DEVELOPMENT OF THE DIGESTIVE-TRACT IN LARVAL SUMMER FLOUNDER

Histological changes of the digestive system and its associated glands, and structures of the jaw were studied in summer flounder Paralichthys dentatus from hatching (day 0) until day 44. Specimens for this study were hatched from artificially spawned broodstock and maintained in the laboratory (20 +/- 1 degrees C). During the first 3 days after hatching, the formation of the oral jaw apparatus, lengthening of the digestive tube, yolk resorption, and mucosae differentiation are the most conspicuous elements of development. The larval digestive system is morphologically ready to process external food at the time of mouth opening (3-4 days after hatching). Epithelial cells of the anteromedian and the posterior intestine show evidence of lipid and protein absorption, respectively, after first feeding. The most noticeable events occurring during the next month of independent life are an increase in mucosal folding, cellular differentiation in the luminal epithelia, gut segmentation and looping, and liver growth. Gastric glands and pyloric caeca appear by day 31 and complete the morphological digestive features characteristic of the juvenile stage. (C) 1995 The Fisheries Society of the British Isles