Live-bearing aquarium fish

Certain species of fish are ovo-viviparous. This means that while the fry still incubate inside eggs, these eggs are not expelled at an early stage of their development, but are retained within the female's body and the fry are allowed to hatch inside her before they are expelled. These fish, for example the guppy, are among the most popular aquarium fish.

Placental mammals, on the other hand, are viviparous, and have dropped the egg stage altogether.

Almost all popular live-bearing aquarium fish belong to the family Poecilidae,that is, the guppies, mollies, platies and swordtails[?], but some species of sharks and rays also bear live young.