Introduction to livebearing toothcarps

Livebearing fish species occur in a range of fish families and are to be found in multiple parts of the world. All have their own requirements and behaviors. Most known livebearers are endemic to South- and Central-America and Southeast Asia. But because of the fact that endemic locations seem tropical to a lot of people, livebearers are considered being tropical fish. Well, that's an incorrect statement! Yes, there are tropical livebearers but there are also subtropical livebearers. And even within the tropical zone on this globe there are sub areas that have a subtropical climate. Specifically, certain altitudes won't have the tropical conditions in a tropical globe zone. Most livebearers found elsewhere on this globe are mostly feral strains.
By most people most known livebearers are guppies, mollies, swordtails and platies which belong to the Poeciliidae family. However, there are a lot more livebearer species (poeciliid fish and others) which are kept by aquarists. The majority of them are considered being rare livebearers. And most are kept by serious and experienced aquarists.
Most people do keep the cultivated domestic species of the Poeciliidae family at home.

In general fish are wether oviparous or viviparous. When fish are oviparous, the females will spawn the eggs. The fertilization can happen internal (true oviparity) or external (ovuliparity).

When it comes to livebearers, they won't lay eggs (oviparous) but have embryonic development within the mother after an internal fertilization. And when the female is due, she'll give birth to  free swimming offspring. 

To most people, livebearers are all alike when it comes to reproduction. But that ain't correct for there are two ways how these kinds of fish reproduce. Most known are the ovoviviparous livebearers like e.g., mollies, guppies, swordtails, platies and so on... But there are also the true livebearers like e.g., goodeids (splitfins). These true livebearers do have a reproduction which is pretty similar to mammals.
True viviparous livebearers like goodeids in the embryonic state are being nourished by the mother through the socalled trophotaenia (or also spelled as trophotaeniae). It's in some way similar to the umbilical cord (placental viviparity) in mammals. These trophotaenia will be shed or absorbed (they'll nourish themselves through the trophotaenia) in the first till two days after birth. These trophotaenia do look like a cluster of small umbilical cords.

With ovoviviparous livebearers, the embryos have no placental attachment and will be totally nourished from the yolk sac. So, the mother does not nourish the embryos, she's just a safe place before the eggs will hatch. By the time that the mother is ready to give birth, the eggs hatch and the fry will pop out of the female's body. Mostly they're curled up when they leave the mother's body. But sometimes when it takes a bit longer before fry can leave the mother's body, the rear end of the fry will come out first.
The fry do have a big belly. They'll nourish themselves the first till two days after birth with what they've stored in their belly. And than they'll slim down and than they'll look for food themselves.

A lot of livebearers are known to reproduce well in captivity. There are exceptions to this rule and some livebearers are very hard to breed in captivity. They might require particular circumstances in order to get them to breed. If the right conditions are not given to them, some female livebearing species will often miscarry, give birth to the embryos too soon or even absorb the embryos.

Since the male livebearer has to fertilize the eggs internally, all male livebearers have an anal fin and this has developed into a reproductive organ that can be inserted into the female. With males of an ovoviviparous species, the reproductive organ on the fish is called "gonopodium". Males of a viviparous species e.g., from the families Goodeidae and Hemirhamphidae, the reproductive organ will be called "andropodium". Note that the andropodium is just a sort of notch on the front part of the anal fin.

One of the main differences between the two male sexual organs is that an andropodium will fold when the male inserts it into the female, while a gonopodium will flip forward to insert the female as shown in the picture below.

Many people think that keeping multiple kinds of livebearers in one tank will create inbreeding. Most of the time that will not be the case. First of all, when it comes to ovoviviparous livebearers, the shape of the gonopodium of each species do differ. And the female sexual organ is adjusted to the gonopodium of a male of her own kind. In order to interbreed with another livebearing species, the sexual organs need to be compatible. But if two species are closely related to eachother, the bigger the chance the sexual organs will be compatible.
Are there two or more different livebearing species in one tank, their first preference is always to mate with one of their own kind if both genders of that particular species are present. Is there just one gender of a specific species present, only than it's most likely that a crossbreed with a sexual compatible tankmate of the opposite sex can be established.

The opening of the sexual organs of both genders are called gonopores.

The difference between the shape of the anal fin between male and female of both ovoviviparous and vivivaprous livebearers will be shown by these pictures below.

Looking at these pictures shown above, it's obvious that there is a clear difference in the shape of the anal fin of both genders. And also wether it concerns an ovoviviparous livebearer or a viviparous livebearer. So  again, don't think that all male livebearer specimens have a gonopodium. And that not all female  livebearer specimens have a V-shaped anal fin. For that would be a complete misunderstanding!

And to refer to the different shapes of the gonopodium again... Here are some pictures which show how different those gonopodiums are shaped depending on the livebearer specimen.

If you'd enlarge these pictures, you'll be able to see clearly that gonopodiums do differ in shape from the start to the tip of the gonopodium. This is very essential to the gonopore of the female to enable a mating. This is one of the main reasons why not all ovoviviparous livebearers can crossbreed with another.

Note: When an anal fin of a young male will develop into a gonopodium, the anal fin starts to change from the third, fourth and the fifth fin ray.

Well, most female livebearers do have a gravid spot. This is the most translucent part of the skin, close to he anal fin and where the sexual organ of the female is situated. Most females do show this spot already a couple of days after birth. It just needs a good eye to spot this gravid spot at such an early stage. Of course, with female livebearers which are very dark based like e.g., black mollies, even with a very good eye sight, you probably won't be able to spot the gravid spot.
Note: I've mentioned that most female livebearers have a gravid spot for there are also species of which the females don't have a gravid spot. 
It's also a misunderstanding that when a female's got a gravid spot, that she must be pregnant. It's a misunderstanding that can be read at a lot of forums. Having a gravid spot does not mean pregnancy by all means. For actually, females are born with it. It takes a bit before they show it clearly to repeat myself. As already mentioned, the gravid spot is the most translucent part of the skin. But when the embryonic state develops with a pregnant female, the skin will stretch more and more and so this part of the skin becomes more translucent. What we call the dark gravid spot is basically nothing more than developing embryos that will shine through the translucent gravid spot. And that's why it seems to get darker. At a certain moment you'll be able to see those eyes of the unborn fry through the female's skin.

Another phenomenon that may occur with xiphophorus strains (mostly platies), is that a juvenile male is devloping a socalled "pseudo gravid spot". In general the pseudo gravid spot shows up during the transformation of the anal fin into a gonopodium or after the gonopodium has been formed. A pseudo gravid spot hardly shows up before the transition of the anal fin. This has got nothing to do with a female turning into a male. For that's a complete other story.
Note: When a pseudo gravid spot shows up on a male's body, it will never dissapear.

Above: An adult male Xiphophorus evelynae with a pseudo gravid spot.

Another misunderstandig is that you'll need a male:female ratio of 1:2 or 1:3 at all times. The socalled harrasment behavior of males goes for just a certain number of livebearers. And most of the times, you see this happen with a certain number of cultivated domestic species. There are a lot of livebearers which can be kept without a problem at an 1:1 ratio in a tank.
For instance, do all male guppies harrass females? No! Definitely not! There are a lot of wild but also fancy guppy strains which are just fine at a ratio 1:1. The problem overhere is that over the years we've been taught a simple rule which we've been generalizing over the years for all guppies (also other livebearers of course). Experienced serious aquarists and breeders do know that this is a fact. Unfortunately, this rule and so many others have been copies of statements that someone made up once just to ensure to put the harassment to a low level and we've been continuing these rules as being true.

Although, a number of male livebearers tend to chase females all day... that doesn't mean that a mating will take place. Eventhough, a male is trying to use his sexual organ. For it's the female who decides if a mating will actually happen.

If a mating does happen, depending on what kind of ovoviviparous livebearer it concerns, the female is able to store sperm in packages for a certain time period. This means that when a mating has happened that the female does not have to be pregnant. She'll decide if she let's a sperm package fertilize her eggs. And only if that happens, she'll be pregnant. The time period of sperm storing can take as long as over a year. So, not just a couple of months as some like to claim. But I do have to mention that the longer those sperm packages are stored, the lesser the quality of the sperm will get. This all means also that such a female can become pregnant multiple times, from 1 -7 times (mostly less). So, not maximum 3-4 times as I do read from time to time.
But as I've already mentioned "depending on what kind of ovoviviparous livebearer it concerns", means that not all ovoviviparous female livebearers are able to store sperm. Those ovoviviparous female specimens that can't store sperm and the true viviparous female livebearers, do need a new mating in order to start a new pregnancy. So, a resumé: Not all female livebearers are able to store sperm packages!

There's also another way of reproduction with a certain number of livebearer species. The superfetative livebearers. Superfetation means multiple pregnancies with embryonic development in different stages. Such pregnant females will drop 1-3  fry every single day or 2-3 days during a period of approximately 10-14 days. It's also a different way of sperm storage. Sperm from one mating is only used for one batch of fry during this labour period of 10-14 days. So, a new mating needs to happen in order to establish a new cycle of pregnancy.

A unique reproduction method is gynogenesis. It's a bit similar to parthenogenesis but with parthenogenesis it involves the development of a female sex cell (gamete) without fertilization. But with gynogenesis, insemination of an ovum (egg cell) is needed to activate the development of the egg but without the fusion of sperm and the egg nuclei. Which means that the genetic package of the male will not be passed on and so the gynogenetic embryo develops at the expense of the ovum nucleus only. Because of this reproduction method, the fish are asexual (only females). And while the genetic input of a male is of no longer value, the offspring will naturally be all female and identical clones of the mother. 
Because of the fact that such fish are females, they need to use sperm from a male related to their own species. A good example of a livebearer which reproduces itself by gynogenesis is the Poecilia formosa also known as the Amazon molly.

Overhere an overview of all fins of a male and female ovoviviparous livebearer.

A "majority" of the livebearing species will prefer alkaline water where the pH is between 7.0 and 7.8. In general, the water should be hard or medium hard.