Summary The effects of thyroid hormone (TH) level changes on skeletal development were studied in cyprinids Labeobarbus intermedius (Barbinae) and Danio rerio (Danioninae). TH was revealed to influence development of a skull, axial skeleton, paired fins and girdles, and squamation. Usually, high TH‐level promotes the premature onset and acceleration of ontogenetic processes, whereas TH‐deficiency delays and retards them. However, skeletal elements differ in TH‐responsiveness. Alterations of TH level induce developmental changes in some structures but do not cause any changes in others. As a result, the normal sequence of ontogenetic events and the temporal interrelations between ontogenetic processes undergo changes resulting in changes in adult morphology.

The variability of pharyngeal dentition in a natural population of B. intermedius and effects of genetic, hormonal and environmental factors on the number of tooth rows in the pharyngeal dentition in offspring from wild-caught parents have been investigated. It was revealed that: (i) about 10% of fish from natural population have four-rowed dentition instead of three-rowed dentition characteristic for this species; (ii) the presence of the additional tooth row is not an abnormality of tooth replacement since it occurs symmetrically on both sides; (iii) occurrence of the fourth row of teeth is heritable since laboratory-reared offspring from parents with four-rowed dentition have the same dentition. Even if one of the parents had four-rowed dentition the percentage of four-rowed individuals in the progeny was significantly higher than in progeny from parents with normal (three rowed) dentition; (iv) the number of tooth rows appears to be hormonally controlled: high levels of thyroid hormone result in a decrease in the number of tooth rows to two. In contrast, deficiency of this hormone results in an increase to four rows; (v) no differences in the number of tooth rows were found in fish reared under 17°C, 24°C, 30°C and room temperature (20-26°C).

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