We commonly see eels in their "youthful years" as females, but after spawning they become males, as nine out of ten large eels are males. Why this is so is not clear. Some people have researched the "mystery of sex change" in fish, and it is believed that the purpose of sex change in fish is mainly to be able to maximize the reproduction of offspring and enable individuals to obtain the stimulation of the opposite sex. Michael, a marine biologist at the University of Utah, believes that in a female fish or a male fish, which is larger, almost monopolize the opportunity to mate with all the opposite sex. Thus, when females are smaller they are guaranteed a chance to mate, and when they grow up to become males, they have more chances to breed, and their chances of mating and reproducing are relatively increased compared to their gender-neutral counterparts. Similarly, in fish that change from male to female, the female is often larger than the male. Although the males are small, the millions of sperm carried by small adult males are enough to fertilize all the eggs carried by even larger females. In addition these females are capable of mating with mature males of any size. Thus, they are males when they are small and females when they grow up, and not only do they get a double chance to mate, but they also produce twice as many fertilized eggs as those fish that never become unsexed, which is a great benefit in reproducing their offspring.

The phenomenon of sex change, which occurs frequently in the animal kingdom, still does not have a satisfactory, scientific explanation, and requires further research and exploration by human beings.