Mexican Molly, Poecilia sphenopsis

Mexican Molly, Poecilia sphenopsis, Female. Fish caught from a stream in Rogers Spring, Nevada, September 2019. Length: 5.8 cm (2.3 inches). Catch, photograph and identification courtesy of Ben Cantrell, San Diego, California.

Mexican Molly, Poecilia sphenopsis, Breeding Male. Fish caught from a freshwater pond within the Pahrump Valley, Nevada, August 2019. Length: 5.8 cm (2.3 inches). Catch, photograph and identification courtesy of Luke Ovgard, Klamath Falls, Oregon.

The Mexican Molly, Poecilia sphenopsis, is a member of the Livebearer or Poeciliidae Family, that is also known as the Common Molly, the Molly, and the Short-finned Molly, and in Mexico as topote mexicano. This fish has been bred for centuries that has produced several color variations and different body shapes which include the commonly called Balloon Molly, the Black Molly, the Dalmatian Molly, the Gold Dust Molly, the Golden Molly, the Liberty Molly, the Lyretail Molly and the White Molly. Globally, there are forty species in the genus Poecilia, of which six are found in Mexican freshwater systems that drain into both the Atlantic and the Pacific Oceans. The Mexican Molly is closely related to the Sailfin Molly, Poecilia latipinna, with which it will cross breed.

The Mexican Molly has an oblong moderately robust body that is compressed at the rear. The wild-type are dull and silvery in color with dispersed black dots on its skin and a bright yellow fringe on the ends of its rounded dorsal and caudal fins. Individuals exhibit wide ranges of color variation that mainly incorporates silver, black and yellow-orange. The males are normally pale gray brown dorsally transitioning to silvery white ventrally; they have rows of orange spots on the upper two-thirds if the body; their caudal fin is yellow with a red wash with several rows of black dots; and, their dorsal fin is yellowish with a red margin and 2 to 4 rows of black spots on the inner two-thirds. The females are pale gray-brown with dark spots dorsally transitioning to silvery white ventrally; their fins are clear except the dorsal fin has a row of dark spots. Their head is moderately robust and flattened dorsally and mouth is small and slightly oblique that opens at the front with a projecting lower jaw and a protractile upper jaw. The Mexican Molly is sexually dimorphic with females being larger than males (12 cm or 4.7 inches versus 8 cm or 3.1 inches), less colorful and smaller caudal fins. Males also have highly modified anal fins called a gonopodium that functions to transfer sperm during mating. Females have a dark gravid spot between the abdomen and the anal fin when pregnant. Their anal fin has 9 rays with its origin being behind the dorsal fin in females and before in males; their caudal peduncle is round and their caudal fin is rounded; their dorsal fin has 9 or 10 rays, has a short base and is located behind the pelvic fins; the pectoral fins are high on the flank and are in front of the pelvic fins; the tips of the pelvic fins of males are swollen and elongated. They are covered with large smooth scales.

The Mexican Molly is non-migratory and found in both coastal brackish and freshwater habitats including rivers and mangroves in schools under floating vegetation or near structures in the water at depths up to 3 m (10 feet) in water temperatures between 18°C (65°F) and 28°C (82°F). They are able to source oxygen from the water surface and can survive in oxygen-depleted habitats. Reproduction involves internal fertilization with gestation periods of 3 or 4 weeks. Each female can produce broods of 10 to 140 live young. The females have the ability to store sperm and give birth on multiple occasions throughout the year. The young mollies aggregate in schools of similarly-sized fish. They feed on crustaceans, insects, insect larvae, plant matter and worms. In turn they are preyed upon by various pelagic and surface predatory fish as well as numerous seabirds, including herons. The Mexican Molly has a lifespan of up to 5 years.

The Mexican Molly is similar in appearance to the Guppy, Platyfish and Swordtails with the females being difficult to separate visually. They are most likely confused with the Shortfin Molly, Poecilia mexicana (orange margin of the caudal fin).

The Mexican Molly is a resident of the freshwater systems of mainland Mexico being found from Veracruz to Belize on the Atlantic Slope and from Oaxaca to Guatemala on the Pacific Slope.

From a conservation perspective the Mexican Molly is currently considered to be of Least Concern with stable, widely distributed populations. They have been introduced throughout the world from escapes, intentional releases from fish farms to control mosquito populations, and releases by aquarists raising havoc of native fishes and their environments. In some locations have become a nuisance pest causing ecological damage because of its ability to rapidly reproduce in high numbers. The Black Molly color variation is one of the most popular aquarium fishes of the world due to their high growth rate, birth size, and brood size. They are very hardy and highly adaptable and are suitable for community tanks. Long term they are threatened by human development of their native habitat.