The difference between males and females can be seen in more than one way. The female is plumper than the male and also the male has a gonopodium, which is a modified anal fin. The gonopodium is used to insert sperm into the females. Sadly Livebearers, if hungry, will eat their fry. When buying Livebearers of both sexes, make sure you have more females to males. Livebearers mouth is angled upwardly therefore they eat food, which is above them. Needless to say, they swim from mid-top levels.
Guppies (Poecilia reticulata) have many different colours and shape of the caudal fin. There is no relevance to males to choose females close to them. In fact, the female does not have the colour to the males. Often females might have a colourful tail, but often not. A guppy is mature usually 3 months from being a fry and therefore females can produce many fries. In the wild, they are in Central America to Brazil and regularly they eat mosquito larvae, which obviously helps humans. Their growth goes to 5cm (2") however in tanks, they are usually 2.5cm (1").
Mollies (Poecilia sphenops) come in various forms, Sailfin, Marbled, Lyre-Tailed and in different colours. Even though I have not listed in this section of Livebearers, they have a different scientific name, however Sailfin Molly is Poecilia velifera. A Molly in wild, comes from Mexico to Columbia and can growth go 12.5cm (5"). Some of them go into brackish waters and it has been said to add some salt into a tank. I have not and have not seen any problem.
Platies (Xiphophorus maculates) are the same with variants. Red, High-Fin, Wagtail, Moon, Tiger and Tuxedo are some. They are found in Southern Mexico and can grow to 7.5cm (3"). Like all Livebearers, Platy can be the first fish in a new tank set-up as they tolerance non-mature tanks. Again like Livebearers, they require plants, which equally protects the fry by swimming in them.
As you can tell by their name, Swordtail (Xiphophorus helleri), the males have that tail, however females do not. Again a number of variants such as Green, Marbled, Black and Lyre-Tailed. In Central America, their colour is green, but not easily seen. They can grow to 10cm (4"), but less in a tank.
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