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The progressive expansion of the Amur sleeper in Europe may have serious consequences for the native ichthyofauna and results in the depletion of autochthonous biodiversity. It can be manifested by food competition with native taxa and the displacement from natural habitats or breeding grounds. Studies of the biology of reproduction of alien species reflect their adaptivity to new environmental conditions. Therefore, the aim of the study was to analyse changes during Amur sleeper embryogenesis, a representative alien invasive fish species. Mature oocytes of the Amur sleeper just after fertilisation had an elongated, symmetrically-ellipsoid shape, with a large perivitelline space. Their average total length was 3.53±0.14 mm, and the total width was 1.35±0.11 mm. On the vegetative pole of the egg, a sticky filiform plait with a height and width of approximately 1 mm, helpful during oviposition to the substrate, was noted. The yolk of the eggs had a homogeneous dark yellow structure with an average diameter of 1.1±0.1 mm. Cleavage took place during the first day of incubation, while gastrulation was initiated around the 38D of development and finished on the 60D when the embryo’s body was elongated and clearly divided into a head, trunk and caudal parts. The onset of the eye-colour stage correlated with the phase of the embryo moving away from the yolk sac, which occurred in the 96 hour of development (76D). On the other hand, hatching with the head part facing forward (preceded by a 180-degree inversion of the embryo) began on the 115D of development. The mean total length of the larvae, unable to swim freely and feed, was 2.95±0.24 mm. An analysis of the specificity of the Amur sleeper’s early ontogenetic development and a description of its sensitive stages may contribute to an understanding of the spread of this alien fish species.

Early Ontogenetic Development of the Amur Sleeper Perccottus glenii Dybowski, 1877, an Alien Invasive Fish Species Outside Its Natural Range