In many of the wrasses, sex change correlates with social hierarchy and social behavior. In many wrasse species, social structure includes a large dominant male and many smaller, subordinate females. Removing the male from the group triggers the largest female to begin transforming into a male. In some species, females can revert back to males. In a few species, self-fertilization is possible.
Some species are unisexual.
In unisexual species, there is no fusion of sperm and egg. The sperm is necessary to trigger the egg cell to develop into an adult, but the sperm cell ultimately degenerates and does not take any part in heredity. The resulting young always are females, with no trace of paternal characteristics. Thus, unisexual species are entirely female. They mate with males of related species to produce female offspring.