Gambusia affinis 

western mosquitofish



Type Locality

Rio Medina and Rio Salado (San Antonio River drainages), TX (Baird and Girard 1853).


Etymology/Derivation of Scientific Name

Gambusia, derived from a provincial Cuban word, Gambusino; affinus, Latin, meaning “related” (Pflieger 1997).



Heterandia affinis Baird and Girard 1853:390.

Zygonectes melanops Hay 1881:501.

Gambusia patruelis Hay 1883:66; Hildebrand and Towers 1928:124.

Gambusia affinis Evermann 1899:309; Cook 1959:156.



Maximum size: Female, 53 mm SL (Hughes 1985b); male, 28 mm SL (Hughes 1985).


Coloration: Top of head dusky; bluish black spot below eye; with a conspicuous dark, elongate spot between anterior and posterior nares; back olivaceous with scattered black spots and a few scattered fine black spots; abdomen silvery to whitish, with an anteriorly directed dark triangle above anal origin; post anal region with a dark streak; black pigment surrounding anus and urogenital opening most conspicuous in females with eggs in ovaries; this pigment variable at other times (Peden 1973; Sublette et al. 1990).  Fins dusky with fine melanophores; dorsal fin with 1 or 2 irregular rows; caudal fin with a partial, vertical row of small, dark spots. Scales edged in black. Young yellowish (Sublette et al. 1990). No dark band on sides; median fins without large black spots near their bases (Hubbs et al. 1991).


Counts: 6 (rarely 7) dorsal fin rays (Hubbs et al. 1991); 28-31 lateral line scales; 9-10 anal rays; 11-14 pectoral rays (Ross 2001).


Body shape: Terete. Back nearly straight in profile. Female markedly larger and heavier bodied than the male (Sublette et al. 1990).


Mouth position: Terminal (Goldstein and Simon 1999).


External morphology: Pectoral fins much larger than pelvic fins. Pelvic fins small, ovate. Anal ovate in female; modified into gonopodium in male. Caudal truncate to slightly rounded (Sublette et al. 1990). Distal end of the fourth fin ray of gonopodium in male parallel or curved in only a weak arch; spines at tip of third anal fin ray of male gonopodium (first enlarged ray) one to three times longer than wide; origin of dorsal fin well behind origin of anal fin (Hubbs et al 1991).


Internal morphology: Teeth barely moveable (Hubbs et al. 1991), in broad villiform bands (Sublette et al. 1990). Peritonium black (Sublette et al. 1990); intestinal canal short with few convolutions (Hubbs et al. 1991).


Distribution (Native and Introduced)

U.S. distribution: Wide ranging species in the southern half of the United States east of the Rocky Mountains and west of the Appalachians (Hubbs et al 1991).


Texas distribution: Found throughout the state (Hubbs et al. 1991).Warren et al. (2000) list the following drainage units for distribution of Gambusia affinis in the state: Red River (from the mouth upstream to and including the Kiamichi River), Sabine Lake (including minor coastal drainages west to Galveston Bay), Galveston Bay (including minor coastal drainages west to mouth of Brazos River), Brazos River, Colorado River, San Antonio Bay (including minor coastal drainages west of mouth of Colorado River to mouth of Nueces River), Nueces River.


Abundance/Conservation status (Federal, State, NGO)

Populations in southern drainages are currently stable (Warren et al. 2000).


Habitat Associations

Macrohabitat: Common to abundant in vegetated ponds, lakes, drainage ditches, and backwaters and oxbows of sluggish streams (Lee and Burgess 1980).


Mesohabitat: Tends to swim near surface. Less common in moderate gradient streams. Often found in brackish or marine environments (Lee and Burgess 1980). Densest populations occur in shallow water having thick vegetation. Species does not adapt to extremely cold environments, selecting thermally fluctuating sites in summer and more thermally stable areas in winter (Hubbs 1971). At very low oxygen levels, individuals rise to water surface and respire the thin film of oxygenated surface water (Cech et al. 1985). Fish exposed to an extract of mosquitofish skin and water, exhibited a fright response, freezing at the water surface, and in extreme cases, darting to bottom and attempting to dig into the substrate in an effort to hide themselves (Reed 1969).




Spawning season: Spawns in warm months (Lee and Burgess 1980). In south central Texas, young occur from March –October with a peak in April. Photoperiod was more significant in reproduction initiation than water temperature (Davis 1978).


Spawning Habitat:


Spawning behavior: Viviparous; have internally fertilized eggs, special organs called gonopodia are developed to facilitate sperm transfer. Mating does not necessarily coincide with fertilization. After copulation, sperm can be stored for a lifetime (Simon 1999). In the process of fertilization, the significantly smaller males approach female from the side or behind; male will dart forward, and quickly thrust the gonopodium forward to an angle of 140-150 degrees, momentarily insert the tip into genital opening of female. Sperm are transferred in a spermatophore. The forward bendign of the gonopodium is accomplished by both intrinsic muscles and a concave arching of the back in the male. The spermatophore is conveyed from the genetal opening down the tubularly folded anal fin (gonopodium) to the tip just prior to intromission (Collier 1936; Paden 1973; 1975). Dark spots surrounding the anus and urogenital areas on the female motivate and facilitate male courtship (Peden 1975). Young born alive after 21-28 days of gestation (Krumholz 1948).


Fecundity: Female mosquitofish can store sperm for extended periods of time so that multiple broods can be produced from a single mating (Haynes 1993). A female can produce 14-218 embryos per brood depending on body size, and can produce two to six broods over the reproductive season, with the number of young per brood decreasing as the season progresses (Krumholz 1948; Haynes and Cashner 1995). Larger females produced larger numbers of offspring than smaller fish (Krumholz 1948). Up to 226 young reported in a single brood (Barney and Anson 1921).


Age at maturation: Females born near the beginning of the reproductive season may become gravid in 21-28 days at only 10-16 mm SL. Females born later in the season delay maturity for six to seven months until the following spring (Haynes and Cashner 1995). Vondracek et al. (1988) reported that females matured in 48-59 days at a water temperature of 30 degrees C, in an introduced California population.




Growth and Population structure: Sex ratios about 1:1 at time of birth, although in older fish the number of males gradually declines due to longer life span and greater hardiness of females. Newborn young average 8-9 mm TL and growth may be rapid, averaging about 0.2 mm/day for juveniles (Krumholz 1948). Growth greatest at water temperatures of 24-30 degrees C (Wurtsbaugh and Cech 1983; Haynes 1993). Growth rates decline as water temperatures approach the upper lethal temperature of 35 degrees C; at temperatures below 10 degrees C growth also stops (Wurtsbaugh and Cech 1983).


Longevity: Life span is short with only a few fish surviving longer than 1 year, with a maximum lifespan for females being 1.5 years. Male life spans are much shorter than those of females (Daniels and Feeley 1992; Haynes and Cashner 1995).


Food habits: Principally carnivorous, feeding on insect larvae, crustaceans, algae and fish fry, including its own progeny (Meffe and Crump 1987; Sublette et al. 1990). Harrington and Harrington (1961) reported species to be omnivorous; prefering mosquito larvae and pupae but consuming other invertebrates, zooplankton, fishes and algae. Goldstein and Simon (1999) list the first and second level trophic classifications as invertivore and drift, respectively; trophic mode: surface feeder. Feeding activity greatest at water temperatures of 24-30 degrees C (Wurtsbaugh and Cech 1983; Haynes 1993).


Phylogeny and morphologically similar fishes

The eight freshwater species of Gambusia in N.A., north of Mexico, are similar in appearance, but only the Mosquitofish occurs outside TX and New Mexico. The Pecos gambusia (Gambusia nobilis), is deeper bodied; has dark edges on median fins, usually 8 dorsal rays; lacks rows of dark spots on middle caudal fin. The following similar species: the blotched gambusia (Gambusia senilis), the Big Bend gambusia (Gambusia gaigei), and the Amistad gambusia (Gambusia amistadensis) have black spots and crescents on side, dark stripe along side; the largespring gambusia (Gambusia geiseri), has many black spots on side, lacks black to dusky teardrop, and black anal spot on female; the Clear Creek gambusia (Gambusia heterochir), has distinct notch in pectoral fin of male, no dark stripe along back, no row of black spots on caudal fin, and is deeper bodied; the San Marcos gambusia (Gambusia georgei), has lemon yellow fins and no row of black spots on middle of caudal fin (Page and Burr 1991).


Host Records

Cestoda (4); Trematoda (14); Nemata (7); Acanthocephala (2; Mayberry et al. 2000).


Commercial or Environmental Importance

Species has been widely introduced throughout the world for mosquito control (Hubbs et al. 1991); however stocking of Gambusia affinis in American southwest has extirpated many rare, localized populations of native species (Lee and Burgess 1980); Meffe (1984) reported that mosquitofish effectively eliminated the Sonoran topminnow (Poeciliopsis occidentalis) in 1-3 years, in some Arizona habitats; G. affinis has been instrumental in the elimination of native populations of Poeciliopsis occidentalis in New Mexico (Sublette et al. 1990). According to Courtenay and Meffe (1989) utilization of this species in mosquito control is ineffective; further, as a result of predation on eggs, larvae and juveniles, their introductions world-wide (except on the continent of Antaractica) have negatively impacted native fishes by significantly reducing or eliminating populations. Courtenay and Meffe (1989) suggest that this species is too aggressive and predatory to warrant indiscriminate introduction throughout the world, and support a biologically appropriate and warranted ban of its use as a control agent; this thought being documented experimentally by Lydeard and Belk (1993) and Belk and Lydeard (1994).



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