Sub-lethal and lethal toxicities of elevated CO on embryonic, juvenile, and adult stages of marine medaka Oryzias melastigma.

The potential leakage from marine CO storage sites is of increasing concern, but few studies have evaluated the probable adverse effects on marine organisms. Fish, one of the top predators in marine environments, should be an essential representative species used for water column toxicity testing in response to waterborne CO exposure. In the present study, we conducted fish life cycle toxicity tests to fully elucidate CO toxicity mechanism effects. We tested sub-lethal and lethal toxicities of elevated CO concentrations on marine medaka (Oryzias melastigma) at different developmental stages. At each developmental stage, the test species was exposed to varying concentrations of gaseous CO (control air, 5%, 10%, 20%, and 30%), with 96 h of exposure at 0-4 d (early stage), 4-8 d (middle stage), and 8-12 d (late stage). Sub-lethal and lethal effects, including early developmental delays, cardiac edema, tail abnormalities, abnormal pigmentation, and mortality were monitored daily during the 14 d exposure period. At the embryonic stage, significant sub-lethal and lethal effects were observed at pH < 6.30. Hypercapnia can cause long-term and/or delayed developmental embryonic problems, even after transfer back to clean seawater. At fish juvenile and adult stages, significant mortality was observed at pH < 5.70, indicating elevated CO exposure might cause various adverse effects, even during short-term exposure periods. It should be noted the early embryonic stage was found more sensitive to CO exposure than other developmental stages of the fish life cycle. Overall, the present study provided baseline information for potential adverse effects of high CO concentration exposure on fish developmental processes at different life cycle stages in marine ecosystems.