What are egg layers and why can't they get pregnant?

Answer 1

An 'egglayer' is simply a fish that lays eggs. This is in contrast to the 'livebearers' which give birth to live young. The livebearing fish that are common in the aquarium hobby are all in the family Poeciliidae, and they include platys, guppies, mollies, swordtails, limia etc. Virtually everything else - all our catfishes, tetras, barbs, danios, rasboras, and countless more common species - are egglayers.

Egglaying fish cannot become 'pregnant' - ie carrying their embryos - because their eggs are not fertilised until they leave the female's body. A female egglayer, when ready to breed, may look 'pregnant' ie noticeably swollen in the abdomen because of all the eggs that are ready to be laid. But if she does not have the opportunity to mate, she will usually reabsorb the eggs - if she lays them, they will be infertile and not develop.

This can be contrasted with livebearers, where the male fertilises the female internally by means of a modified anal fin - this tube-shaped organ, called a gonopodium, delivers his sperm directly into the genital opening of the female. The baby fish, called fry, develop inside the female, and are born alive after a gestation period. When born, livebearer fry are usually largeish (several millimetres long) and fully developed miniature fish.

Egglayer fry, on the other hand, begin life as larvae - little wriggling blobs with no eyes or fins, that usually stay put. After several days of development they become 'free swimming' fry (meaning that they have eyes, fins, and take a horizontal position in the water, and are able to swim.) At this point, they are usually barely 2mm long. Because egglayer fry are so tiny, it is a lot more difficult to raise them - they usually need special liquid foods. It is far more common for the aquarium hobbyist to breed livebearers in a home aquarium because they will breed without any assistance or interference from the aquarist.