Fish
Molly
Poecilia · also called mollies, molly fish
A molly is one of the oldest standbys of the home aquarium, a hardy grazer that shrugs off conditions that would kill a fussier fish and breeds so readily it barely needs help. It is also the group that produced one of the strangest animals in fresh water: the Amazon molly, a fish with no males at all, every individual a clone in a female line hundreds of thousands of years old.
Overview
Mollies are livebearing fish of the genus Poecilia, from the warm coastal lowlands of the Americas. They graze algae and biofilm with comb-like teeth and give live birth to broods of 20 to 100. They are notably salt-tolerant — wild sailfin mollies move between fresh streams, brackish marsh, and the sea, and many aquarists keep them in lightly salted water.
Sails and clones
Male sailfin mollies raise a huge dorsal fin like a flag in courtship and combat, and females prefer the biggest sails, which is why the fin grew so exaggerated. The Amazon molly reproduces by gynogenesis: it mates with males of other molly species, but their sperm only switches on egg development and passes no genes, so every daughter is a clone of her mother. It began as a single hybrid over 100,000 years ago and, against expectation for a sexless lineage, has flourished ever since.
In the aquarium
Today’s common molly is a century-old blend of two or three wild species, bred into black, lyretail, Dalmatian, sailfin, and balloon forms. They are easy fish given hard, alkaline water, space (a common molly reaches 6 cm, a sailfin over 10), some vegetable food, and good filtration — and given males and females, a steady stream of fry.
Status
Least Concern — the widespread species are common and several have become invasive where released. A few mollies confined to single Mexican springs or caves are genuinely threatened by groundwater loss and pollution.
Molly: common questions
Are mollies easy to keep?
Mollies are hardy and generally beginner-friendly, but they have two needs people often miss. First, they are big grazers that do best in hard, alkaline water and often benefit from a little aquarium salt, so they struggle in the soft, acidic water that suits tetras. Second, they grow larger than many expect, up to 6 cm for a common molly and well over 10 cm for a sailfin, and they are active, so they need a tank of at least 75 to 115 litres, more for sailfins. Given hard water, space, some vegetable food, and good filtration, they are tough and long-lived. Balloon mollies, with their deformed spine, are less robust.
Why do my mollies keep having babies?
Because they are livebearers and, like their relatives the guppies, they breed continuously. If you have males and females together you will get broods of 20 to 100 fry roughly every four to six weeks, and a female that has mated can keep producing broods for months from stored sperm even after the male is removed. In a community tank most fry are eaten, which keeps numbers in check, but in a well-planted or species tank the population can climb fast. Keeping only males, or only females, is the usual way to avoid a population explosion.
What is the Amazon molly?
The Amazon molly is a wild fish, native to Texas and northeastern Mexico, that consists entirely of females. It reproduces by gynogenesis, meaning a female mates with a male of a different molly species and the sperm activates her eggs but does not fertilise them, so her offspring are all female clones of herself. The species began as a one-off hybrid between two other mollies over 100,000 years ago. It fascinates biologists because clonal, sexless lineages are expected to be short-lived, yet the Amazon molly has survived and thrived for an extraordinarily long time.
Can mollies live in a saltwater tank?
Mollies can be acclimatised to full marine salinity and are sometimes used to help cycle a new saltwater aquarium because they tolerate shifting water chemistry better than most fish. Wild sailfin mollies naturally move between fresh and salt water. That said, a molly is not a reef fish and looks out of place in a marine display, and moving it between very different salinities must be done slowly, over days, to avoid shock. Most molly keepers settle on hard fresh water or lightly brackish water, which keeps the fish healthy without the complexity of a marine setup.