Molly

First identified by naturalists during the 1820s, Molly fish are American fish that are primarily found in the wild in areas of North and South America. Since around 1899, the Molly has been a popular fish to keep in freshwater aquariums. All species of molly fish can hybridize with one another, so their appearances span many colors, patterns, and configurations. Most species still have large populations in the wild, so this fish is not considered to be endangered. Technically omnivores, they feed primarily on plant materials.

5 Amazing Facts

• Male molly fish, who tend to be much smaller than female mollies, have a specialized anal fin called a gonopodium. This rod-shaped organ is used to deposit sperm in female mollies, who can retain it for extended periods of time.
• The selective breeding of Molly fish through the years has produced a dazzling array of colors and patterns. These fish also vary widely in terms of the shape of their dorsal and tail fins.
• Hybridized versions of Molly fish started appearing during the 1920s, and they quickly became popular with aquarium enthusiasts. Although hybridized Mollies are abundant in artificial environments, they aren’t typically seen in the wild.
• Mollies are shoaling fish, not schooling fish, which means that although they tend to travel in groups, they do so more for social purposes than for traveling in specific directions with each other.
• The most common varieties of molly fish typically available for aquariums are the common molly, the black molly, the white molly, the sailfin molly, and the Mexican molly.

Classification and Scientific Name

The Molly fish belongs to the Poeciliidae family and is part of the Poecilia genus. The term “Poecilia” means “many colored.” It is derived from the Greek word “poikilos,” which means “with a lot of colors.” This reflects the fact that these fish tend to vary considerably in terms of the colors and patterns that they exhibit. Examples of different species of molly fish include:

• The common, or short-finned, molly, Poecilia sphenops. “Sphenops” means “wedge appearance” and refers to the fact that this species’ fins are stubby and wedge-shaped.
• The sailfin molly, or topsail molly, Poecilia latipinna. “Latipinna” means “broad fin,” which reflects the fin shape of this particular species.
• The Mexican sailfin molly, Poecilia velifera. “Velifera” means “sail-bearing,” which refers to the unique sailfin that is found on this species.

Mollies are related to the platy, swordtail, and guppy fish. They have a similar appearance to these cousins, but they are a bit larger and more lively.

Appearance and Behavior

In the wild, the common molly exhibits a dull, silvery color. While their bodies are normally light gray in color, mating males may exhibit more of a greenish-blue color. The sides, backs, and dorsal fins of many mollies are dotted with rows of spots. These spots can be so crowded together that they almost become one large pattern.

Most species of molly fish have chunky, thick-set bodies; rounded fins; tall, angular dorsal fins; and pointed heads and mouths. Their upturned mouths are designed to help them to skim the upper layer of the water, which is rich in oxygen. Their tails are convex.

In the wild, mollies can grow to be as large as 20 cm or so in length. However, most Molly fish average between 5 and 13 cm in length. Males tend to be smaller than females, and female mollies usually have rounder bodies. One of the biggest distinguishing characteristics of male mollies is their gonopodia. These are specialized anal fins that double as narrow copulatory organs. Shaped like rods, gonopodia are used during reproduction to deposit sperm into female mollies.

Unlike many fish, mollies don’t travel in schools; rather, they travel in shoals. In a shoal, multiple fish band together for social reasons. In a school, they do so to travel in the same direction together. Approximately one-quarter of all fish shoal their whole lives, and the molly is one of them. They tend to shoal in the hundreds, and the male-to-female ratio is usually around one to two.

Top feeders, mollies use their upturned mouths to exploit films of oxygen-rich surface water. As a result, they are tolerant and able to survive in oxygen-depleted habitats. Although peaceful, mollies are also active swimmers. Male mollies occasionally exhibit aggressive behavior, and they are known to aggressively pursue females. They provide no parental care to their young, and adult mollies often prey on young mollies.

Mollies are diurnal, which means that they are awake by day and asleep by night. They are good community fish, although males tend to establish hierarchies.

Habitat

American fish, mollies are mostly found in the wild in areas of North and South America. However, over the last few decades, they have been introduced in countries like Japan, Taiwan, Singapore, Israel, and even parts of Eastern Europe, and more recently, regions such as northern Egypt.

Molly fish prefer to live in shallow surface waters. In the wild, they are usually found in loosely vegetated coastal streams. However, these fish are known to do well in lakes, estuaries, deltas, swamps, ponds, marshes, and even in ephemeral places like roadside ditches. They prefer to stay under floating vegetation or near structures to stay out of view of predators.

Different species of mollies are found in different places. The common molly, for example, is typically found throughout Mexico down through northern South America. The sailfin molly is found along the Gulf of Mexico and along the southeastern coast of the U.S. The Mexican molly is located in southeastern Mexico and is primarily found in the coastal Yucatan.

Diet

Molly fish are technically omnivores because they not only consume plant materials but also are known to consume crustaceans, insects, and worms. However, their diets overwhelmingly consist of plant materials, so these fish are practically herbivores. Mollies particularly enjoy algae, but they are known to consume vast amounts of other phytoplankton.

Predators and Threats

Most Mollies are not endangered. The Mexican molly, for example, is listed on the IUCN Red List of Threatened Species as Vulnerable, and the common molly is listed as Least Concern. However, it is believed that most species have fairly large, steady populations in the wild. The sailfin molly has a status of Least Concern, which means that it is unlikely to be considered an endangered species any time soon.

In terms of predators, the molly’s biggest threat comes from larger fish. Because many species of mollies are quite small — especially male mollies — they tend to be easy pickings for larger fish like largemouth bass. Aquatic birds, aquatic insects, amphibians, reptiles, and even mammals may all prey on fish. That is especially true because mollies are surface feeders. Although they tend to hide under vegetation and other objects, they must emerge to feed — and that is when they become vulnerable to predators.

Other common predators of the molly fish include giant water bugs, American alligators, snowy egrets, bullfrogs, and raccoons.

Additionally, mollies occasionally fall prey to other mollies. These fish are not particularly aggressive, but they are known to fin-nip — a common problem in aquarium settings. Their fry are also vulnerable to being attacked and eaten by adult mollies, so young mollies must be kept separate from adults in aquariums.

Reproduction, Babies, and Lifespan

Male mollies use specialized anal fins called gonopodia to deposit sperm into female mollies. Female fish are fertilized internally, and they can store sperm for up to a few months at a time. This adaptation is designed to ensure that populations stay strong even if male mollies are scared away from the group.

Prolific breeders, Molly fish have 28-day gestation periods and typically have anywhere from 50 to 100 live fry per period. However, gestation periods may vary depending on water temperatures. Females can produce live fry every 30 to 40 days or so, and 70 percent of the fry tend to be female. Male mollies tend to aggressively pursue female mollies, and it is less taxing for females if at least two are kept for every one male in aquariums. Mollies are live bearers, which means that they give birth to live fish instead of laying eggs.

Young mollies reach sexual maturity at very young ages, so their populations can grow rapidly. In the wild, Molly fish tend to live for about a year after reaching sexual maturity. However, in aquariums, these fish may live for an average of three years; it’s not unheard of for some to live for as long as five years. Male mollies have higher mortality rates, which is largely explained by the fact that they are smaller and therefore more susceptible to predators. However, they also tend to be more susceptible to diseases.

Young Molly fish must immediately fend for themselves in the wild. Parents do not provide any sort of care for them after birth.

These fish can hybridize with all other species of molly fish, which is why selective breeding of these fish is so popular. Hybridization of Molly species through the years has resulted in a huge assortment of colors, patterns, and tail shapes — another reason that these fish are so popular in aquariums.

Population

The molly populations in the wild are unknown, but not endangered. Population levels of different species of mollies vary, of course. It is known, however, that the sailfin molly is particularly abundant in the wild.

Although many mollies are found in the wild, many others are kept in aquariums. It’s possible that pet Molly fish, bred in captivity, are generations away from life in the wild.

Because these fish are prolific breeders, it is unlikely that their populations will dwindle. Nonetheless, environmental changes can affect their ability to breed because the gestational period of these fish varies depending on things like water temperature and salinity.