Poecilia formosa

Amazon molly




Type Locality

Palo Alto, México; Girard 1860. (Martin 1978).


Etymology/Derivation of Scientific Name

Poecilia, from Greek poikilos (Latinized stem poecil), meaning “variegated, speckled” (Boschung and Mayden 2004).






Maximum size: 9.6 cm (Page an Burr 1991).


Coloration: Rows of brown spots on side (may have rows of dusky black spots) (Page and Burr 1991).


Counts: 10 or more dorsal fin rays (Hubbs et al 1991).


Body shape:


Mouth position:


External morphology: Dorsal fin rays 10 to 12; dorsal fin base more than one-half predorsal length, dark spots on scales do not obscure diamond-shaped color pattern; exists only as females. Origin of dorsal fin anterior to origin of anal fin (Hubbs et al 1991).


Additional info from Hubbs key: Intestinal canal long with many convulsions; teeth not moveable.


Additional info from Page and Burr 1991: The Amazon Molly is an all-female species thought to have originated as a result of hybridization between Sailfin and Shortfin mollies, and is intermediate between these two species in its characteristics. Eggs develop in Amazon Mollies following stimulation by sperm of males from either parental species.


Distribution (Native and Introduced)

U.S. distribution: Native to north Mexico and south Texas, current geographic distribution reflects a rapid northward expansion via efficient colonizing ability (Turner, 1982).


Texas distribution: Native to lower Rio Grande Valley and coastal streams and possibly native to coastal portions of Nueces River. Introduced populations found in San Marcos River and San Antonio River. (Martin 1978). Found in conjunctions with P. latipinna in coastal waters and P. mexicana in inland rivers (Dawes 1991).


Abundance/Conservation status (Federal, State, NGO)



Habitat Associations

Macrohabitat: prefers sluggish streams and ditches with fresh or brackish water. . (Martin 1978) Has to remain near populations of P. mexicana and P. latipinna in order to be able to reproduce (Avise et al., 1991). Water clear to muddy; currents none to moderate, sometimes rapid; substrates mud, silt, clay sand, gravel, rock, boulder, vegetation, and oyster shells; vegetation none to abundant (Rauchenberger 2005).





Spawning season:


Spawning habitat:


Spawning Behavior: All female species that reproduces by gynogenesis (Hubbs et al 1991). Reproduces by mating with one of the males of P. mexicana or P. latipinna (Balsano et all 1972). The first recognized vertebrate with unisexual reproduction (Hubbs and Hubbs, 1932). This all-female species produces diploid apomictic eggs (Rasch et al., 1982; Monaco et al., 1984). Embryogenesis is subsequently activated by sperm from a related bisexual species, normally either P. mexicana or P. latipinna. (Turner et al., 1980a, 1980b). The process apparently takes place without syngamy, such that genetic transmission is exclusively clonal (Balsano et al 1989). There is extensive literature on the reproduction of this fish. This molly is maintained by mating with males of either parental species, the sperm activatin the egg but taking no part in inheritance. Thus the offspring are invariably daughters and each is a genetic copy of its mother (Miller et al., 2005).


Fecundity: Broods of about 30 fry are produced sporadically (Dawes 1991).


Age at maturation:




Growth and Population structure:




Food habits:


Phylogeny and morphologically similar fishes

Species from the genus Poecilia inhabiting eastern Mexico fall naturally into two groups which differ in respect to numerous morphological characters but which may simply by referred to here as those with long dorsal fins and those with short dorsals. In respect to its morphological characters, P. formosa is intermediate between these two groups (Darnell and Abramoff 1968). It occurs as a hybrid between P. latipinna and P. mexicana (Hubbs and Hubbs 1932; Turner 1982).


Host Records

Environmental factors influencing male formation and reproductive ecology, including sexual parasitism on parental species (Hubbs 1964).


Commercial or Environmental Importance

The species is opportunistic and tolerant of degraded habitat and water quality conditions, has been collected in stressed aquatic environments by San Antonio river Authority biologists throughout the lower San Antonio River Basin. (Gonzales and Moran 2005).



Abramoff, P., R. M. Darnell and J. S. Balsano. 1968. Electrophoretic demonstration of the hybrid origin of the gynogenetic teleost Poecilia formosa. American Naturalist. 102:555-558

Avise, J. C., J. C. Trexler, J. Travis and W. S. Nelson. 1991. Poecilia mexicana is the Recent Female Parent of the Unisexual Fish P. formosa. Evolution. 45(6): 1530-1533.

Balsano, J. S., E. M. Rasch and P. J. Monaco. 1989. The evolutionary ecology of Poecilia formosa and its triploid associate, pp. 277-297. In G. K. Meffe and F. F. Snelson, Jr (eds.), Ecology and Evolution of Livebearing Fishes (Poeciliidae). Prentice Hall, Englewood Cliffs, NJ.

Boschung, H. T., Jr., and Richard L. Mayden. 2004. Fishes of Alabama. Smithsonian Books, Washington, 736 pp.

Dawes, J. A. 1991. Livebearing Fishes. A Guide to Their Aquarium Care, Biology and Classification. Blandforn, London, England. 240 pp.

Gonzales, M. and E. Moran. 2005. An inventory of fish species within the San Antonio Missions National Historical Park. San Antonio River Authority, Final Report.

Hubbs, C. 1964. Interactions between a bisexual fish species and its gynogenetic sexual parasite. Texas Memorial Museum Bulletin 8: 1-72.

Hubbs, C. L. and L. C. Hubbs. 1932. Apparent parthenogenesis in nature, in a form of fish hybrid origin. Science 76:628-630.

Hubbs, C., R. J. Edwards and G. P. Garrett. 1991. An annotated checklist of freshwater fishes of Texas, with key to identification of species. Texas Journal of Science, Supplement. 43(4):1-56.

Martin, F. D. 1980. Poecilia formosa (Girard) Amazon Molly pp 548 in D.S. Lee et al. Atlas of North American Freshwater fishes. N.C. State Mus. Nat. Hist., Raleigh, i-r+854.

Miller, R. R., W. L. Minckley and S. M. Norris. 2005. Freshwater Fishes of Mexico. The University of Chicago Press. Chicago, IL. pp 234.

Page, L. M., and B. M. Burr.  1991.  A Field Guide to Freshwater Fishes of North America, north of Mexico.  Houghton Mifflin Company, Boston, 432 pp.

Rasch, E. M., P. J. Monaco and J. S. Balssano. 1982. Cytophometric and autoradiographic evidence for funtional apoxomixis in gynogenetic fish, Poecilia formosa, and its related triploid hybrids. Histochemistry. 73: 515-533.

Rauchenberger, M. 2005 Artificial Key to Mexican Poeciliidae. In Miller, R. R. et al. 2005. Freshwater Fishes of Mexico. The University of Chicago Press. Chicago. Illinois.

Turner, B. J. 1982. The evolutionary genetics of a unisexual fish, Poecilia formosa pp. 265-305. In C. Barigozzi (ed.), Mechanisms of Speciation. Alan R. Liss, New York.

Turner, B. J. and B. H. Brett, E. M. Rasch and J. S. Balsano. 1980b. Evolutionary genetics of gynogenetic fish, Poecilia formosa, the Amazon molly. Evolution. 34:246-258.

Turner, B. J., and B. H. Brett, and R. R. Miller. 1980a. Interspecific hybridization and the evolutionary origin of gynogenetic fish, Poecilia formosa. Evolution. 34: 917-922.