The digestive organs of postlarvae in 13 teleost species chiefly reared in laboratory were compared with one another to make clear their developmental processes.Characters differentiated during postlarval stage are teeth on jaws, pharyngeal teeth, taste buds, hepatopancreas, goblet cells in intestinal epithelium, gastric gland, pyloric caeca, and others.The time when teeth on jaws and pharyngeal teeth are formed varies from species to species.In general the jaw teeth in the species with stomach differentiate and develop at about the time when the gastric gland differentiates, and the pharyngeal teeth appear earlier than the former except for the puffer.The development of the lower pharyngeal teeth in cyprinoids is followed by the enlargement of upper cornified portion, or the cornified pad.Taste buds, which have already differentiated during prelarval stage in many species with demersal eggs, are formed within a week after feeding in every species.Pancreatic tissues which have already dispersed at prelarval stage partly invade into the liver along the hepatic portal vein to form hepatopancreas during the middle period of the postlarval stage.
After the start of feeding two conspicuous features appear in the epithelium of the intestine.One is the appearance of many vacuoles in the antero-median part of intestine, and the other is acidophilic granules in the posterior part of intestine.The posterior constriction with valvular structure forms the border between the two parts of the intestine.Their histological features and relationships with feeding conditions carry conviction that they are both absorptive features, and the vacuoles are fat, and granules are protein absorbed.These points will be explained in detail in reports which will be published in the future.
Many larvae change their body forms, and their fins except the pectoral differentiate during the final period of postlarval stage, when the gastric gland appears in the mucosa of the stomach.At the first stage of the formation of the gland, cuboidal cells well stained with hematoxylin constitute glandular acinuses without ducts to lumen in the anterior part of stomach, and afterward vesicles with secretory granules increase to construct a few layers in the whole part of the stomach.This remarkable event is attended with appearance of teeth on jaws and goblet cells with PAS positive materials in the epithelium of the intestine.
The age when gastric gland differentiates varies from species to species, for example, 90 to 120 days after hatching in
Plecoglossus altivelis, 25 days in
Acanthopagrus schlegeli and 20 days in
Pagrus major. But the time corresponds with length of larval stage, and the relative differentiation time, which is nearly three fourths of larval period, hardly varies from species to species.This fact indicates that the differentiation of the gastric gland is strongly related to the transition from larvae to juveniles, which require more developing digestive mechanisms preparing for new foods in a new habitat.
Pyloric caeca appear following the completion of gastric gland.They are formed through projection of the intestinal wall at the most anterior part, so that their epithelium is identical with that of intestine.At the period when pyloric caeca differentiate, fishes are at the transitional stage from larvae to juveniles.At this time body forms are regulated to approach basic forms of their adults, and all fins are completed.
The digestive system of postlarvae is stomachless in structure, and may be at the undif-ferentiated state in function.Therefore it may be speculated that larvae have characteristic digestive mechanisms different from those of adults.
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