The purpose of this study was to characterize in detail the reproductive strategy, course of embryogenesis, and development of larvae in three species of fishes of the genus Cichlasoma: the green terror (Andinoacara rivulatus), the red discus (Symphysodon discus), and the jaguar cichlid (Parachromis managuensis). Eggs for the study were obtained from five pairs of each species (300 eggs from each female) and incubated at 26 °C. The developing eggs were observed under a microscope (Carl Zeiss Stereo Discovery. V12 and Nikon 2000SE software (NIS-Elements F 4.30.01 64-bit) from fertilization to larval hatching until complete yolk-sac resorption. The largest average number of eggs per female was found in the jaguar cichlid ($\overline{\mathrm{x}}$ = 2991 eggs), a smaller average number of eggs was shown in the green terror ($\overline{\mathrm{x}}$ = 922 eggs), and the red discus showed the smallest average number of eggs ($\overline{\mathrm{x}}$ = 300 eggs). There were significant differences in the sizes of the eggs of the studied species: jaguar cichlid eggs were the smallest (1.060 ± 0.05 mm³), red discus eggs were larger (1.070 ± 0.07 mm³), and green terror eggs were the largest (1.365 ± 0.16 mm³). The embryogenesis time in the red discus was 2132 °H (82 Hpf), in the green terror it was 2158 °H (83 Hpf), and the longest in the jaguar cichlid was 2470 °H (87 Hpf). At the end of embryogenesis, the average size of the larvae after hatching was measured (red discus $\overline{\mathrm{x}}$ = 4.346 mm, green terror $\overline{\mathrm{x}}$ = 5.203 mm, and jaguar cichlid $\overline{\mathrm{x}}$ = 5.301 mm) and the time of yolk-sac resorption from the moment of hatching to the transition from endogenous to exogenous feeding was determined (jaguar cichlid 5 days, green terror 6 days, and red discus 3 days). The results of this study may contribute to the development of reproductive biotechnology for the studied fishes that could be used in aquaculture and, thus, help protect them in their natural habitats.

Urbanization and technological advancements result in the dispersion of antropogenic electromagnetic fields (EMF) that can affect on ecosystems. Therefore, it is important to understand their impact on the environment. Aquatic ecosystems are subject to EMF as part of various electricity sources, e.g., high-voltage transmission lines (HVTL). We examined the impact of EMF generated by HVTL on the spatial arrangement and survival of pike (Esox lucius) embryos. Fertilized eggs were incubated under two HVTL configurations 110 kV and 220 kV compared with a control group devoid of anthropogenic EMF. Embryo orientation and survival were monitored until blastopore closure. The control group showed dominance in the arrangement of embryos along the N-S, NNW-SSE, and NNE-SSW axes, with a slight prevalence of northern directions. EMF originating from HVTL did not exert a significant influence on the spatial arrangement of pike embryos, although some deviations from the arrangement noticed in the control group were observed. Increased embryo mortality was observed only at 110 kV site, but probably due to factors unrelated to EMF. In conclusion, EMF generated by HVTL did not significantly change pike embryo orientation or chances of survival. However, longer exposure or higher EMF levels could provoke notable reactions, requiring ongoing evaluation as power networks continue to spread more widely.

This article has examined the effects of magnetic fields with intensities of 1 mT, 3 mT, and 5 mT on two species of cichlid fish (family Cichlidae). This study focused specifically on assessing the influence of these magnetic fields on larval development, the survival rate, and the occurrence of deformities in the larvae of the jaguar cichlid (Parachromis managuensis) and the green terror (Andinoacara rivulatus). The analysis of the study results indicates that the effects of magnetic field exposure depend both on its intensity and on the fish species being studied. Magnetic fields, depending on the applied intensity, can influence changes in body size as well as the size of the yolk sac. In jaguar cichlid larvae, increased survival was observed in the groups exposed to the magnetic field; however, this was accompanied by a higher number of deformities compared with the control group. In the case of the green terror, the highest survival rates were recorded in the control group and in the group exposed to a magnetic field of 3 mT, while increased numbers of deformities were also noted in the exposed groups. The obtained results may be of significant importance for ornamental fish aquaculture, where appropriately selected magnetic field parameters could support larval growth and survival, reducing losses in breeding.

The aim of this study was to determine the influence of static magnetic fields with intensities of 1 mT, 3 mT, and 5 mT on the embryonic and postembryonic development of the rainbow cichlid ( Herotilapia multispinosa ). The experiments were conducted using eggs obtained from sexually mature, actively spawning pairs of this species. Analysis of the results showed that the applied magnetic fields significantly affected the rate of embryogenesis, which, depending on the intensity, could be either accelerated or slowed down. In particular, exposure to a 5 mT field accelerated development and produced larger hatchlings with smaller yolk sacs but lower subsequent growth, whereas exposure to a 3 mT field prolonged embryogenesis, resulting in smaller hatchlings with relatively larger yolk sacs and faster posthatching growth. The findings suggest that appropriately selected magnetic field parameters may positively influence the development of eggs and larvae of the rainbow cichlid, indicating the potential application of this method in ornamental fish aquaculture to reduce breeding losses.