A livebearer is a fish that utlizes internal fertilization, becomes pregnant, and gives birth to live young. Livebearers fall into two categories – ovoviviparous and viviparous. Ovoviviparous livebearers keep the eggs inside their body. The embyros develop within the eggs until they hatch and come out of the mother’s body fully formed. Viviparous livebearers keep young inside their body and birth live young as well. But the egg cell divides into the embryo and is attached to the mother by a placenta (like in mammals.)
An easy way to tell if almost any fish is a livebearer (gets pregnant) or an egglayer (can’t get pregnant) is to look for something called a gonopodium (or gonopod.) This is a thin, long, controllable fin underneath the fish’s body that delivers sperm to females.
All livebearers (at least as far as science is aware) have this or a similar appendage to deliver sperm while egglayers do not. Most male livebearers can be seen, at one point in time or another, “flexing” this fin.
An example of a livebearer without a gonopod is halfbeaks – they have a modified anal fin called an andropodium, but it acts quite similarly. We see these modified fins in sharks and cousins like skates and rays as well, however, they’re called claspers. In nature, there are apparently few ways to effectively deliver sperm to internal eggs.
Interesting sidebar: this appendage (or appendages in the case of sharks and cousins) can have some gnarly hooks that cause inflammation. It’s an interesting mechanism to lock in their sperm and lockout the sperm of potential rivals.
Without this fin – or a modified version of it – they’re almost certainly egg layers.
Examples Of Livebearing Fish
This is by no means a definitive list, but here are a few fish that can give birth to live young that are commonly kept in aquariums: