Hybridization plays a pivotal role in evolution, influencing local adaptation and speciation. However, it can also reduce biodiversity, which is especially damaging when native and non-native species meet. Hybridization can threaten native species via competition (with vigorous hybrids), reproductive resource wastage and gene introgression. The latter, in particular, could result in increased fitness in invasive species, decreased fitness of natives and compromise reintroduction or recovery conservation practices. In this study, we use a combination of RAD sequencing and microsatellites for a range-wide sample set of 1366 fish to evaluate the potential for hybridization and introgression between native crucian carp (Carassius carassius) and three non-native taxa (Carassius auratus auratus, Carassius auratus gibelio and Cyprinus carpio) in European water bodies. We found hybridization between native and non-native taxa in 82% of populations with non-natives present, highlighting the potential for substantial ecological impacts from hybrids on crucian carp populations. However, despite such high rates of hybridization, we could find no evidence of introgression between these taxa. The presence of triploid backcrosses in at least two populations suggests that the lack of introgression among these taxa is likely due to meiotic dysfunction in hybrids, leading to the production of polyploid offspring which are unable to reproduce sexually. This result is promising for crucian reintroduction programs, as it implies limited risk to the genetic integrity of source populations. Future research should investigate the reproductive potential of triploid hybrids and the ecological pressures hybrids impose on C. carassius.

The crucian carp (Carassius carassius) is a cyprinid fish with its natural distribution in Europe and the western part of Asia. Due to its hardiness and unique ability to survive winter anoxia, it has been translocated to small lakes and ponds, and in Northern Europe since medieval times has been used as a food source. Crucian carp was the only fish in the pond that survived anoxia. Small lakes and ponds with winter anoxia result in dense populations of stunted and slender fish. In lakes with other fish species present, the crucians' numbers and densities are low and they grow to large sizes. In the presence of piscivores such as pike, crucians are deep bodied. The presence of pike-eating crucians, or the pike odors, induce a change in the body form of crucians. The change in body form makes it more difficult for pikes to swallow crucians and the handling time increases. Closely related invasive Carassius species have become a serious threat to crucian carp populations in Central-Eastern Europe and South-Eastern England through competition for space and food resources and hybridization. The crucian carp's close relationship to goldfish (Carassius auratus), the most studied species concerning sex pheromones, has made it possible to demonstrate that sex pheromones are also present in a wild Carassius species and in their natural environment. The results indicate that two species use the same sex hormonal pheromone system. The crucian carp has become an important model for laboratory studies of olfaction and taste.

Selective serotonin reuptake inhibitors (SSRIs) are psychotropic pharmaceuticals used as antidepressants. SSRIs are commonly found in surface waters in populated areas across the globe. They exert their efect by blocking the serotonin re-uptake transporter in the presynaptic nerve ending. The present study examined whether behavioural efects to exposure to SSRI citalopram depend on personality and sex in the stickleback (Gasterosteus aculeatus). Three aspects of stickleback behaviour are examined: feeding behaviour, aggression, and boldness. We exposed sticklebacks to 350–380 ng/l citalopram for 3 weeks. Feeding and aggressive behaviour were recorded before and after exposure, whereas scototaxis behaviour was tested after exposure. The results show treatment efects in feeding and aggressive behaviour. Feeding is suppressed only in the male group (χ2=20.4, P<0.001) but not in the females (χ2=0.91, P=0.339). Aggressive behaviour was signifcantly afected by treatment (χ2=161.9, P<0.001), sex (χ2=86.3, P<0.001), and baseline value (χ2=58.8, P<0.001). Aggressiveness was suppressed by citalopram treatment. In addition, the fsh showed no change in aggression and feeding behaviour over time regardless of sex and treatment, which indicate personality traits. Only females are afected by treatment in the scototaxis test. The exposed females spent signifcantly (χ2=5.02, P=0.050) less time in the white zone than the female controls.

Low concentrations of psychiatric drugs are commonly found in surface waters around the world, and their ability to alter behaviors in aquatic fauna has raised concerns about ecological effects. One critical effect that contamination can have in aquatic systems is modifications of reproductive behaviors. In this study, we hypothesized that oxazepam, a common anxiolytic drug and aquatic contaminant previously shown to induce anti-anxiety behavior in salmonids, also affects the reproductive behavior of Atlantic salmon (Salmo salar). To test this hypothesis, small sexually mature Atlantic salmon males (mature parr) were exposed to concentrations of oxazepam comparable to that of effluent water (1.9 mu g/L). After 5 days of exposure, their reproductive behavior was compared with that of unexposed control individuals. Behavior was monitored in a large-scale flume aquarium that also contained a large male and a female to mimic natural spawning conditions for Atlantic salmon. Three behaviors were quantified for the mature parr; (a) amount of courting behavior towards female, (b) proximity to female, and (c) amount of attacks received from the large male. In contrast to our hypothesis, no difference in either courting behavior, proximity to female, or received aggression could be found between exposed and control male parr. Hence, our results indicate that there is no acute effect of the highest levels of oxazepam concentration reported for urban environments on spawning behavior of mature male parr, although chronic effects could not be ruled out.

Previous studies with olfactory-disturbing pesticides resulted after exposure in disturbed behavior and physiology in fish. In the present experiment, reproductive behavior and milt volumes of precocious brown trout (Salmo trutta L.) male parr were studied in a large stream aquarium after exposure to the olfactory-disturbing fungicide 15 μg l^-1 IPBC (iodocarb; 3-iodo-2-propynyl butyl carbamate) for 96 h. The statistical analyses did not reveal any significant differences for time attending females between controls and IPBC-exposed males. Furthermore, there were no significant differences in milt volumes. However, when taking all fish into consideration, there were significant differences in milt volumes between parr that had been attending females and those had not been attending females. Controls that had attended females had significantly higher milt volumes than controls or IPBC-exposed males that had not attended females. Taking all control and IPBC parr into consideration, there was a statistically significant positive correlation between time attended females and volume of milt and gonadosomatic index (GSI), respectively. In summary, 15 μg l^-1 IPBC did not have any significant effects on mature male parr reproductive behavior and milt volumes.

In fish aquaculture, great efforts are made to develop feed that excludes marine fish proteins and oils, or at least to decrease these parts to a minimum but the fish might not be attracted to or might not eat the alternative feed. Molecules detected by the olfactory sense can induce arousal and search behavior that attract the fish and induce foraging. In the present study, we have examined whether mussel extracts can induce foraging behavior in crucian carp, Carassius carassius. Free amino acids are present in high concentrations in mussels and they are detected at low concentrations by olfaction and induce food search behavior in several fish species. We studied the behavior responses to water extracts of mussel powder compared to extracts of a plant-based commercial carp feed. Extracts of different concentrations were administered in glass aquaria with individual fish. The tests began with 5 min with only tap water and this was followed by 5 min with extracts or amino acid solutions. The behavior was recorded with a video camera. The initial control period was compared to the following period with odors. No significant foraging response was observed with extracts of mussels, but the commercial carp feed extract induced increased bottom snapping. Synthetic mixtures of five nutritional amino acids present at high concentrations in both extracts did not give any significant foraging response. Our results show that mussels are not suitable as a protein source in feed to crucian carp. 

Selective Serotonin Re-uptake Inhibitors (SSRIs) are a class of psychotropic drugs used to treat depression in both adolescents and pregnant or breast-feeding mothers as well as in the general population. Recent research on rodents points to persistent behavioural effects of pre- and perinatal exposure to SSRI which last into adulthood. To study effects of developmental exposure in fish, three-spine sticklebacks were exposed to 1.5 µg/l of the SSRI citalopram in the ambient water for 30 days, starting two days post-fertilisation. After 100 days of remediation in clean water the fish were put through an extensive test battery. Feeding behaviour was tested as the number of bites against a piece of food and found to be increased in the exposed fish. Aggression levels were measured as the number of bites against a mirror image during 10 minutes and was also found to be significantly increased in the exposed fish. Novel tank behaviour and locomotor activity was tested in an aquarium that had a horizontal line drawn half-way between the bottom and the surface. Neither the latency to the first transition to the upper half, nor the number of transitions or the total time spent in the upper half was affected by treatment. Locomotor activity was significantly reduced in the exposed fish. The light/dark preference was tested in an aquarium where the bottom and walls were black on one side and white on the other. The number of transitions to the white side was significantly reduced in the exposed fish but there was no effect on the latency to the first transition or the total time spent in the white half. The results in the current study indicate that developmental SSRI exposure causes persistent behavioural effects in fish and contribute to the existing knowledge about SSRIs as environmental pollutants.

Selective serotonin re-uptake inhibitors are pharmaceuticals used to treat a range of psychological disorders. They are frequently found in surface waters in populated areas. In recent years, they have been shown to affect the behaviour of various aquatic organisms in a way that can have ecological effects. In this study, we exposed zebrafish of both sexes to nominally 0.00, 0.15 and 1.50 µg L−1 Escitalopram in flow-through tanks for three weeks. Subsequently, ten swimming behaviour parameters were quantified using high-resolution video tracking. There were noticeable gender differences in the behaviour responses to Escitalopram. Female fish exposed to 1.50 µg L−1 Escitalopram had a lower maximum swimming velocity, stopped less often and exhibited increased boldness (reduced thigmotaxis) compared to controls. Male fish exposed to 1.50 µg L−1 had a lower maximum swimming velocity compared to control fish. At the end of exposures, both length and weight of the females exposed to 1.50 µg L−1 Escitalopram were significantly less than the group of control fish. In addition, males exposed to 1.50 µg L−1 Escitalopram were significantly shorter than control fish. The behaviour, weight and body length of the fish exposed to nominally 0.15 µg L−1 was not significantly different from control fish in either sex. The results of this study demonstrate that Escitalopram can affect subtle but ecologically important aspects of fish behaviour and lends further credibility to the assumption that Escitalopram is an environmentally active pharmaceutical.

Sewage treatment plant effluents contain a complex mixture of pharmaceuticals, personal care products and industrial chemicals, thus exposing aquatic organisms. Still, the consequences of exposure to combinations of different classes of drugs is largely unknown. In this study, we expose adult zebrafish (Danio rerio) males and females to low, environmentally relevant concentrations of the endocrine disrupting chemical 17α-ethinyl estradiol (EE₂) and the selective serotonin re-uptake inhibitor (SSRI) citalopram, alone and in combination, and analyse three non-reproductive behaviours of importance for population fitness.

Two weeks exposure to 0.1 and 0.5 ng/LEE₂ resulted in increased anxiety in males in the scototaxis (light/dark preference) test. Significantly longer latency periods before entering the white zone and fewer visits in the white zone were observed in males exposed to both 0.1 and 0.5 ng/LEE₂ compared to unexposed males. No significant effects of citalopram alone (0.1 and 0.5 µg/L) were observed in the scototaxis test. The combined exposures (0.1 ng/L EE₂ + 0.1 µg/L citalopram and 0.5 ng/L EE₂ + 0.5 µg/L citalopram) resulted in abolishment of the anxiogenic effects of EE₂, with significantly shorter latency period (low dose) and more transitions to white (high and low dose) than in fish exposed to EE₂ alone. No significant effects of either EE₂, citalopramor the combination of the two were observed in females. In the novel tank test, significantly more transitions to the upper half of the tank were observed in males exposed to 0.1 µg/L citalopram alone compared to unexposed males while males exposed to 0.1 ng/lEE₂ had significantly shorter latency period to enter the upper half. Exposure to the combination of the two low concentrations did, however, result in a significantly longer latency and fewer transitions to upper half compared to both control, EE₂- and citalopram-exposed males. These males also spent significantly less time in the upper half than the fish exposed to 0.1 ng/l EE₂ or 0.1 µg/l citalopram alone. No significant effects on novel tank behaviour were observed in females or males exposed to the higher concentrations. In the shoaling test, males exposed to 0.1 µg/L citalopram and females exposed to 0.5 ng/l EE₂ made significantly fewer transitions away from peers while males exposed to 0.1 µg/L citalopram + 0.1 ng/l EE₂ performed significantly more transitions than the fish exposed to 0.1 µg/L citalopram alone.

In conclusion, this study shows that very low concentrations ofEE_2, at or slightly above the predicted noeffect concentration (NOEC), affects anxiety in zebrafish males. Furthermore, citalopram, in spite of marginal effect of its own at such low levels, counteracts the response to EE₂. This study represents an initial effort to understand the effects on water-living organisms of the cocktails of anthropogenic substances contaminating aquatic environments.