Sexing certain species of animals is pretty easy, what with clear as day differences such as female kangaroos having pouches unlike their male counterparts. Not all animals are this easy to classify, however. Although goldfish are indeed sexually dimorphic animals, it often takes more time and effort to be able to correctly identify the gender of one.
Male and female goldfish do not look exactly the same, and because of that are considered to be sexually dimorphic, according to FishChannel.com. Although marked physical differences do exist between the sexes, they sometimes are quite difficult to discern. It may take a little patience to be able to come up with the correct gender of a goldfish.
In some cases, you may be able to identify the gender just by looking at its overall size. The female varieties tend to be slightly more rotund than males, and just bigger in general. If a goldfish seems to be more elongated and lithe, then it may just be a male. The vents of female goldfish are usually larger and more circular than those of males. Male vents are long and often have a triangular outline.
In fully grown male goldfish, you may notice that the pectoral fins not only are usually more significant in length, but often are more thick, jagged and sharp in appearance, as well.
Anal Region Differences
You may be able to distinguish differences in the anal regions of male and female goldfish. In female goldfish, the opening tends to jut out, while it is usually more hollowed in the males. The opening in female goldfish usually also has an oval form to it, unlike males.
Breeding Season Changes
Identifying the gender of goldfish is typically a much more simple process when mature specimens are involved -- particularly during breeding season, which usually occurs in the heart of the springtime each year. Around this time the belly regions of female goldfish usually seem a little bit thicker. Male varieties also display tubercles, which are small white raised spots that show up on the plates of their gills, as well as on parts of their anal and pectoral fins.