Body Shape Variability of the Minnow Phoxinus phoxinus (Linnaeus, 1758) (Cyprinidae, Actinopterygii) in Large and Small Watercourses of the Sylva River Basin (Middle Urals)

Abstract The variability of the body shape of minnows in populations from different-size watercourses of the Sylva River basin (Middle Urals) is studied by geometric morphometrics. Twenty-six homologous landmarks were used to describe the body shape variability of the fish. Four geographically adjacent minnow populations isolated by artificial and natural dams were investigated. More than 56% of the variance of intergroup differences in the body shape is determined by the habitation of minnows in the large watercourse of the main flow (the Sylva River) and its small tributaries (the Lomovka, Musorka, and Sarga rivers). The main-flow minnow is characterized by relatively large eyes, low body, and small short jaws. On the contrary, the tributary minnow is characterized by a relatively high body, small eyes, and long jaws. Along with the significant differentiation of the isolated minnow populations from the main flow and its tributaries, a high level of individual morphological differences was discovered, which indicates some directional change in the fish development trajectory in the population from each watercourse. The discrimination level of individuals reached 97.9%. A low intragroup morphological diversity was revealed for the minnow population in the large watercourse of the Sylva River with a multispecies ichthyological community. Minnows from the small watercourses of the Lomovka, Musorka, and Sarga rivers with a low species diversity have a high intragroup morphological diversity in the body shape. This may indirectly show some ontogenetic change in the individuals from the small tributaries due to the formation of a wide range of ontogenetic trajectories and suggests a high phenotypic plasticity of the species. The role of the hydrological regime in the small watercourses and the role of the taxonomic diversity of fish communities in the formation of the morphological diversity of minnows is hypothesized.