Exposure to oxidative by-products during metamorphosis affects pigmentation patterns in flounder, Paralichthys olivaceus

Hatcheries often employ disinfection by electrolysis for water; however, this produces free oxidants that pose practical problems in the production of flounder, Paralichthys olivaceus. Rearing juvenile flounder in electrolyzed seawater (EL-SW) causes pseudo-albinism. To elucidate the mechanism of the onset of EL-SW-induced pseudo-albinism, we compared thyroid hormone (TH) metabolism, melanoblasts, and skin differentiation in flounder reared in normal seawater (normal SW) and EL-SW and determined the critical period for the onset of pseudo-albinism. The number of thyroxin (T4)-positive thyroid follicles increased toward the metamorphic climax. The expression of deiodinase 1 (dio1), which converts plasma T4 to triiodothyronine (T3), in the liver peaked synchronously with the increase in T4-positive thyroid follicles. EL-SW exposure had no effect on TH metabolism. Instead, EL-SW inhibited differentiation of adult-type melanoblasts in the skin of the ocular side together with a loss of structural asymmetry in the bilateral skins. Although EL-SW does not affect TH metabolism, our data indicate that it does inhibit the signaling system that controls differentiation of the blind and ocular lateral halves during the establishment of metamorphic asymmetry. The early-metamorphic stage, E- and F-stages, was demonstrated to be the critical period for the onset of pseudo-albinism in EL-SW-reared larvae. These results suggest that long-term exposure to free oxidants, even at low levels, acts as a chronic stressor on flatfish metamorphosis.