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The activity of hydrolases (maltase, saccharase, amylolytic activity) in the intestinal mucosa of the plankton-feeding zope Ballerus ballerus and the benthos-feeding white-eye bream Ballerus sapa was investigated. The temperature characteristics of maltase hydrolysis (T(opt) and E(act)) are similar in both species. The lower K(m) of maltase hydrolysis in the white-eye bream reflects a higher enzyme/substrate affinity and indicates a more effective carbohydrate hydrolysis in the benthos-versus plankton-feeding species. The glycosidase activity in the white-eye bream is twice as high as in the zope. This may be due not only to different feeding spectra and biochemical food contents but also to the differences in thyroid status of species under consideration.

Fish scale cover forms in late ontogenesis. Therefore, the conditions of development significantly affect its characteristics (number of scales). This study is aimed at considering the influence of external and internal factors on variation of the number of scales in fish. Acceleration of development results in decrease of the number of scales, while it increases with deceleration. Experiments on regulation of thyroid status of fish showed that the certain mechanism of alteration of the number of scales is related with heterochrony, such as a shift of the laying period of scale cover. Accelerated development is caused by early scale development at smaller body length, while decelerated development is characterized with later scale development and greater body length. Data considering heterochrony as the possible reason for deviations in the number of scales in related fish species are represented. Moreover, alterations of the distance between scales (morphogenetic calculation) can serve as another alteration mechanism of the number of scales in fish (especially phyletically far species).

An experiment on acceleration and retardation of ontogenesis with thyroid manipulation has revealed direct changes in definitive dentition of pharyngeal bones in Abramis brama bream. As development pace accelerates, the number of teeth reduces to the formula 5-4. When development pace slows down, this number increases to the formula 6-5. Moreover, an additional minor row of teeth (1.6-5.1, 2.6-5.2) is formed. The observed changes transcend typical changes happening in nature. It is assumed that heterochronies provoke changes in the number of teeth.