Koi have been a part of Asian culture for a very long time. In recent years, they have become popular in other parts of the world. There are different types of koi, but all koi are a form of carp, just like goldfish. As with most fish, telling the difference between males and females can be tricky, but there are some key variants that can help.
One of the most obvious differences between male and female koi of any age is the fins. Fins on female koi generally are less colorful than those of the males. Female koi fins also tend to be larger. The edges of a fins on female koi are more rounded and more opaque than the fins of males. The fins on the male koi have a more pointed appearance and tend toward more solid coloration.
Female koi are, in general, larger than their male counterparts. That aspect of their physiology is the result of selective breeding to protect the females when put in with groups of male koi. In other words, koi breeders would usually put one female with a group of males to create a more diverse gene pool. In order to protect the female fish from injury, they used larger females and smaller males, and those traits were passed on to their offspring.
There are definite shape differences in adult koi based on the gender. However, those differences are not as apparent in the young fish. That's because it's caused by the eggs carried by the female koi, which don't develop until maturity. In general, male koi have a narrower, sleeker appearance. Female koi, on the other hand, are shaped more like a blimp with a more rounded body. That shape is caused by the eggs that they carry.