The eel has two sexual maturities, which means that the eel has one sex change in its life. When eels are small, they are female, and when they mature and lay eggs, they change from female to male, and then when they grow up, the big eels are male.

The first sexual maturity is from embryo to spawning, which means that most eels under 24 centimeters are female. After the eel lays eggs, its ovaries gradually turn into spermathecae, a period called the female parthenogenesis, during which the eel's body length is about 24 to 40 centimeters. Then as the individual grows, the eel reaches its second sexual maturity after its ovary completely turns into a spermatheca.

In short, the eel is female from the stage of embryonic development to the first sexual maturity, and after the second sexual maturity, the eel becomes male. This is the secret of eel hermaphroditism. This process of male-female transformation is called sex reversal in biology.