I now know they are livebearers, and that this species ranges from the humble guppy right through to the Great white shark. You will be aware of the fact that the Great white shark has live fry. I think that is what attracted me to the guppy was the delight of seeing the young in the tank. You will find livebearers from Lake Baikal in Russia, to Cuba, the Amazon and various other parts of the world. Lake Baikal is on my bucket list for next year!
The different locations these fish live in the world means that species have evolved into many varieties of livebearers with their own means of reproduction. The guppy evolved, as have the Platies and mollies, into fish where the male anal fin becomes a rod type appendage when the fish reaches sexual maturity.The gonopodium is not hollow, instead it forms a groove created by the rays of the fin, allowing the male to send the sperm to the female. The end of the gonodium has small hooks that catch the female in the correct position; this ensures sperm is directed into the female.
Amazingly, the female can store sperm for using later. Therefore, when you buy a pregnant female from the store or a specialist it is very likely you will have several broods of fry without the presence of the male in the tank. If you have two or three females, it makes a quiet aquarium for them. I think half the pleasure in having a male is watching his desperate attempts at mating!
Having just females allows you time to take note of when they are due to drop their fry. It is a lot easier to notice when they swim quietly with out being hassled by the male. You then have time to move the female into another fry tank if you wish to increase your stock, or start a breeding scheme. Separating the mother allows you to pick out color varieties that you fancy specializing in once the fry have grown. Keep in mind this can take a long time and a lot of patience. The females breeding every month to six weeks keeps you busy and interested.
If you have a tank with no restraints on who breeds with whom, keep a look out for what is called a sport. This term means you may see a fish that is very different to the rest of the aquarium. It will still be a guppy, if that is what you have in the tank. The difference will show in its size, color, fins or tail style. If it attracts you, make efforts to separate it and consider how you could change or what improvements would be possible. This is where some of the fascination comes from in having fish.
Over my years of keeping fish, I only ever struck one of these fish, so it does not happen very often. When you consider the number of fry that are born it becomes quite rare.