Extreme environments characterized by harsh physiochemical stressors provide ideal natural laboratories for testing a priori hypotheses about the mechanisms behind adaptation and ecological speciation. Hydrogen sulfide (H₂S) is a potent respiratory toxicant that inhibits aerobic ATP production. H₂S-rich springs throughout Latin America have been independently colonized by fishes within the family Poeciliidae, mediated largely by convergent adaptation to physiochemical stress from H₂S exposure. Mounting evidence suggests that environmental conditions experienced by the mother prior to birth can shape the phenotype of offspring through maternal effects. Maternal effects can lead to adaptive change if the mother’s environment leads to a shift in offspring phenotype that improves the match between offspring phenotype and its own environment, but these effects can also be neutral or maladaptive if this match is not affected or worsened. In this study, we quantified the impact of maternal food access during gestation and maternal population of origin (sulfidic or non-sulfidic) on numerous life history (size at birth, growth rate, and age at sexual maturity) and behavioral phenotypes (boldness, predator avoidance, and feeding efficiency) related to competitive ability throughout development in Atlantic mollies (Poecilia mexicana). We predicted that, for each phenotype tested, neonates from non-sulfidic mothers would perform significantly better than neonates from sulfidic mothers, and that neonates from well-fed mothers would outperform those from poorly fed mothers, representing a tradeoff between tolerance to abiotic stress and competitive ability in the sulfidic population. This work represents a novel approach to investigating the impacts of maternal effects on adaptation to environmental stress.