Synchronously Mature Intersex Japanese Flounder (<i>Paralichthys olivaceus</i>): A Rare Case

Japanese flounder is usually gonochoristic, with gonads that are either testes or ovaries. Here, we report an unusual case of hermaphroditism in Japanese flounder captured from the Bohai Sea. In the intersex flounder, the membrane of the upper ovary was closely connected to the abdominal muscles and internal organs, and the eggs filled the entire abdomen. The lower ovary was small and closely connected to the testes. The testes contained few fully mature sperm. Both eggs and sperm were capable of fertilization. The levels of several reproduction-related hormones (17β-estradiol, 11-ketotestosterone, 17α, 20β-dihydroxyprogesterone, luteinizing hormone, follicle-stimulating hormone, and testosterone) in the intersex flounder were intermediate, between those in females and males. The results showed that the heterozygosity of the intersex flounder was 0.632, and there were 28 single-nucleotide polymorphisms in the <i>cyp21a</i> gene. Compared with that of wild flounder, the activity of 21-hydroxylase was reduced by approximately 20.0%, and expressions of <i>cyp19a</i>, <i>amh</i>, and <i>dmrt1</i> differed. We present the first report of its kind, detailing the anatomy, hormonal endocrinology, molecular biology, and physiology of the intersex Japanese flounder.