Differences in swimming performance and energetic costs between an endangered native toothcarp ( Aphanius iberus ) and an invasive mosquitofish ( Gambusia holbrooki )

Swimming performance is a key feature that mediates fitness and survival in many fish species. Using a swim tunnel respirometer, we compared prolonged swimming performance and energy use for two competing species: an endangered, endemic toothcarp ( Aphanius iberus ) and a worldwide invasive mosquitofish ( Gambusia holbrooki ). Critical ( U crit ) and optimal swimming speeds, standard and maximal metabolic rates, absolute aerobic scope, as well as the minimum cost of transport were estimated and compared between species and sexes. Body streamlining and caudal peduncle depth were also measured to explain the differences in swimming performance and efficiency. Both sexes of A. iberus presented similar swimming capacity and metabolic traits, whereas males of G. holbrooki showed higher critical swimming speeds, maximal metabolic rate and absolute aerobic scope than females. We also found marked differences between species in most of the response variables examined. Aphanius iberus showed lower swimming capacity ( U crit mean <10 cm s −1 ), higher maximal metabolic rate and absolute aerobic scope than the invasive species. By contrast, G holbrooki swam faster and had lower cost of transport at a given fish mass and speed, thereby leading to a higher swimming efficiency. The observed differences in swimming efficiency were closely related to differences in morphological characteristics and therefore to drag pressures and propulsion. Our results add a mechanistic basis to the ecological understanding of these two species and suggest that although both are poor swimmers compared to many other similarly sized species, the native species likely has more restricted water flow tolerance and dispersal capacities.