Of all the tropical fish varieties kept as pets, the freshwater molly is perhaps the most diverse.
Over centuries of captive breeding, it has developed an incredible range of colors, shapes, and fin types.
There are golden mollies, short-finned mollies, sailfin mollies, black mollies, lyretail mollies, and many more varieties spread over three interrelated species.
Molly fish are not only beautiful to look at, they’re delightfully easy to maintain and breed. They’re what’s known as a livebearer which means their babies develop in eggs that are internally gestated.
These babies (or fry) emerge from the mother fully formed having been fertilized, grown, and set loose from their eggs while still inside her uterus.
Molly fish stay pregnant for around sixty days and can give birth to as many as a hundred babies per pregnancy.
First-time breeders and young mothers tend to produce fewer fry than this, but should still be expected to birth between forty and seventy live babies.
A Quick Guide to Breeding Molly
Mollies breed via internal fertilization whereby the male inseminates the female’s eggs while they’re still inside her.
It’s a straightforward, low maintenance process for breeders but it can lead to some surprises for pet owners.
Female mollies can store sperm for many months and use it to fertilize eggs every thirty days.
Female mollies bought as pets for tropical aquariums commonly fall pregnant despite there being no males in their environment. This can be bewildering for fish owners who don’t understand the species’ reproductive habits.
How to Tell If Your Molly Fish Is Pregnant
It’s common for molly to be bought as pets while pregnant already so owners should learn to recognize the signs.
Only professional breeders can be certain – showing the following symptoms is not a definite sign of pregnancy – but look out for a swollen stomach, a dark triangular patch around the fish’s anal vent, and changes in behavior.
Pregnant mollies seek warmth and will often spend time close to an aquarium’s heater.
They move around less, eat more food, and spend increasing amounts of time away from their tankmates.
The average gestation period for a molly fish is sixty days though some varieties, like the Dalmatian molly, stay pregnant for seventy days.
Owners are advised to separate pregnant females from their tank mates in a breeding cage around the fifty-day mark.
This provides security and calm for the expectant mother and enables the fry to be born into a safe environment free of potential predators.
Molly fry can be born into the same space as other fish in an aquarium but there’s a risk of predation while they’re very young and extremely small in size.
If you cannot separate the mother and her babies, make sure there’s plenty of plants for the tiny fry to hide in. It won’t guarantee survival, but it will increase their chances of living and growing.
Add broad, leafy plants to your aquarium to provide ample cover.