Sex-swapping fish.

Hey, Matthew:

My friend swears that he heard that all goldfish are born female but some of them eventually turn into males. Could this possibly be true?

-- Ron, Santee

I hope your pal's quit school and his job and is now just hanging around the tank so he doesn't miss the big moment. Sexual assignment can be a changeable thing in the lower orders of animals, including some fish. If the pond gets too crowded or there's some other stress on the population, they can do a male-female swap to save the group. Gender swapping in fish amounts to rerouting some minor internal plumbing, nothing very dramatic. Some lower animals dispense with gender altogether and are both male and female, saving themselves a lot of dreary evenings in singles bars. Your friend's latched onto a fact-like story but applied it to the wrong species. His goldfish aren't going to do anything except swim, eat, and die.