Mollie Fish Species Profile

Characteristics, Origin, and Helpful Information for Hobbyists

Mollie

ho80 / Flickr / CC By 2.0

The genus mollie or mollienesia, as it was originally classified, has some of the most beautiful and unusual varieties of live-bearer fish. The sailfin mollie is arguably one of the most exotic fish available of any genus. However, mollies are perhaps the most difficult of all of the common live-bearers to keep as they are the most delicate of the live-bearers. When chilled or otherwise exposed to environmental stress of any kind, they tend to develop an ailment known commonly as “shimmies.” In reality, this is not an infectious disease but merely a compounding of symptoms of stress. 

Species Overview

Common Names: Short-finned molly, common molly, sailfin molly, black molly, white molly, golden molly, lyretail molly, dalmatian molly 

Scientific Name: Poecilia sp.

Adult Size: 4 inches

Life Expectancy: 5 years

Characteristics

Family Poeciliidae
Origin southern North America, Mexico
Social Peaceful
Tank Level Mid-dweller
Minimum Tank Size 10 gallons
Diet Omnivores
Breeding Live-bearer
Care Easy
pH 7.5 to 8.5
Hardness 15-30 dGH
Temperature 78F

Origin and Distribution

Mollies are mostly found from Southern United States down into Central America. The native habitat of these fish extends from the southern United States to the Yucatan peninsula in Mexico, and they thrive mainly in freshwater environments, sometimes venturing into brackish estuaries. Mollies, like guppies, are capable of acclimating to full strength sea water temporarily, but it is not their natural environment by any means.

The genus Mollienesia, from which the name mollie was derived, has been re-classified as Poecilia and is considered to be in the same genus as the guppy. For years it had been noticed that the guppy will, under certain circumstances, interbreed with the mollie, and the close relationship between the two has been confirmed genetically.

Colors and Markings

All captive mollie species have been interbred over centuries, resulting in many different color varieties and fin shapes. Solid black mollies and sail fin types with orange-bordered dorsal fins are common varieties. No other freshwater fish approaches the velvety black color of the black mollie. There are also albino forms with red eyes, true lyre tails in most every color, and even chocolate, gold dust and orange varieties.

Diet and Feeding

In nature mollies are nearly exclusively plant and algae eaters, so they must be fed lots of spirulina, even boiled spinach finely chopped, to remain healthy. If your tank lacks algae, you must provide “spirulina” flakes, special commercial Mollie food, or small amounts of chopped boiled spinach. They enjoy eating algae growth in the aquarium, and will graze endlessly on it, looking for choice patches to nibble. 

Sail fin Mollies are available in several color varieties. Although these fish breed readily, young raised in tanks rarely develop the “sail fin” and in fact only reach about 3-31/2 inches in length. But raised in a back yard pond, astounding, jeweled sail fins will appear at about 4 months.

Gender Differences

Only males have the “sail fin” and in the wild, can grow up to 5 inches long

In most live-bearers, the pregnant mother swells unmistakably and also presents the well-known “gravid spot” which is a dark spot near the base of the anal fin caused by the stretching of the peritoneal wall. The female has a normal-shaped anal fin.​

Slightly forward of this, inside her body appears a dark area which is known as the gravid spot. It is, in fact, equivalent to the womb, but unlike mammals, the egg is not attached to the mother’s body and fed by her directly.

Each egg contains an embryo and is well furnished with nutritive elements, provided by the mother's system, on which the developing embryo feeds during its development. However, recent studies show that there is a symbiotic relationship between the mother and the developing fry with an exchange of fluids, the total extent if which is still under investigation.

More Pet Fish Species and Further Research

If mollies appeal to you, and you are interested in some compatible fish for your aquarium, read up on:

Check out additional fish species profiles for more information on other freshwater fish.

Article Sources
The Spruce Pets uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Mexican Molly (Poecilia sphenops). U.S. Fish & Wildlife Service, 2020

  2. Poecilia LatipinnaFlorida Museum