 2022. The morphological, histological, and histochemical structure of the oesophagus, stomach, and intestine of fish were photographed and described. The results showed that the esophagus of K. pelamis was described as a flexible tube with irregular mucosal folds, comprising multiple layers of mucus cells containing strong acid mucopolysaccharides (MPS) and weak sulphomucins. Submucosa consisted of fibrous connective tissue with variations in mesh size, and the muscularis layer was dense with circular muscle fibers. The stomach has a bag shape. It is separated into three regions cardiac, fundic, and pyloric. The cardiac region exhibited broad folds with dense stomach glands containing weakly acid MPS and sulphomucins. Its submucosa contained blood vessels dispersed in fibrous connective tissue, while the muscular layer comprised of inner circular and outer longitudinal muscle fibers. In the fundic region, numerous primary rugae were observed with abundant gastric glands. The pyloric region showed irregular mucosa without gastric glands. Its submucosa consisted of thicker areolar connective tissue. The intestine appeared as a small tube with folded duodenal mucosa containing deep crypts. Its epithelial mucosa consisted of columnar and mucus cells containing strong acid MPS and sulphomucins. The submucosa was thinner compared to other parts and contained fibrous connective tissue with lymphatic vessels and blood vessels. The muscular layer was dense, especially in ilial regions with long folds and abundant mucus glands. It was concluded that the tubular parts of digestive tube skipjack tuna were adapted to diet and food behaviors, as evidenced from the related functional mechanisms of fish digestive physiology.