Guppies have a special place in the aquarium hobby. You would be hard-pressed to find a pet shop that doesn't have several varieties of guppy for sale. Part of their charm is that guppies easily breed in captivity, which makes selective breeding less complicated. This has led to the production of dozens, if not hundreds of fancy guppy varieties. If you want to breed guppies yourself, you need to learn how to tell male guppies from females.
The Dead Giveaways
You can tell a male fancy guppy is a male by his gonapodium. The gonapodium is a copulatory organ that only male guppies have. The gonapodium is formed when the anal fin rolls into a tube. A female has a regular or triangular anal fin. The anal fin is on the underside of the fish, where the abdomen meets the tail, between the paired pelvic fins. This organ will look like a small tube or stick on a male. Keep in mind that when a female guppy has her fins clamped against her body, the anal fin may resemble a gonapodium. However, a healthy female won't clamp her fins for long.
In all types of fancy guppies, males are more colorful than their female counterparts. While fancy guppies come in many variations, the females almost always resemble wild livebearers with drab silvery bodies. However, some females have color in the caudal or tail fins and in the dorsal or back fins. But if you compare these females to males of the same variety, you will find them bland by comparison. Some females have a dark spot near the anal fin called a gravid spot. However, this spot may not be obvious unless the guppy is pregnant.
In addition to bright colors, male guppies usually have longer fins than females. Fancy guppies have many different fin shapes, including elongated "veil tails" or unusually shaped tails like "lyre tails," whereby the top and bottom of the fin jut out. Females rarely have any sort of changes to their fin shape, and usually have standard or wildtype tail and dorsal fins. Some female fancy guppies have somewhat longer tails than wildtype guppies but, if you compare them with males of their same variety, they are shorter.
Size and Shape
You can tell males from females by the general shape of their bodies. Fish breeders sometimes use shape nuances to spot males and females early in development, before fish develop their full adult coloration and fins. Female guppies are always larger than males of the same age, variety and diet. Female guppies can grow twice the length of males. Additionally, females have more rounded bodies, notable at the lower rear margin of the abdomen or when viewed from above. Males are more slender than females.