Desert fishes are among the most imperiled yet poorly studied groups of fishes on Earth. Human modifications of the environment and introductions of invasive species frequently drive population declines and extinctions of fishes in arid regions of North America. One recent example is the extirpation of Gambusia senilis from the Devils River in Texas following the construction of Amistad Reservoir on the Rio Grande. The direct cause of this extirpation is unclear, but it has been hypothesized that the introduction of predatory bass (Micropterus spp.) to the river played at least some role. We tested this hypothesis using a geometric morphometric analysis as well as examining other specific morphological traits considered adaptive to high-predation conditions. We found variation among populations in all traits examined, but the relationship between these traits and predation risk was relatively ambiguous. Two populations with introduced sunfish (Lepomis spp.), the Río San Pedro at Pedro Meoquí and Río Chuviscar at Aldama, exhibited the combinations of traits that might be most likely to facilitate success in a high-predation environment, but it is not clear that any population of G. senilis could coexist with predatory bass.
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