”Poecilia mexicana” is a slim, laterally flattened molly.<ref name=”Franz”/> The maximum known [[standard length]] (SL) is 95 mm.<ref name=”Miller”/> The head is flat on top and triangular in side view, narrowing to a pointed snout. The mouth opens at the front, is straight, and can be pushed forward. Both jaws carry narrow bands of teeth; the outer teeth are long, fine, and curved backward, forming a brush-like edge. Eyes are large, about one third of head length, and set high on the head.<ref name=”Franz”>{{cite journal | last=Steindachner | first=Franz|author-link=Franz Steindachner| title=Beiträge zur Kenntniss der Sciaenoiden Brasiliens und der Cyprinodonten Mejicos | journal=Sitzungsberichte der Akademie der Wissenschaften mathematisch-naturwissenschaftliche Klasse|language=de| volume=48 | date=1863 | pages=162–185 | url=https://biostor.org/reference/219481 | access-date=2025-12-14}}</ref>
”Poecilia mexicana” is a slim, laterally flattened molly.<ref name=”Franz”/> The maximum known [[standard length]] (SL) is 95 mm.<ref name=”Miller”/> The head is flat on top and triangular in side view, narrowing to a pointed snout. The mouth opens at the front, is straight, and can be pushed forward. Both jaws carry narrow bands of teeth; the outer teeth are long, fine, and curved backward, forming a brush-like edge. Eyes are large, about one third of head length, and set high on the head.<ref name=”Franz”>{{cite journal | last=Steindachner | first=Franz|author-link=Franz Steindachner| title=Beiträge zur Kenntniss der Sciaenoiden Brasiliens und der Cyprinodonten Mejicos | journal=Sitzungsberichte der Akademie der Wissenschaften mathematisch-naturwissenschaftliche Klasse|language=de| volume=48 | date=1863 | pages=162–185 | url=https://biostor.org/reference/219481 | access-date=2025-12-14}}</ref>
There are [[sexual dimorphism|clear differences between the sexes]]. Males are more slender, while females are deeper-bodied. Females have a broader back and a strongly rounded belly, whereas males are less curved. The tail base is relatively deep, especially in females. The [[dorsal fin]] begins slightly before the middle of the body and is rounded. [[Pectoral fin]]s are large and nearly as long as the head; [[pelvic fin]]s 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 much of its base.<ref name=”Franz”/>
There are [[sexual dimorphism|clear differences between the sexes]]. Males are more slender, while females are deeper-bodied. Females have a broader back and a strongly rounded belly, whereas males are less curved. The tail base is relatively deep, especially in females. The [[dorsal fin]] begins slightly before the middle of the body and is rounded. [[Pectoral fin]]s are large and nearly as long as the head; [[pelvic fin]]s 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 much of its base.<ref name=”Franz”/>
[[Fish scales|Scales]] are fairly large and cover most of the head except the jaws. Along the side of the body there are about 28 scales, with 10–11 between the dorsal and pelvic fins and about eight around the tail base. Most scales show a small central pore linked to the [[lateral line]] sensory system. The upper body is brown, gradually changing to golden yellow below. Each scale is darker at the base and along its rear edge. Dark spots on the sides vary: many males lack them, but some individuals show a row of spots, while females may have several rows running lengthwise 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.<ref name=”Franz”/>
[[Fish scales|Scales]] are fairly large and cover most of the head except the jaws. Along the side of the body there are about 28 scales, with 10–11 between the dorsal and pelvic fins and about eight around the tail base. Most scales show a small central pore linked to the [[lateral line]] sensory system. The upper body is brown, gradually changing to golden yellow below. Each scale is darker at the base and along its rear edge. Dark spots on the sides vary: many males lack them, but some individuals show a row of spots, while females may have several rows running lengthwise 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.<ref name=”Franz”/>
Species of fish
Poecilia mexicana, commonly known as the Atlantic molly, is a species of poeciliid fish native to fresh and brackish water in Middle America. One population is found in caves.
Description
Poecilia mexicana is a slim, laterally flattened molly.[3] The maximum known standard length (SL) is 95 mm.[4] The head is flat on top and triangular in side view, narrowing to a pointed snout. The mouth opens at the front, is straight, and can be pushed forward. Both jaws carry narrow bands of teeth; the outer teeth are long, fine, and curved backward, forming a brush-like edge. Eyes are large, about one third of head length, and set high on the head.[3]
There are clear differences between the sexes. Males are more slender, while females are deeper-bodied. Females have a broader back and a strongly rounded belly, whereas males are less curved. The tail base is relatively deep, especially in females. The dorsal fin begins slightly before the middle of the body and is rounded. Pectoral fins are large and nearly as long as the head; 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 much of its base.[3]
Scales are fairly large and cover most of the head except the jaws. Along the side of the body there are about 28 scales, with 10–11 between the dorsal and pelvic fins and about eight around the tail base. Most scales show a small central pore linked to the lateral line sensory system. The upper body is brown, gradually changing to golden yellow below. Each scale is darker at the base and along its rear edge. Dark spots on the sides vary: many males lack them, but some individuals show a row of spots, while females may have several rows running lengthwise 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.[3]
Distribution and habitat
Poecilia mexicana occupies much of the Atlantic slope of Middle America, extending 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 occur on the Bay Islands of Honduras. Within Mexico it has been recorded from Campeche, Chiapas, Hidalgo, Nuevo León, Oaxaca, Puebla, Querétaro, Quintana Roo, San Luis Potosí, Tabasco, Tamaulipas, Veracruz, and Yucatán. On the Pacific slope it is known only from the upper Río Choluteca basin in Honduras. The type locality was given as Orizaba, but the specimens almost certainly came from farther east, probably the Río Blanco or one of its tributaries. The species has also been introduced into the Río Lerma basin on the Pacific slope of Mexico.[4]
Poecilia mexicana inhabits a wide range of lowland aquatic environments, including coastal lagoons, estuaries, ponds, and rivers, and in Mexico it also penetrates upland streams to at least 600 m elevation. The species tolerates fresh, brackish, and saltwater, having been recorded at salinities up to 32.4 ppt. It typically occurs in shallow water, often less than 1 m deep, occupying pools, riffles, or still habitats with little or no current. Substrates vary widely and include rock, mud, silt, sand, and rubble, though the fish is especially common over rocky bottoms coated with dense films of filamentous algae, diatoms, protozoans, and decomposing plant material. During the rainy season, dense aggregations often disperse into temporary waters such as roadside ditches, sometimes several kilometers from permanent habitats. Water clarity ranges from clear to highly turbid, and vegetation may be dense, particularly in oxbows of lowland rivers. Its broad ecological tolerance makes it a common fish along the Atlantic slope.[4]
Biology
The Atlantic molly is primarily benthic, feeding on material scraped from exposed surfaces. Stomach contents consist largely of filamentous algae, diatoms, fragments of vascular plants, detritus, and other decaying organic matter. Feeding is accomplished by brushing or rasping surfaces with the lower lip and tooth row.[4]
Reproduction is prolonged through the year: collections containing juveniles 8.0–13 mm SL from December through August indicnear-continuous breeding, likely producing young monthly. Brood sizes recorded from adult females range from 13 to 35, though very large individuals may carry over 100 embryos.[4]
A cross between the female of Poecilia mexicana and the male of P. latipinna gave rise to the all-female species P. formosa.[5] The species is also known to produce natural hybrids with P. latipunctata, P. petenensis, P. sphenops, and P. velifera, though such hybrids are rare.[4]
Females avoid parasitised males.[6]
References
- ^ Daniels, A.; Maiz-Tome, L. (2019). “Poecilia mexicana“. IUCN Red List of Threatened Species. 2019 e.T191750A2002413. doi:10.2305/IUCN.UK.2019-2.RLTS.T191750A2002413.en. Retrieved 19 November 2021.
- ^ Froese, Rainer; Pauly, Daniel (eds.). “Poecilia mexicana“. FishBase. November 2014 version.
- ^ a b c d Steindachner, Franz (1863). “Beiträge zur Kenntniss der Sciaenoiden Brasiliens und der Cyprinodonten Mejicos”. Sitzungsberichte der Akademie der Wissenschaften mathematisch-naturwissenschaftliche Klasse (in German). 48: 162–185. Retrieved 2025-12-14.
- ^ a b c d e f Miller, Robert Rush (2005). Freshwater fishes of México. Chicago, Illinois, United States: University of Chicago Press. pp. 236–7.
- ^ Avise, John C.; Trexler, Joel C.; Travis, Joseph; Nelson, William S. (1991). “Poecilia mexicana is the recent female parent of the unisexual fish P. formosa“. Evolution. 45 (6). Oxford University Press (OUP): 1530–1533. doi:10.1111/j.1558-5646.1991.tb02657.x. ISSN 0014-3820.
- ^ Plath, M. (2004). “Cave molly females (Poecilia mexicana) avoid parasitised males”. Acta Ethologica. 6 (2): 47–51. doi:10.1007/s10211-004-0085-1. S2CID 1138263.
