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How to Tell the Difference Between a Male & Female Goldfish

Updated February 21, 2017

If you or your child is eager to name the new family fish, it will be helpful to know its sex. Knowing the sex of your goldfish can also help you if attempting to breed them. If you want to buy mature goldfish that are ready to breed, it will be easy to select a male and female. Before goldfish mature, however, it is much more difficult to tell which sex they belong to.

Shape

When both goldfish are swimming through the tank or bowl, take a bird's eye view of them. Mature goldfish will have a distinct body shape at maturity because the female needs to be wider and softer to carry eggs. Male goldfish at maturity will retain a thinner, torpedo shape that is characteristic of both sexes when young. Female goldfish may also appear larger on one side of the body rather than plumper overall.

Body

The fins, gills and vents are the only truly defining characteristics on any goldfish. Under and behind the gills of the goldfish, you will find its fin. On a male goldfish, the fin will be thicker, pointier and more rigid than the female's. In comparison, the female's will be shorter and rounder. The vent can be found at the back of the fish and is the anal opening. When a female fish is ready to breed, her vent will become larger and rounder, from concave to convex. The male's will remain small and concave.

Spawning

Only the male will develop new growths when ready to spawn. This growths are white bumps called tubercles. You can find tubercles on the male's head, gill covers and pectoral fins. The fins develop tubercles only when the fish have begun to breed, however.

Behaviour

Male fish are aggressive when it comes time to breed. A male fish will chase the female around the tank or bowl to initiate breeding. The male may go as far as knocking against the female to force release of the eggs. You will find that this chasing is relentless, making it an easy behaviour to catch and observe.

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About the Author

Michael Monet has been writing professionally since 2006. At the San Francisco School of the Arts, he studied under writers Octavio Solis and Michelle Tea, performed his work in Bay Area theaters and was published in literary journals such as "Paradox," "Umlaut" and "Transfer." Monet also studied creative writing at Eugene Lang College in New York and Mills College in Oakland.