Poecilia sphenops is a species of fish, of the genus Poecilia, known under the common name molly; to distinguish it from its congeners, it is sometimes called short-finned molly or common molly. They inhabit fresh water streams and coastal brackish and marine waters of Mexico. The wild-type fish are a dull silvery color, often sprinkled black all over.
Contrary to popular belief, this species of fish is actually a freshwater species, spending little time in brackish water before swimming back to their freshwater biotope. However, fish of the same species have been found in coastal sea waters, brackish swamps and freshwater streams, living and breeding. Mollies appear to be a hardy and highly adaptable species (this has been diluted over years of interbreeding in tank-bred specimens).
This species is one of the ancestors of the black mollies, a number of melanistic breeds which are black all over. It is one of the most well-known aquarium fishes and nearly as easy to keep and prolific as guppies (for optimal health and breeding success, they demand fresh vegetable food like algae). There are several other popular breeds, like the golden molly, nicknamed "24 karat", or the balloon molly, which has a deformed spine and a decreased lifespan due to the associated health problems. Also, breeds with altered caudal fin structures such as lyretails exist. The wild form is in fact quite rarely kept, as it has a rather plain silvery coloration suffused with brown and green hues. If given good care with ample sunlight, high water temperatures and fresh vegetables, they will, however, prove charming fish who make up for their somewhat plain coloration with their lively behavior.
The common molly can produce fertile hybrids with many Poecilia species, most importantly the sailfin molly. In the case of black a bit more attention and have a somewhat decreased lifespan - though certainly not as much as the deformed breeds.
The male black mollies generally tend to be mildly aggressive. Although they are compatible tankmates with fish such as tiger barbs, they will chase them.
Mollies rank as one of the most popular feeder fish due to high growth rate, birth size, and brood number.