The Atlantic molly, or cave molly, is a small, freshwater livebearing fish that feeds on invertebrates and, in sunlit environments, algae. Its range extends from northeastern Mexico down through Central America (Tobler et al, 2008). P. mexicana has undergone a divergent shift between sub-populations in terms of environmental preferences: while most of the species resides in tropical streams and stream pools, distinct populations have colonized cave environments and undergone shifts in morphology and behavior (Menzel and Darnell, 1973).
Poecilia mexicana exhibits a large range of sizes that vary based on habitat- the standard length is 24-54 mm for females and slightly smaller for males, around 20-42mm (Menzel and Darnell, 1973). Although many traits vary from population to population, P. mexicana is generally a slender fish with broad caudal and dorsal fins (Plath et al, 2003). Coloration varies, but it is often intricate with beautiful markings on the fins (Tober and Plath, 2010).
P. mexicana is thought to be the female parent of Poecilia formosa, an all-female poeciliid species known as the Amazon molly (Avise et al, 1991).
occurs (regularly, as a native taxon) in multiple nations
Regularity: Regularly occurring
Type of Residency: Year-round
Global Range: Native to Atlantic slope from Rio San Juan, Mexico, to Guatemala. Established in a drainage canal south of Mecca, Riverside County, California; Bruneau Hot Springs, Owyhee County, Idaho; and in springs and the Moapa River, Clark and Lincoln counties, Nevada; common at some localities (Page and Burr 1991). Widespread and solidly established in Nevada waters (Minckley 1973; Moyle 1976). Established in fresh waters in Hawaii (Robins et al. 1991).
- 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 p. (Ref. 5723)
P. mexicana inhabits aquatic tropical habitats, including streams and ponds, from northern Mexico to the river systems in the southern provinces such as the Rio Jampa system around Veracruz City (Menzel and Darnell, 1973). The species is also present in the Atlantic areas of Central America (Tobler et al, 2008).
In P. mexicana, morphology varies based on population. Cave populations exhibit different traits than epigean populations, which live in open environments in streams and ponds.
The epigean form is laterally compressed and has a deep body, with a gentle, steady slope descending from the anterior tip of the dorsal fin to the snout (Menzel and Darnell, 1973). The fish can be quite deep in the trunk: females swell when they are carrying young. In males, the anal fin is modified and elongated into a gonopodium, a reproductive structure (Meefe and Snelson, 1989). P. mexicana possesses obvious spots and bands in a pattern along its body. (Parzefall, 2001).
The cave form is much more slender and has smaller fins. It shows a slightly concave curve from the anterior tip of its dorsal fin to its snout. It also exihibits reduced eyes and a much higher incidence of free neuromasts (neuromasts are a sensory structure in fish that occur exposed on the body or in canals). Some canal segments develop without a complete covering and there are several large free neuromasts on the head. It's head is also smaller and it has highly reduced pigmentation, sometimes resulting in a yellow-colored fish (Parzefall, 2001).
- Greenfield, D.W. and J.E. Thomerson 1997 Fishes of the continental waters of Belize. University Press of Florida, Florida. 311 p. (Ref. 30499)
The size ranges based on population: females tend to be larger than males with a standard length of 24-54mm. Males are 20-42mm (Plath et al, 2003).
Poecilia mexicana in many ways resembles other poeciliid species, exhibiting many typical "molly-like" characteristics. P. mexicana can be distinguished in certain instances by its habitat- the cave species is the only poeciliid species in the sulfidic cave environments in which it lives. The cave species exhibits a reduced head size and less obvious lateral compression, in addition to reduced eyes (Tober and Plath, 2010).
In all populations, the anal fin originates directly under the dorsal fin. The epigean forms exhibits bright colors on the caudal and dorsal fins and a distinctive pattern of banded spots that span the entire body in multiple lines from the gill region to the caudal fin (Parzefall, 2001).
Catalog Number: USNM 120285
Collection: Smithsonian Institution, National Museum of Natural History, Department of Vertebrate Zoology, Division of Fishes
Locality: Turbo, Gulf of Darien, Pan., Darien Province, Panama, North America
- Syntype: Garman, S. 1895. Memoirs of the Museum of Comparative Zoology at Harvard College. 19 (1): 62, pl. 5.
Habitat Type: Freshwater
Comments: Warm springs and their effluents, canals, weedy ditches, and stream pools (Page and Burr 1991). In Nevada, found in warm springs. In California, has been collected in freshwater ditches around Salton Sea.
Depth range (m): 0.35 - 1.5
Depth range (m): 0.35 - 1.5
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P. mexicana occurs in either the surface environments of streams and ponds or the dark environment of of subterranean caves. It can be found in stream environments throughout its range, in which adults live with mixed schools in deeper regions and juveniles gravitate toward shallower areas (Parzefall, 2001). It is well known for its colonization of the sulfidic cave and its surroundings in Cueva de Azufre. P. mexicana has been able to adapt to the complete range of hydrogen sulfide levels and darkness found in and around the Cueva de Azufre system (Tobler and Plath, 2010).
Non-Migrant: No. All populations of this species make significant seasonal migrations.
Locally Migrant: No. No populations of this species make local extended movements (generally less than 200 km) at particular times of the year (e.g., to breeding or wintering grounds, to hibernation sites).
Locally Migrant: No. No populations of this species make annual migrations of over 200 km.
Cave forms remain localized in their habitat, and do not display territorial behavior. Surface forms move freely within their habitats, although dominant males and females establish loose territories in which subordinates also live and try to compete for food and mates (Plath et al, 2003)
Comments: Diet varied. Feeds on both plant and animal matter; vascular plants and mosquito larvae are important items in the diet (Brown 1971).
P. mexicana eats an omnivorous diet. In a "typical molly habitat", a clear stream, the primary nutrition sources available are algae and small invertebrates. In cave environments, where food is thought to be plentiful, P. mexicana subsists on chironomid flies and larvae, chemoautotrophic sulfur bacteria, and bat guano. (Plath et al, 2005).
Diseases and Parasites
Has contributed to the decline of native, spring-inhabiting species in Nevada (Minckley 1973).
Life History and Behavior
Adult stream-dwelling fish generally live in large, mixed schools in the deeper regions of rivers and streams, while juvenile fish tend to reside in the calmer shallows where they can take cover in plant growth. Within populations of mature fish, hierarchies appear in both sexes: the females compete for food, and males compete for females. To maintain and establish these hierarchies, aggressive behavior occurs in both sexes, including chasing and biting (Parzefall, 2001).
Cave mollies, on the contrary, exhibit reduced aggressiveness in all situations and rarely initiate aggressive behavior. It seems that a lack of light interrupts many of the standard aggressive techniques. Furthermore, the cave populations of P. mexicana do not form schools at all. This suggests the role of visual input in schooling behavior and coordination (Parzefall, 2001).
Reaches sexual maturity at about 1.5 to 2" in length. Under suitable conditions may produce several broods a year (Brown 1971). Fertilization internal; young born alive.
P. mexicana is a viviparous fish that undergoes internal fertilization and live birth with unique morphological adaptions. The female has fused ovaries and exhibits superfetation, the condition of supporting offspring that differ in age and resulted from different matings. The male has fused testes that form a tubular organ that transports cyst-like bundles of sperm. These sperm travel down the gonopodium, which is a specialized organ for transferring sperm into the female. This organ is actually a modified anal fin consisting of thickened fin rays and an elongated structure.
During copulation, the gonopodium is positioned at the entrance of a female's oviduct and transfers the bundles of sperm. Females can store sperm in the ovary and gonoduct for up to eight months and have short interbrood intervals (Meefe and Snelson, 1989),
Molecular Biology and Genetics
Barcode data: Poecilia mexicana
Below is a sequence of the barcode region Cytochrome oxidase subunit 1 (COI or COX1) from a member of the species.
See the BOLD taxonomy browser for more complete information about this specimen and other sequences.
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Download FASTA File
Statistics of barcoding coverage: Poecilia mexicana
Public Records: 39
Specimens with Barcodes: 110
Species With Barcodes: 1
National NatureServe Conservation Status
Rounded National Status Rank: NNA - Not Applicable
NatureServe Conservation Status
Rounded Global Status Rank: G5 - Secure
No special status (IUCN.org)
Not listed (IUCN.org)
Poecilia mexicana seems to be growing in number as nonnative populations become established, especially in the United States. The release of aquarium specimens and escapes from fish farms is the likely cause of this ((Poecilia mexicana (shortfin molly") ).
Poecilia mexicana is actually proving to be a threat to Unites States ecosystems as it begins to act as a harmful invasive species. It affects ecosystems by preying on native larval fish and decimating local populations of damselflies ("Poecilia mexicana (shortfin molly") )
Relevance to Humans and Ecosystems
Poecilia mexicana is a very important research subject for investigating the effects of adaptations to extreme environments on a population's morphology and behavior. Many studies have been done to describe these effects, and the existence of cave-dwelling populations (the extremophile form, or the population dwelling in an environment of biological extremes such as low light and high hydrogen sulfide content) and more typical epigean populations allow for comparison between the two forms and a thorough assessment of the evolutionary and behavioral implications of these adaptions (Tober and Plath, 2010)
Poecilia mexicana, commonly known as the shortfin molly or Atlantic molly, is a species of fish from the genus Poecilia.
Distribution and habitat
The fish lives in tropical freshwater and brackish water habitats. The shortfin molly is considered benthopelagic. It lives in a pH range between 7.0 and 7.5 at temperatures between 22 and 28 degrees Celsius.
The fishing industry has no interest in harvesting the species. However, it is sold commercially for aquariums. Occasionally, the shortfin molly is used as bait.
Etymology, taxonomy, and history
Franz Steindachner first described the species in 1863. Poecilia refers to the Greek word poikilos, which means "with a lot of colours". Common names include "shortfin molly" and "Atlantic molly."
Names and Taxonomy
Comments: Included in the POECILIA "SPHENOPS complex" by Lee et al. (1980). Included in order Cyprinodontiformes by Parenti (1981).
See Breden et al. (1999) for a molecular phylogeny of POECILIA.
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