Shortfin Molly, Poecilia mexicana

Shortfin Molly, Poecilia mexicana. Fish caught from the Meadow Hotspings, Meadow, Utah, August 2020. Length: 5.1 cm (2.0 inches). Catch, photograph and identification courtesy of Luke Ovgard, Klamath Falls, Oregon.

Shortfin Molly, Poecilia mexicana. Fish caught from the Río Bravo, Tamaulipas, September 2019. Length: 5.8 cm (2.3 inches). Catch, photograph and identification courtesy of Eli (obsessiveangling.wordpress.com).

The Shortfin Molly, Poecilia mexicana, is a member of the Livebearer or Poeciliidae Family, that is also known as the Atlantic Molly, the Mexican Molly, and the Orangefin Molly, and in Mexico as topote del Atlántico. 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 Shortfin Molly has an elongated torpedo shaped compressed body. They vary significantly in color based on location but are generally an olive-green color dorsally that transitions to white or pale orange ventrally. They have rows of greenish spots on their sides. Their anal and pectoral fins have orange spots; the caudal fin is a metallic blue with orange spots. The males are dark green with black spots on the dorsal fin. 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 that is equipped with large backward curved teeth and several rows of small pointed teeth. They reach a maximum of 14.4 cm (5.7 inches) in length. Their anal fin has 9 rays with its origin being under the dorsal fin in females and after in males with the third ray modified into an elongated sex organ; their caudal peduncle is thick and their caudal fin is stubby and rounded; their dorsal fin has 8 to 11 rays, a short base and originates behind the pelvic fins and small in males; their pectoral fins are high on the flank; and, the pelvic fins are swollen in males. They have 21 to 26 gill rakers. They have 18 scales around the caudal fin base and 25 to 30 large scales on the lateral line.

The Shortfin Molly is a benthopelagic non-migratory species that are found in both coastal brackish and freshwater habitats in lagoons, estuaries and steams at depths up to 1 m (3 feet) in water temperatures between 22°C (72°F) and 28°C (82°F). They can also survive in brackish waters with salinities up to 18 ppt. They are also found in caves and known as the Cave Molly with these fish still retaining their functional eyes. Reproduction involves internal fertilization with gestation periods of 4 weeks. Each female can produce broods of 30 to 80 live young. They are primarily herbivores that feed on bacterial, chironomid larvae and detritus. In turn they are preyed upon by various pelagic and surface predatory fish as well as numerous seabirds, including herons. The Shortfin Molly is poorly studied with very limited information available about their lifestyle and behavioral patterns including specific details on age, growth, longevity, movement patterns, diet, habitat use, and reproduction.

The Shortfin Molly is easily confused with the Mangrove Molly, Poecilia orri (smaller; 16 scales around the caudal base), the Mexican Molly, Poecilia sphenops (elongated body), the Petén Molly, Poecilia petenensis (14 to 16 dorsal fin rays), and the Yucatán Molly, Poecilia velifera (16 to 20 dorsal fin rays).

The Shortfin Molly is a resident of the shallow freshwater systems of mainland Mexico being found within the Río Bravo basin southward to Belize on the Atlantic Slope within the States of Campeche, Chiapas, Hidalgo and Jalisco. They have been introduced in Nuevo León, Oaxaca, Puebla, Querétaro, Quintana Roo, San Luis Potosí, Tabasco, Tamaulipas, Veracruz and the Yucatán. They are known to be present in dense populations in the caves of Tabasco.

From a conservation perspective the Shortfin 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. They are utilized by the aquarium trade extensively. They have also been utilized in some regions as a live bait for Skipjack Tuna. Their long-term viability is threatened by human development of their native habitats.