Functional hermaphroditism has been confirmed in 94 teleost genera within 27 families and seven orders (Mitcheson & Liu, 2008), of which the hermaphroditic patterns are diverse, including simultaneous, protogynous, protandrous, bidirectional sex change and androdioecy (Erisman et al., 2013;Kobayashi et al., 2013;Mitcheson & Liu, 2008). Of these hermaphroditic fish, ricefield eel (Monopterus albus) is a protogynous hermaphroditic Synbranchiform species that commonly changes sex from functional female to functional male after ovulation (Liu, 1944). The ricefield eel offers an opportunity to unveil the molecular mechanism of sexual reversal.Ever since the ricefield eel was found to be sequential hermaphroditic, researchers have examined its mechanisms of sex determination and sex change. At the cellular level, histological and ultrastructural changes in the gonad during sex reversal from female to male in ricefield eel were observed under an optical microscope and electron microscope (