As has been noted above, Spanovskaya and co-authors (1964) noted in rotan from the European part of the USSR differentiation in growth rate and assigned it to the consequences of acclimitaiztion. Krysanov and Elovenko (1981) also paid attention to the growing variation of these fishes at long isolation of the water body. However the authors came across the same differentiation in a study of rotan from natural Far Eastern water bodies. Unlike populations from flood plain lakes of rivers Amur, Ussuri, Mo, Razdolnaya in water bodies having no relations with river for many years, as for instance clay pits in Khabarovsk, isolated lakes of the European part of Russia and Eastern Kazakhstan, distinct ecological dimorphism is observed. Two forms that the authors conventionally denoted as light and dark ones differ significantly by 12 of 45 examined plastic characters.
Rotan of the light rapidly growing form feeds mostly on fish. It has a more elongated body shape, strong head, terminal mouth, anal and second dorsal fin are better developed and shifted backwards. Rotan of the dark form feeds on all groups of aquatic invertebrates. Differentiation becomes noticeable by the end of the first year of life when stage III of gonad maturity (start of yolk development) is attained. Ratio of abundance of these forms in the population changes with growth. Fishes of the light form in the age group 50-100 mm are also rare, as well as dark individuals among individuals longer than 200 mm.
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