Damage to survival, development, and growth of Japanese flounder (Paralichthys olivaceus) at early developmental stages by Ulva prolifera decay

Large-scale Ulva prolifera green tides decay in the Southern Yellow Sea has affected coastal ecological environment and caused substantial economic losses. This decay period overlaps with the critical early developmental stages of numerous marine fishes. However, research on the impacts and underlying mechanisms of U. prolifera decay on fish is still underexplored, we therefore conducted a stress experiment to investigate the effects of U. prolifera decay on Japanese flounder (Paralichthys olivaceus), an important marine economic flatfish. Our results showed the mortality of embryos, larvae and juveniles exposed to U. prolifera decay solution respectively increased by 39 %, 18.1 % and 19.4 %, and juveniles exposed to sediment from areas of U. prolifera accumulation increased by 16 %, compared to controls. Hatchlings exhibited a malformation rate of 31.1 % in the test group compared to 5.9 % in the controls, and metamorphosis was delayed about 6 days in larvae. The weight of juveniles was reduced by approximately 40 % in the test group compared to controls. Moreover, the tgf-β1, tab1 and smad2/3 genes showed up-regulated expression from embryo to juvenile stages and th, thrb and trh genes demonstrated down-regulated expression in larvae, probably affecting skeletal development in embryos and delaying larval metamorphosis, respectively. Additionally, genes in the GH-IGF-1 axis were down-regulated, potentially inhibiting growth in juveniles. Our findings provide valuable insights into the effects of U. prolifera decay at the early developmental stages of Japanese flounder, and provide supportive evidence for assessing the risks of U. prolifera decay to fishery resources and germplasm recruitment.