When my know-it-all college friend saw my new fish tank he started saying how goldfish are females when they're young and then turn into males later in life, just like that. Ha, I wasn't born yesterday. Cecil, tell this bookbrain to cut the bullshit and pay up on our fish story bet.
No dice, junior, although maybe you can call it a draw. Sex reversal is fairly common among some of your lower order critters, fish among them. Changing from female to male is called protogynous hermaphroditism, and the other way around is called protandrous hermaphroditism. If you’ve got both male and female appurtenances at the same time you’re equipped for synchronous hermaphroditism, in which case fertilization takes place by means of mutual copulation, the erotic possibilities of which I leave to the imagination of the reader.
Goldfish (Carassius auratus) are generally pretty straitlaced, but a Russian researcher some years ago reported he had discovered a protogynous version of C. auratus gibelio in a shallow lake on the steppes. Most of the fish were born female, but at about the three-year point a small percentage converted to males, enabling the species to perpetuate itself.
One character out there claims that many varieties of goldfish routinely change from male to female at the two-year point. This may conjure up a pretty racy idea of partying down with the goldfish. ("All right, kids, it’s midnight! Everybody switch!") However, other researchers don’t buy the idea, and in any case the piscine sexual apparatus is not as impressive as the human version–sex reversal is mostly a matter of switching ducts on a sort of all-purpose gonad. Pretty low on the hubba-hubba quotient, and imagine the arguments in the bedroom: "Keep your fins off me, buster, unless you agree to be the mom."
Send questions to Cecil via firstname.lastname@example.org.