Chronic Toxicity of Deep-Sea Mining Heavy Metals: Evidence from Fecundity and Offspring Development in Marine Medaka (Oryzias melastigma)

The release of heavy metals from commercial deep-sea mining is likely to be a long-term process lasting decade, and therefore fish communities are actually exposed to a chronic, ongoing stressor. However, toxicity assessment data for marine fishes related to heavy metals have predominantly been derived from acute toxicity experiments to date. To evaluate this potential risk, this study selected four heavy metals (Cu2+, Cd2+, Mn2+, VO4³-) and with potential for release from deep-sea mining and determined their 14-day median lethal concentration (LC50) for embryos and 96 hour (96 hr) LC50 for newly hatched larvae of marine medaka. Subsequently, a 60-day low-concentration, long-term exposure experiment was conducted based on the 96 hr LC50 of Cu2 + and Cd2 + for larvae. This experiment measured parental reproductive indices, such as spawning output and fertilization rate, and offspring developmental parameters, including hatching rate, malformation rate, hatching delay rate, and egg diameter, under stress from Cu2 + and Cd2 + at concentrations of 1/25 or 1/5 of 96 hr LC50. The results indicated that at long-term exposure concentrations even as low as 1/25 of 96 hr LC50 (Cu2+: 0.36 mg L-1, Cd2+: 0.25 mg L-1), both parental reproductive capacity and offspring development were significantly inhibited. This was manifested by an approximately 20% decrease in parental spawning output, a 5-14% reduction in offspring hatching rate, a 12-29% increase in hatching delay rate, alongside significant elevations in malformation rate and heart rate. In contrast, the impact on fertilization rate was relatively minor. These findings demonstrate that marine medaka are more sensitive to the long-term stress induced by heavy metals compared to short-term acute exposure, particularly regarding key indicators related to population recruitment such as parental reproductive fitness and offspring development. This study provides long-term toxicological data under stress conditions, so far largely lacking, for establishing environmental impact assessment thresholds for deep-sea mining activities.