Toxic Effects of Arsenic on the Embryonic Development of Marine Medaka ( <i>Oryzias melastigma</i> ) Under Different Salinity Conditions

Arsenic, a toxic metalloid widely distributed in nature, stands as one of the most hazardous environmental pollutants in estuarine and coastal ecosystems. The biotoxicity of arsenic is highly dependent on its chemical speciation and environmental conditions, where salinity acts as a critical factor that markedly influences its toxicological effects on aquatic organisms. To date, few studies have reported the combined effects of arsenic and salinity on fish, and the mechanisms of arsenic-induced toxicity on early fish development under varying salinity conditions remain poorly understood. In our study, the toxic effects of arsenic at different concentrations (0, 0.1, 1 and 10 mg/L) on the embryonic development of marine medaka (Oryzias melastigma) under three salinities (10, 20 and 30 ppt) were investigated. Our research results showed that arsenic exposure reduced the hatching rate of medaka embryos. At salinities of 10 and 20 ppt, the exposure to arsenic induced additional developmental toxicities, including decreased heart rate, shortened hatching time, increased malformation rate, and reduced body length of newly hatched larvae. Additionally, arsenic exposure altered the expression levels of cardiac development-related genes and immune-related genes, with varying expression changes observed across different salinity levels. These findings, taken together, demonstrate that salinity influences the arsenic-induced toxicity (cardiotoxicity, oxidative damage) on the embryonic development of medaka. Our study provides additional data support for the toxicity research and ecological risk assessment of arsenic in estuarine and coastal ecosystems.