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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 38° D of development and finished on the 60° D 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 96thhour of development (76° D). On the other hand, hatching with the head part facing forward (preceded by a 180-degree inversion of the embryo) began on the 115° D 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.

Gametogenesis and the temporal changes occurring in the ovaries and testes throughout the reproductive cycle in the invasive alien bivalve, Chinese pond mussel Sinanodonta woodiana(Lea), from the heated Konin lakes system (central Poland) were studied using histological techniques. S. woodianawas confirmed to be a gonochoristic species with overall sex ratio of 1:1. The examined morphological parameters of Chinese pond mussel spermatozoa, i.e. 42 μm mean total length; 4.3 μm mean head length and the maximum size of previtellogenic (34-43 μm) and vitellogenic oocytes (75-83 μm) are consistent with values established for closely related members of the Unionidae family. Our results suggest that S. woodianain the Konin lakes system are able to spawn throughout March to October, with a season of higher reproductive activity in females extending from March to April. This type of reproductive biology may contribute to the Chinese pond mussel's success in thriving in freshwater ecosystems.

The effect of Ovaprim (salmon GnRH analogue and a dopamine antagonist) treatment on the quantity and quality of smelt (Osmerus eperlanusL.) sperm was studied in relation to time after hormonal stimulation. Sperm was obtained at 0, 24, 48 and 72 h after treatment (n=13/each time point). Computer Assisted Sperm Analysis (CASA) was used to evaluate sperm motility parameters and histological analysis was used to examine the testis morphology. Only a small volume of semen (1–5 µl) was collected at the beginning of the experiment (time 0) but it dramatically increased 24 h after hormonal treatment. A further increase in semen volume was recorded 48 h after hormonal stimulation. CASA parameters, such as percentage of motile cells, progressive motility, curvilinear velocity, straightlinear velocity, straightness and amplitude of lateral head displacement of stripped sperm increased 48 h after hormonal treatment, which indicates high quality of sperm. No further increase in sperm quality was recorded at 72 h. Compared to stripped semen, testicular semen was characterized by a slightly lower quality. In addition, histological analysis indicated that 24 h after hormonal treatment, a high number of spermatozoa was released from the testis. Ovaprim-stimulated smelt became clearly darker than the control fish. In conclusion, our results suggest that smelt semen should be collected 48 h after hormonal stimulation to ensure high quality of semen. This time may vary depending on maturation status of testis.

Common carp spawning characteristics were compared between season and out-of-season periods. There were no differences in the percentage of ovulation, spermatozoa motility and embryo survival to the eyed-egg-stage between the spawning periods. Fish spawned during the season produced oocytes of higher total weight than those spawned out-of-season. On the day exogenous feeding began, larvae obtained in out-of-season spawning were shorter than those obtained during the spawning season. However, the out-of-season larvae started feeding two days earlier than in-season larvae. As a result, seven days after fertilization, larvae obtained during out-of-season had the same length as larvae obtained during season. It appears to be possible to obtain common carp larvae half a year before the natural spawning period without decreasing the quality of gametes and larvae.

•The study of effects of three commercial diets on growth, cannibalism and survival in Eurasian perch post-larvae was performed.•Proximate and fatty acid content of perch body, liver histology and vertebral column anomalies in relation to tested diets were analyzed.•Eurasian perch fed by diet with the highest fat content and the lowest protein content achieved significantly higher weight and body length.•Probably different palatability of the feeds (understood in terms of its availability) could have caused difference in cannibalistic behavior.•No effect of tested feeds on the histological structure of the liver and the prevalence of skeletal malformation in Eurasian perch was found.