Metabolism, movement behaviour, and salinity responses of invasive mosquitofish compared with a threatened toothcarp

The success and impact of invasive non-native species depend on how they cope with local abiotic conditions, especially in comparison to co-occurring native taxa. In aquatic systems, salinity serves as a key environmental filter, mediating the establishment of invasive species and influencing biotic interactions. However, the mechanistic basis behind these context-dependent responses remains poorly understood. In this study, we examined interspecific differences in metabolic rate, movement behaviour, and their relationship between the worldwide invasive Gambusia holbrooki and a threatened Spanish endemic fish Aphanius iberus across an experimentally-manipulated salinity gradient. Using intermittent-flow respirometry and automated tracking of movement patterns, we compared aerobic metabolic traits and movement behaviours under various salinities. Toothcarp maintained a stable aerobic scope across salinity treatments, whereas invasive mosquitofish exhibited a markedly lower aerobic scope with higher salinity, primarily due to a decline in maximum metabolic rate. This stress response was not linked to increased osmoregulatory costs, as baseline metabolism decreased. Behavioural tests demonstrated consistent species differences in routine locomotion, with mosquitofish showing more exploratory behaviour and toothcarp showing more stop-go behaviour and remaining more stationary overall. Notably, in toothcarps, we found a negative link between standard metabolic rate and space use, suggesting that individuals with higher baseline metabolism may be constrained in their movement. Conversely, in mosquitofish, although salinity affected metabolic capacities, this effect was not reflected in their movement, indicating a weak relationship between metabolism and behaviour, likely supported by trait flexibility. By integrating metabolic traits with behavioural data, our results reveal mechanisms underlying invasive species responses and strengthen predictions of their performance relative to native fishes under changing conditions, such as salinity in inland waters. This highlights the importance of trait-based approaches for predicting responses to abiotic stressors and for evaluating the ecological impacts of invasive taxa.