Abstract
Relationships between the developmental process of the digestive system and changes in activities of digestive enzymes during larval and juvenile stage of Ayu, Plecoglossus altivelis, were investigated. Two important periods are recognized in the developmental process of the digestive system. One occurs in the late stage of prelarvae, when the basic structure is established, the other, in the transitional stage from larvae to juveniles, when gastric glands develop and pyloric caeca differentiate. The development of the digestive system to one adult-like in function is accompanied with changes in body form and depth. The activity of amylase was recognized in larvae measuring about 25mm after which it increased to a fixed level in the juvenile stage. Though pepsin-like enzyme is scarcely detected in larvae measuring about 25mm, it abruptly increased in activity in juveniles measuring 35mm in which gastric gland is fully developed functionally. On the other hand, trypsin-like enzyme hardly increased in activity during transition from larvae to juveniles. The increase in pepsin-like enzyme activity and stagnancy of trypsin-like enzyme activity indicate that the digestive mechanism of proteins converts to a higher developmental level during the transitional stage from larvae to juveniles.
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