Behavioral and physiological dynamics of a native minnow exposed to invasive fish species

The introduction of invasive alien species is a major threat to freshwater biodiversity, particularly in Mediterranean ecosystems where seasonal droughts increase population density and social stress. This study investigated the effects of the presence and of variables densities of two widespread invasive fish species Gambusia holbrooki (Girard, 1859) and Lepomis gibbosus (Linnaeus, 1758), on the behaviour and physiology of the Iberian endemic Squalius alburnoides (Steindachner, 1866). Under baseline density, exposure to G. holbrooki significantly altered the behaviour of S. alburnoides, leading to increased aggression towards conspecifics, enhanced evasion, and a rise in the number of attacks suffered. These behavioral changes were accompanied by a reduction in forebrain dopamine levels, suggesting that dopaminergic influence stress-related responses. In contrast, interactions with L. gibbosus under the same conditions did not produce significant behavioral or physiological effects, although conspecific aggression showed temporal fluctuations. Under high-density conditions, both invaders intensified antagonistic interactions with S. alburnoides. Significant neurochemical alterations occurred solely in fish exposed to G. holbrooki, which showed elevated 5-HIAA concentrations and increased DOPAC/DA and 5-HIAA/5-HT ratios, indicating activation of serotonergic and dopaminergic pathways. Unexpectedly, plasma cortisol levels in S. alburnoides decreased in the presence of both invader species, suggesting a possible downregulation of the hypothalamic-pituitary-interrenal axis under putative social stress. Overall, our results demonstrate that invasive species differentially modulate behavioral and physiological stress responses in S. alburnoides, with G. holbrooki exerting stronger effects than L. gibbosus, and that higher densities amplify these species-specific interactions.