## 4 Discussion  
A wide range of pharmaceuticals are continuously released into the aquatic ecosystem and subsequently detected in matrices such as water, sediment, or biota [61; 62; 63; 64; 65; 66]. They appear in the aquatic environment as parent substances or their metabolites, mainly via effluent water, after their incomplete removal by wastewater treatment plants [67]. Throughout the developed world, antidepressants are an important and widely prescribed class of drugs [14]. Recently reported data show that these pharmaceuticals, even at relatively low, detectable but environmentally relevant concentrations, can significantly affect the physiological functions of various non-target aquatic organisms at different trophic levels. Importantly, aquatic organisms are exposed to these doses throughout their lifetime. These concentrations usually do not cause mortality but they can induce a number of sublethal effects that are often passed on to future generations of organisms and can affect \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Group** & **Concentration** & _Danio rerio_ (48 hpf) & _Xenopus laevis_ (56 hpf) \\ \hline control & - & 158.3 \(\pm\) 14.6 & 120.0 \(\pm\) 7.4 \\ \hline  & 0.1 \(\mu\)g/L & **175.0 \(\pm\) 14.1 \({}^{**}\)** & 119.9 \(\pm\) 11.0 \\  & 1 \(\mu\)g/L & **176.4 \(\pm\) 12.6 \({}^{**}\)** & 121.7 \(\pm\) 6.9 \\ F & 10 \(\mu\)g/L & **175.9 \(\pm\) 16.1 \({}^{**}\)** & 124.9 \(\pm\) 5.7 \\  & 100 \(\mu\)g/L & **173.3 \(\pm\) 17.0 \({}^{**}\)** & 119.7 \(\pm\) 6.2 \\  & 1000 \(\mu\)g/L & **181.1 \(\pm\) 14.1 \({}^{**}\)** & 123.5 \(\pm\) 12.4 \\  & 10,000 \(\mu\)g/L & 155.1 \(\pm\) 25.7 & - \\ \hline  & 0.01 \(\mu\)g/L & 155.8 \(\pm\) 9.4 & 122.1 \(\pm\) 9.7 \\  & 0.1 \(\mu\)g/L & 157.0 \(\pm\) 10.9 & 124.0 \(\pm\) 9.6 \\  & 1 \(\mu\)g/L & 157.4 \(\pm\) 9.4 & **130.1 \(\pm\) 5.6 \({}^{*}\)** \\ C & 10 \(\mu\)g/L & 157.7 \(\pm\) 10.6 & 126.3 \(\pm\) 10.0 \\  & 100 \(\mu\)g/L & 164.2 \(\pm\) 10.2 & 120.7 \(\pm\) 8.4 \\  & 1000 \(\mu\)g/L & **174.9 \(\pm\) 15.7 \({}^{**}\)** & 124.3 \(\pm\) 8.3 \\  & 10,000 \(\mu\)g/L & **179.2 \(\pm\) 12.5 \({}^{**}\)** & 112.7 \(\pm\) 7.5 \\  & 100,000 \(\mu\)g/L & 140.3 \(\pm\) 10.3 & - \\ \hline  & low & **182.3 \(\pm\) 14.2 \({}^{**}\)** & 118.1 \(\pm\) 6.2 \\ F + C & medium & **186.6 \(\pm\) 11.8 \({}^{**}\)** & 120.0 \(\pm\) 8.5 \\  & high & **180.0 \(\pm\) 14.1 \({}^{**}\)** & 122.6 \(\pm\) 9.5 \\ \hline \hline \end{tabular} Abbreviations: F—fluoxetine hydrochloride; C—citalopram hydrochloride; hpf—hours post-fertilization; low—fluoxetine hydrochloride 0.1 \(\mu\)g/L, citalopram hydrochloride 0.01 \(\mu\)g/L; medium—fluoxetine hydrochloride 10 \(\mu\)g/L, citalopram hydrochloride 1 \(\mu\)g/L; high—fluoxetine hydrochloride 100 \(\mu\)g/L, citalopram hydrochloride 100 \(\mu\)g/L, citalopram hydrochloride 100 \(\mu\)g/L, citalopram hydrochloride 100 \(\mu\)g/L; high—hours post-fertilization.  
\end{table} 
Table 5: Embryonic heart rate (beats per minute—ppm) of zebrafish (_Danio rerio_) and African clawed frog (_Xenopus laevis_) embryos exposed to antidepressants as single substances and as a mixture. The results are expressed as mean \(\pm\) standard deviation. Bold font and asterisks indicate a significant difference (\({}^{*}p<0.05\), \({}^{**}p<0.01\)) between the control and experimental groups at the same time of exposure. fundamental ecological processes [12; 68; 69; 70]. Many ecotoxicological studies also confirmed the sensitivity of various aquatic organisms to different antidepressants. For example, [PERSON] et al. [71] reported that venlafaxine can affect snails (_Planobarrius correus_) at levels ten times lower than citalopram. In recent years, we have increasingly encountered the use of embryonic stages of various aquatic organisms in t toxicity tests. Embryotoxicity is evaluated through the monitoring of traditional endpoints (e.g., mortality, hatching rate, malformations).