AbstractSpecies interactions can contribute to species turnover when the outcomes of the interactions are context dependent (e.g., change along environmental gradients). Plasticity may change this dynamic by altering the environmental tolerances of the species interacting. Here, we explored how the competitive interaction between two euryhaline fish, Poecilia reticulata and Poecilia picta, is influenced by acute and developmental responses to salinity. In Trinidad, P. reticulata is confined to freshwater despite being tolerant of brackish water. P. reticulata may fail to occupy brackish water because of reduced tolerance to salinity or because P. picta competitively excludes them, and developing in brackish water could alter the dynamics of either scenario. To test this, we compared the salinity tolerances of both species in the absence of competition, reared P. reticulata individuals in freshwater or brackish water, and tested the consequences of developmental plasticity in experiments in which P. reticulata competed against conspecifics or P. picta during acute exposure to freshwater or brackish water. We found that (1) P. reticulata has a weaker salinity tolerance than P. picta; (2) P. reticulata that developed in freshwater perform best when competing against P. picta in freshwater but perform poorly when competing against P. picta in brackish water, suggesting the species interaction is context dependent; and (3) developing in brackish water did not benefit P. reticulata in brackish water. Our results suggest that P. reticulata's freshwater range limit is in part a product of a lower salinity tolerance leading to a decrease in competitive performance in brackish water. Adaptive plasticity has been suggested to be a crucial part of the colonization process, yet nonadaptive plastic responses as found here can limit range expansion and reinforce range limits.