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Molly Fish Species Profile

Characteristics, Origin, and Helpful Information for Hobbyists

Dalmatian Molly fish with black spots swimming near aquatic plants

The Spruce / Adrienne Legault

These fish are named Molly after the original scientific name of their genus, Mollienisia, as it was originally classified in 1846. Since then, the genus has been renamed Poecilia, after the family name Poeciliidae. This genus has some of the most beautiful and unusual varieties of live-bearer fish. The sailfin molly is arguably one of the most exotic fish available of any genus. Other species of mollies have similar appearances, but don't have the large dorsal sailfin. Mollies come in many color varieties. Mollies are relatively hardy, but prefer warmer aquarium water temperatures. When live-bearers are 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, sailfin molly, black molly, white molly, golden molly, lyretail molly, dalmatian molly 

Scientific Name: Poecilia sp.

Adult Size: Up to 5.5 inches

Life Expectancy: 5 years

Characteristics

Family Poeciliidae
Origin South and Central America, Mexico
Social Peaceful
Tank Level Mid-dweller
Minimum Tank Size 20 gallons
Diet Omnivores
Breeding Live-bearer
Care Easy
pH 7.5 to 8.5
Hardness 15-30 dGH
Temperature 78F
Male Sailfin Molly

The Spruce / Adrienne Legault

Female marble Molly

The Spruce / Adrienne Legault

Origin and Distribution

Mollies are mostly found from Southern United States down into Central America and northern South America. 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 Mollienisia, from which the name mollie was derived, has been re-classified as Poecilia and Mollies are 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 for over a century, resulting in many different color varieties and fin shapes. Solid black mollies and sail fin mollies with orange-bordered dorsal fins are common varieties. No other freshwater fish approaches the velvety black color of the black molly. There are also albino forms with red eyes, true lyre tails in most every color, and even chocolate, gold dust and orange varieties. The males of the molly species P. sphenops have short dorsal fins, while the sailfin mollies of the species Poecilia latipinna  and P. velifera have very large dorsal fins.

Sailfin Mollies are available in several color varieties. Although these fish breed readily, young raised in tanks may only develop the “sail fin” in a large aquarium. They may also be slightly smaller, reaching only 3.5 inches in length. But raised in a back yard pond, males with a large dorsal sail fin will appear at about 5 months of age, and grow up to 5-6 inches.

An abnormal genetic defect of the spinal cord has produced the Balloon molly. Through selective breeding it is now widely available. Balloon mollies can still reproduce, but may be more susceptible to health problems.

Diet and Feeding

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

Gender Differences

It is easy to determine the gender in Mollies as males have a prominent gonopodium, a modified anal fin used to inseminate females. Only males have the large dorsal “sail fin” in the species that do have enlarged dorsal fins. The females have a smaller dorsal fin.

The female has a fan-shaped anal fin. ​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. The female carries the fertilized eggs in her body. Each egg contains an embryo and is well furnished with nutritive elements. The eggs hatch within the female and she releases free-swimming babies. There is no parental care exhibited by this species, and the adults will eat the babies if they don't hide in aquatic plants. They mature quickly and breed readily, with females giving birth to a dozen or more fry every month.

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.

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. Treating Columnaris or Body Fungus – Not a True Fungus. Central Florida Aquarium Society.

  2. Fraser, B.A., Weadick, C.J., Janowitz, I. et al. Sequencing and characterization of the guppy (Poecilia reticulata) transcriptome. BMC Genomics, vol. 12, no. 202, 2011. doi:10.1186/1471-2164-12-202

  3. Poecilia latipinna. Florida Museum of Natural History.