Towards a Better Definition of "Metamorphosis" in Coregonus sp.: Biochemical, Histological, and Physiological Data

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Abstract
In Coregonus sp. the period between hatching and metamorphosis is characterized by an enhanced involvement of glycolysis in energy metabolism (as reflected by oxygen consumption and enzyme activities) and by the differentiation of the red and pink muscle fibers on which the increasing versatility of swimming performance of the larvae appears to depend. The larval weight of Coregonus sp. increased during the first 150 d of development at 10 °C from 6 to 2000 mg, but the average rate of oxygen consumption decreased only from 639 to 419 μg∙g−1∙h−1. Four types of muscle fibers were distinguished, each with a distinct developmental pattern: red and pink fibers first become observable 25–34 d after hatching, the latter growing more slowly than the former; before this, only white muscles and a characteristic layer of small diameter red fibers are present in Coregonus sp. The activities of the two oxidative enzymes, citrate synthase and cytochrome oxidase, and the activity of hexokinase, increased directly after hatching, reaching a peak within 20–47 d. On the other hand, the three glycolytic enzymes, phosphofructokinase, pyruvate kinase, and lactate dehydrogenase, remained at a constantly low level from hatching to day 40, whereafter their rates of activity began to increase rapidly.