It is relatively easy to distinguish between male and female mollies by considering fin shape, body shape and coloration. These small fish hailing from coastal waters in North, Central and South America give birth to live young, rather than depositing eggs. This makes mollies (Poecilia spp.) popular among aquarists interested in breeding fish for feeders, fun or profit. Breeding mollies is as simple as providing a proper habitat and diet, and ensuring the tank contains at least two females for every male.
Secondary Sexual Characteristics
The faster your mollies grow, the quicker you will be able to determine their genders. At birth, all mollies appear female; it may take as long as three months for the small fish to develop secondary sexual characteristics. However, well-trained eyes may be able to distinguish between the sexes when the fish are only three-quarters of an inch in length, by observing minute anatomical differences.
Most adult male mollies have longer and more elaborate tails, pectoral fins and dorsal fins than females do. However, the most accurate way to distinguish male mollies from female mollies is to examine the anal fin, located on the underside of the body, between the pectoral fins and the tail. Those possessing fanlike anal fins are females. Conversely, the anal fins of males are highly modified, and they function as intromittent organs, allowing the males to transfer sperm to the females. Scientists call these fins gonopodia.
Body Shape and Size
Although the differences between the two sexes are subtle, male and female mollies have slightly different body shapes. Males tend to be thinner and more streamlined, while females -- who must carry the developing young -- are bulkier. Even virgin females have more voluminous bodies than males, but pregnant females become especially bulky as they approach the end of their monthlong gestation periods. Mollies grow to between 2 and 4 inches long, females generally longer than males.
While both sexes usually bear the same basic colors, males often display bolder hues than females do. In the wild, this represents a threat to the survival of males. In addition to bolder body colors, male mollies may display more color on their fins than females do.