Studies on the Structure and Function of the Digestive System in Teleost Larvae-II
Characteristics of the Digestive System in Larvae at the Stage of First Feeding
1969 Volume 16 Issue 2 Pages 41-49
Details
1969 Volume 16 Issue 2 Pages 41-49
The digestive organs of 11 marine and9 freshwater teleost larvae were compared one another and with those of adults to speculate their function at the transitional stage (from prelarvae to postlarvae). The digestive tract of larvae is divided into two major types: the straight- and convolute-type. The former is seen in the fishes referred to clupeichthyes and mesoichthyes and the latter in the fishes of percichthyes.
Generally, in parallel with the evolutional trend of fish, the digestive tract tends to become shorter with development of internal mucosal folds, external muscular coats and posterior valvular constriction . The taste buds are obscure and few in number in many marine fish larvae, but, they are well developed in freshwater fish larvae, especially in those having pharyngeal teeth at the prelarval stage. These rules are also applicable to the case of mucous cells in the pharynx and esophagus which produce PAS-positive materials at the present stage.
The larvae whose adults have true stomach possess a shorter and slightly distending part between the esophagus and intestine. This is the early appearance of the stomach without gastric gland and blind sac. No differentiated pyloric caeca are seen, except in Salmo gairdnerii. In spite of some exceptions, intestinal epithelium is exclusively composed of columnar absorptive cells, provided with well developed striated border. Goblet cells, if exist, are extremly few. Glycogen and fat scarcely ac-cumulate in the liver. The pancreatic tissues with eosinophile zymogen granules form the compact organs in all species examined, and hepatopancreas is not yet formed even in those having this peculiar structure in the adults. The fact that basic structure of digestive system in larvae at the stage of first feeding is common with that of stomachless fish suggests the similarity between them in function.
