Goldfish have two sexes, yet spend their youth essentially sexless -- and their gender development is surprisingly controversial. Once they reach sexual maturity, they breed once per year. This breeding season is the only time when there is any behavioral difference between the sexes.
Hot Times in Goldfish City
Water temperature controls every aspect of goldfish sex, from when eggs hatch, to which fish become male and which become female. Temperature must be right for fish to reach sexual maturity, and it controls the onset of the breeding season. Sexual behavior will not happen for goldfish until the mercury hits between 70 and 78 degrees Fahrenheit. After that, there still will be no goldfish action if conditions aren't just right, that is clean, still or gently moving water, low stress, enough food, and plenty of plant cover.
The Spawning Chase
The only difference between male and female goldfish behavior is how they behave when they are spawning. Males are the ones chasing and bumping other fish. Females are the ones swimming away. They will do this for many, many hours at a time. Some males chase both sexes, but it is easy to tell which fleeing fish are females, because they're the ones laying eggs.
Dude Looks Like a Lady
Detecting the difference between male and female goldfish takes some practice. Their differences are subtle. The anal fin is closer to the tail in males than in females, and the female cloaca (genital and anal opening) is more pronounced. Females are plumper than males, who tend to look more streamlined. Confusing the entire issue is that males often are "prettier," with longer, more flowing fins and tails, which of course, will make most visitors to your tank call them "she." Males in breeding condition are easy to spot right before the chasing starts because they grow temporary white bumps on their gill covers. Do not mistake this for a disease! They are afflicted only with love.
Most goldfish breeding manuals will tell you that it is impossible to tell the sex of a goldfish that is under one year old and less than 3 inches long. This may be because young fish don't have one -- goldfish sex is not determined by chromosomes, though fish do have two different sex chromosome genotypes (YY and XY).
Both genotypes can become male or female based on temperature. Exactly when a goldfish's gender is set in stone is an extremely controversial matter. Some experts claim it is before the embryos hatch, but a study from the Mumbai Central Institute of Fisheries shows that goldfish do not differentiate into males and females until they are several days or weeks old, "and that the timing of the sensitive period is itself dependent on temperature."
Some experienced fish keepers insist that even adolescent and adult goldfish can change sex. This isn't as unlikely as it may sound -- sex change is the norm for reef fish, and is common in freshwater fish as well. However, to date there is no scientific proof that this occurs in goldfish.
Angela Libal began writing professionally in 2005. She has published several books, specializing in zoology and animal husbandry. Libal holds a degree in behavioral science: animal science from Moorpark College, a Bachelor of Arts from Sarah Lawrence College and is a graduate student in cryptozoology.