http://www.straightdope.com/classics/a1_071.html
I guess I didn’t know about the controversy over goldfish changing sex. But Cecil’s correct that fish are really quite flexible in these matters. And then there is the issue of estrogen-mimicing pollutants causing imposex and ovotestes.
At least one species actually changes both ways each year, and performs a spawn as a male and as a female in every year. In some coral reef fish, a male fish has a harem of females. If the male dies or is usurped, the dominant female in the group will become a male and take over the harem. All these fish are born as females, and the only way they ever become males is if they take over a harem. There are several species of fish that have no males, and reproduce by parthenogenesis. All young are essentially clones of the mother. One of these species (a molly, a live bearing fish) actually requires fertilization by a male of another, closely related, species to complete the cycle. The male provides no genetic material, but somehow being internally inseminated gives the female the cue to start the reproductive process. This is considered reproductive parasitism, because the male receives no reproductive benefit (although I guess you could contend that he benefits by getting laid).
To add weirdness, some tilapias actually engage in :eek: oral sex. The male builds a spawning bed, the female comes and lays her eggs in it. She then picks up the eggs in her mouth. The male has some yellow dots on his anal fin that look like eggs. As the female tries to take in these faux eggs, he releases his milt, and the eggs are fertilized within the mouth of the female. She then carries the eggs in her mouth until they hatch, fasting until that time.
Fishhead