Poecilia butleri

P. butleri occurs along the Pacific drainage of Mexico and El Salvador, from the Fuerte River basin in Sonora southward to the mouth of the Comasagua River west of La Libertad, El Salvador; its true southern limit remains uncertain and it likely extends farther west in El Salvador. While it occurs in freshwater in northwestern Mexico, east of the Río Balsas it is primarily found in brackish and marine coastal habitats, and it is expected to inhabit many coastal lagoons of El Salvador. The species is abundant across much of its range.^[6] Its range extends further north than that of P. sphenops. In the part of the range it shares with P. sphenops, P. butleri inhabits slower-flowing streams and is therefore more common near the coast. P. butleri and, on the Atlantic slope, P. mexicana, are the only shortfin molly species found in brackish lagoons and river mouths.^[4]

Poecilia butleri is common in the upper reaches of the Ameca and Coahuayana rivers in Jalisco, where those populations appear to be introduced.^[6] Its ovoviparity and preference for estuarine and freshwater habitats may facilitate its spread into Baja California, where it appears to have been accidentally introduced from Sinaloa in 1995 via containers for breeding Pacific white shrimp. The establishment of mollies in this area might affect native mullets, snappers, grunts, and snooks, which rely on salt marsh habitats for reproduction.^[7]

Poecilia butleri occupies a wide range of coastal and inland shallow waters, including low-gradient streams, slow stretches of larger rivers, lagoons, estuaries, ocean bays and coastal pools, tolerating fresh to fully saline conditions. Water clarity ranges from clear to turbid or muddy, currents from still to moderate, and depths generally do not exceed 1 m. Substrates recorded include sand, silt, mud, gravel, rock and boulder, and the species occurs among diverse vegetation types—filamentous algae, Chara, Potamogeton, Eichhornia, Pistia, Typha, Juncus, Nasturtium, Salvinia—as well as in mangrove stands.^[6] In the Papagayo River drainage, the Pacific molly occurs together with the characin Astyanax fasciatus, a Cichlasoma cichlid, and the goby Sicydium multipunctatum in slowly flowing waters over green algae-covered rocks and boulders; P. sphenops is also present in small numbers, apparently invading from the upper reaches.^[4]

Poecilia butleri is herbivorous and detrivorous;^[5][8] a gut analysis of specimens from the Quelite River and a mangrove swamp near Tecoman showed that the fish had fed exclusively on cyanobacteria.^[5] It might also take dipterous larvae.^[7] Its predators include the boat-billed heron.^[9] The species may carry parasites such as trematodes, cestodes, and nematodes.^[10]

Poecilia butleri reaches sexual maturity at five or six months. After a gestation of 28 days, the female gives birth to 20 to 60 young.^[11] Reproductive season appears to be extended: juveniles 7.0–11 mm standard length have been collected from February through late July.^[6]