The water flea, Daphnia magna, serves as a key model organism for investigating the response of aquatic organisms to environmental stressors, including hypoxia. Hypoxia-inducible factor 1-alpha (HIF-1α) is a central regulatory protein involved in the cellular response to hypoxic conditions. In this study, we used CRISPR/Cas9 gene editing to create D. magna mutant lines with targeted alterations in the HIF-1α gene. Mutants demonstrated decreased survival and reproductive output and down-regulated genes for the HIF-1α–mediated pathway in low-oxygen conditions. These findings suggest that the HIF-1α pathway is a critical component of resistance to hypoxia in D. magna. This study provides novel insights into the molecular basis of hypoxia tolerance of HIF-1α in D. magna and expands our understanding of how aquatic organisms can adapt to or be challenged by changing oxygen levels in the face of global …