About Poecilia mexicana Steindachner, 1863
Poecilia mexicana, commonly called the Atlantic molly, is a slim, laterally flattened molly species that displays a wide range of physical forms. The maximum known standard length of the species is 95 mm. Mature males fall into multiple size classes, ranging from 18 mm to over 70 mm in standard length. Its head is flat on top, triangular when viewed from the side, and narrows to a pointed snout. The mouth is positioned at the front of the head, forms a straight opening, and can be extended forward. Both jaws hold narrow bands of teeth; the outer row of teeth are long, fine, and curved backward, creating a brush-like edge. The eyes are large, measuring roughly one third of the head length, and are set high on the head. There are clear physical differences between males and females. Males have a more slender body shape, while females have deeper bodies. Females have a broader back and a strongly rounded belly, while males have less curved body contours. The base of the tail is relatively deep, particularly in females. The dorsal fin starts slightly before the midpoint of the body and has a rounded shape. Pectoral fins are large and nearly as long as the head, while pelvic fins are shorter. The anal fin is tall and somewhat pointed, and the tail fin is broadly rounded and slightly longer than the head, with scales covering most of its base. Scales are fairly large and cover most of the head, with the exception of the jaws. There are around 28 scales along the side of the body, 10–11 scales between the dorsal and pelvic fins, and around eight scales around the tail base. Most scales have a small central pore connected to the lateral line sensory system. The upper body is brown, and color gradually shifts to golden yellow on the lower body. Each individual scale is darker at its base and along its rear edge. Dark spots on the sides of the body are variable: many males have no spots, some males have a single row of spots, and females may have several lengthwise rows of spots along the body. The dorsal fin always has several rows of dark spots; the pelvic and anal fins are golden yellow, and the pectoral fins are dull yellow to brownish. The species has two recognized subspecies that differ primarily in overall body form and fin proportions. P. m. limantouri has a more slender, almost cylindrical body, a relatively short dorsal fin, and a narrow caudal (tail) fin. In contrast, the nominate subspecies P. m. mexicana has a deeper body and is somewhat laterally compressed, with a noticeably longer dorsal fin and a broader caudal fin. In areas where the ranges of the two subspecies overlap, individuals often show intermediate physical features. Poecilia mexicana is distributed across most of the Atlantic slope of Middle America, ranging from the lower basin of the Río Bravo (including the Álamo and San Juan rivers) south through Central America to Costa Rica, where it reaches the Río Matina. Offshore populations live on the Bay Islands of Honduras. Within Mexico, the species has been recorded in the states of Campeche, Chiapas, Hidalgo, Nuevo León, Oaxaca, Puebla, Querétaro, Quintana Roo, San Luis Potosí, Tabasco, Tamaulipas, Veracruz, and Yucatán. On the Pacific slope, it is only known from the upper Río Choluteca basin in Honduras. The species' original type locality was recorded as Orizaba, but the type specimens almost certainly originated farther east, most likely from the Río Blanco or one of its tributaries. This species has also been introduced to the Río Lerma basin on the Pacific slope of Mexico. Poecilia mexicana lives in a wide range of lowland aquatic environments, including coastal lagoons, estuaries, ponds, and rivers. In Mexico, it also occurs in upland streams up to at least 600 m elevation. The species tolerates fresh, brackish, and saltwater, and has been recorded at salinities as high as 32.4 ppt. It typically lives in shallow water, often less than 1 m deep, and occupies pools, riffles, or still habitats with little to no current. It can be found across a wide variety of substrates, including rock, mud, silt, sand, and rubble, but is especially common on rocky bottoms covered in dense films of filamentous algae, diatoms, protozoans, and decomposing plant material. During the rainy season, dense aggregations of the fish often disperse into temporary water bodies such as roadside ditches, sometimes moving several kilometers away from permanent habitats. Water where the species occurs can range from clear to highly turbid, and vegetation can be dense, particularly in oxbows of lowland rivers. Its broad ecological tolerance makes it a common fish along the Atlantic slope of Middle America. Poecilia mexicana limantouri occupies the northern part of the species' Gulf Coast range, extending from the Rio Grande drainage in Nuevo León south to the lower Río Tamesí near Tampico. P. m. mexicana occurs farther south, from the Río Cazones drainage to at least the Río Jamapa system south of the port of Veracruz. A broad zone of intergradation between the two subspecies lies between these regions, running along the coast from the lower Río Soto la Marina in Tamaulipas to the Río Tuxpan in northern Veracruz, and extending inland into nearby drainage basins. The Atlantic molly is primarily benthic, feeding on material scraped from exposed surfaces. Stomach contents are mostly made up of filamentous algae, diatoms, fragments of vascular plants, detritus, and other decaying organic matter. The fish feeds by brushing or rasping surfaces with its lower lip and tooth row. Males do not grow significantly after reaching maturity, so they do not move between size classes. Large males defend females from other males, but neither large nor small males perform courtship displays. Small males, which are almost always more abundant in wild populations, hide near groups of females and ambush them to force copulation. Females avoid males that carry parasites. Reproduction occurs almost continuously throughout the year: collections of juveniles measuring 8.0–13 mm standard length collected from December through August indicate near-continuous breeding, with females likely producing young each month. Recorded brood sizes from adult females range from 13 to 35, though very large females may carry more than 100 embryos. A cross between a female Poecilia mexicana and a male P. latipinna produced the all-female species Poecilia formosa. P. formosa needs to mate with males of other poeciliid species to trigger reproduction, but all offspring are clones of the mother. P. mexicana and P. latipinna are the preferred sperm donors for P. formosa. Poecilia mexicana also forms natural hybrids with P. latipunctata, P. petenensis, P. sphenops, and P. velifera, though these hybrids are rare.
