Ontogeny of Gastrointestinal Tract in Hybrid Flounder Jasum, Paralichthys olivaceus x P-dentatus

The histological development of the digestive tract and associated organs of the hybrid flounder, Jasum (Paralichthys olivaceus xParalichthys dentatus), was studied from hatching through metamorphosis (46 d posthatch [dph]) using light microscopy. At hatching, the larvae possessed a primordial digestive tract comprised of three major sections (buccopharynx, esophagus, and intestine) which was not yet opened to the exterior. During the endogenous feeding period (0-4 dph), the larvae relied primarily upon the yolk and underwent dramatic changes in the digestive system. While exogenous feeding began at 4 dph (4.3 mm total length [TL]), the yolk was not completely absorbed until approximately 5 dph at which time the digestive tract consisted of buccopharynx, esophagus, stomach anlage, anterior intestine, posterior intestine, and rectum. Hepatic, pancreatic, and gall bladder tissues were identifiable at 2 dph, and all of these organs were functional by the exhaustion of endogenous nutrients (including oil globule) at 10 dph. Acidophilic supranuclear vacuoles were first observed in the posterior intestine at 6 dph and had almost disappeared by 19 dph. Coincident with the disappearance of the supranuclear vacuoles at 19 dph (6.6 mm TL), the pyloric ceca began to appear at the junction of the pyloric stomach and the anterior intestine. The glandular stomach was devoid of goblet cells throughout the course of this study, but the first gastric glands were observed at 34 dph (12.9 mm TL), and the stomach was fully developed by 46 dph (14.4 mm TL), coinciding with the completion of metamorphosis. This study has shown that larval Jasum have a similar developmental pattern as that of most marine flatfish, characterized by a relatively slow development of gastric digestion preceded by the earlier formation of pyloric ceca.