## 4 Discussion
[32] investigated the effects of fluoxetineon the embryonic development of zebrafish embryos and observed 100% mortality after exposure to 5.51 mg/L and 15 mg/L at 120 hpf and 96 hpf, respectively. Fluoxetine also significantly affected swimming activity, with a significant reduction in swimming distance. This may have been related to possible neurotoxic effects, which was also suggested by a statistically significant reduction in acetylcholinesteraseic activity in the treated groups. Hatching is a critical period in embryo development and, therefore, its monitoring is an important indicator of embryotoxicity. Accelerated hatching of embryos is described in many toxicological studies after exposure to pharmaceuticals [23; 74], pesticides [1; 2], or emerging pollutants [3]. It can be caused by the increased spontaneous movements of treated embryos. In our study, significantly earlier hatching was found only in the zebrafish toxicity test, especially in the experimental groups exposed to the mixture of both antidepressants. No changes were recorded in the toxicity test on frog embryos, because hatching started earlier and most of the embryos were hatched at 48 hpf. Earlier hatching after antidepressant exposure has also been confirmed in some other toxicological studies. [PERSON] et al. [23] reported that tricyclic antidepressants tested at a concentration range of 10 \(\upmu\)g/L to 500 \(\upmu\)g/L caused a statistically significantly earlier hatching of common carp embryos at both 72 hpf and 96 hpf. However, they observed retarded development of larvae in these experimental groups over the following days of tests. Hatching stimulation of zebrafish embryos was also observed after exposure to environmentally relevant concentrations of fluoxetine by [PERSON] et al. [34]. Similarly, [PERSON] [4] reported earlier hatching (\(p<0.05\)) of zebrafish embryos in the experimental group exposed to fluoxetine at 0.99 \(\upmu\)g/L at both 49 hpf and 57 hpf. The authors also found that fluoxetine exposure significantly affected the larvae survival rates and increased the total length of the larvae. By contrast, different results were found in a study by [PERSON] et al. [34], who found delayed development (\(p<0.05\)) of zebrafish embryos exposed to fluoxetine at a concentration of 15 mg/L at 48 hpf. Similarly, [PERSON] et al. [73] reported that antidepressants such as fluoxetine, serntraline, clomipramine, amitriptyline, and duoxetine exhibited concentration-dependent effects on the embryo-larval development of Pacific ovsters, characterized by a decrease in the rate of normal development. This significant impact was recorded at concentrations of 100 \(\upmu\)g/L and over. The monitoring of morphological malformations belongs to the basic observations during embryonic toxicity testing and it is described in various aquatic organisms after antidepressant exposure [24; 38; 72; 75]. Some toxicological studies reported these anomalies even at environmentally relevant concentrations [73; 74]. [PERSON] et al. [34] reported numerous malformations in the trunk or tail regions, including spinal curvatures, of zebrafish embryos after fluoxetine exposure, which may have been caused by the disturbance of nervous system development. Similarly, we found frequent tail and trunk deformities in zebrafish after fluoxetine exposure. Significant differences were confirmed in the two highest tested concentrations (1000 and 10,000 \(\upmu\)g/L). Suprisingly, the combination of both antidepressants at the lowest concentrations caused the highest incidence of malformations in frog embryos. These paradoxical findings may suggest a possible combined effect of the tested substances, which is not yet described in research and could be a topic for further research. [PERSON] et al. [24] tested the embryotoxicity of three antidepressants with different modes of action on the early life stages of zebrafish and the Western clawed frog.