text: Antioxidant defenses and immune responses of flounder Paralichthys olivaceus larvae under methylmercury exposure.
  Methylmercury (MeHg) is a highly toxic contaminant in coastal environments and poses threats to marine fish in early life stages (ELSs). However, MeHg toxicity to fish embryos and larvae is not well investigated. This study investigated the antioxidant defenses and immune responses of flounder Paralichthys olivaceus larvae exposed to waterborne MeHg (0, 0.1, 1.0 and 10.0 μg L-1) for 35 days, from embryogenesis to settlement. The results revealed that metal accumulation in the larvae was positively correlated with MeHg concentration, reduced larval growth and survival. The activities of catalase and glutathione reductase were significantly increased at 10.0 μg L-1, while glutathione peroxidase activity and lipid peroxidation level were significantly increased at concentrations over 1.0 μg L-1. The corresponding antioxidant-related genes were upregulated under MeHg exposure (cat and gpx at 10.0 μg L-1; gr over 1.0 μg L-1). Lysozyme content was significantly increased, but immunoglobulin M content was significantly decreased at 10.0 μg L-1. The immune-related genes were significantly upregulated (hsp70 at 0.1 and 10.0 μg L-1; lzm and il-1β over 1.0 μg L-1; tnf-α and il-6 at 10.0 μg L-1) or downregulated (igm, over 0.1 μg L-1). Overall, MeHg exposure induced oxidative stress and caused immunotoxicity at concentrations over 1.0 μg L-1 and 10.0 μg L-1, respectively. The transcription of selected genes correlated with the corresponding biochemical markers in response to MeHg toxicity. These findings improve our knowledge to better understand the mechanisms by which marine fish at ELSs cope with oxidative stress and immunotoxicity induced by MeHg.