Ionizing radiation (IR) resulting from both natural and anthropogenic sources is ubiquitous throughout the environment. Historically, studies on the biological impacts of radiation primarily focused on responses to acute doses of radiation, with little advancement in our understanding of environmentally relevant exposures. Epigenetic mechanisms are capable of mediating organismal responses to environmental stressors and DNA methylation plays important roles in gene regulation and promoting chromosomal stability. In this work, we assess broad-scale transcriptional and epigenetic variation resulting from chronic exposure to low doses of ionizing radiation (LDIR
5.78, 53.76, or 520.23 mGy/day) using Japanese medaka fish (Oryzias latipes) in a replicated mesocosm design. We observed significant changes to the hepatic transcriptome induced by a 3-month chronic exposure to IR, whereas global DNA methylation appeared largely unaffected. Our findings reveal a set of genes, including those involved in immune function, responding to environmentally relevant IR exposures, which do not appear to be mediated by a systemic global shift in DNA methylation.