Poecilia sphenops
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 from Mexico to Colombia. The wild-type fishes are dull, silvery in color. The molly can produce fertile hybrids with many "Poecilia" species, most importantly the sailfin molly. The male mollies generally tend to be mildly aggressive. Mollies rank as one of the most popular feeder fish due to high growth rate, birth size , reproduction, and brood number. Biology. 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). Mollies are similar in appearance to their livebearer cousins, the platy, swordtail and guppies; the molly tends to be slightly larger and more energetic. Many aquarists  note the stronger individuality and aggression in keeping mollies versus platies, who appear much more docile. Size. Standard size of male fish is 3.2″ (8 cm) and female fish is 4.8″ (12 cm). Varieties. Selective breeding over centuries has produced several color variations and different body shapes.