The eastern mosquitofish (Gambusia holbrooki) is closely related to
G. affinis, and has previously been classified as subspecies G.
affinis. Some authorities (e.g., ITIS) still consider G. affinis
and G. holbrooki to be subspecies of the same organism.
The mosquitofish, Gambusia affinis , is a small, robust-bellied
fish. The relatively large head is flattened on the upper surface, and the
small mouth is superior (upturned) and protrusible. The eyes are large
relative to the body. Dorsal and caudal fins are rounded and no lateral
line is visible (IGGS 2006).
The body is usually greenish olive to brown above, grey-blue on the sides,
and silvery-white below. The body has a characteristic diamond or net
pattern on the body formed by dark pigment at the scale margins. Small
black dots are also usually present on the body and tail. A small dark bar
below the eye also aids in identification and a black peritoneum (abdominal
cavity lining) can often be observed through the belly in living specimens.
Melanistic (black or nearly black) individuals are common in some
populations. The dorsal fin is single and has only soft rays. Individuals
typically have 7-9 soft dorsal rays, 8-10 anal fins, and 29-32 lateral line
scales (Hoese and Moore 1977, Robins et al 1986, IGGS 2006).
The species is sexually dimorphic, with adult males being considerably
smaller than females and also possessing a gonopodium-an elongated anal fin
that functions as an intromittent organ for sperm transfer during mating.
Mature females have a distinct gravid spot located on the posterior abdomen
above the rear of the anal fin (Hoese and Moore 1977, GLAVCD undated).
Potentially Misidentified Species
As previously noted, Gambusia affinis and Gambusia holbrooki
are sufficiently similar in appearance and behavior to cause considerable
taxonomic difficulties, and these may or may not be distinct species. In
south Florida, Gambusia affinis co-occurs with the congeneric
mangrove gambusia, G. rhizophorae. The latter species is not known
from as far north as the IRL however (Ray et al. 1986).
The characteristic net-like scale pattern and the poster origin of the
dorsal fin relative to the anal fin are typically sufficient to distinguish
Gambusia from the co-occurring poeciliid, the sailfin molly,
Poecilia latipinna (Hoese and Moore 1977).
Poeciliid fishes can generally be differentiated from the potentially
similar looking cyprinodontids (the killifishes) by the presence of either
a gravid spot (mature females) or an intromittent organ (occurring on
mature males) on the former.
HABITAT AND DISTRIBUTION
Gambusia affinis is native to fresh/low-salinity waters of the
eastern and southeastern US and Gulf of Mexico, from New Jersey to central
Mexico (Hoese and Moore 1977). They may be found in fresh water as far
inland as Illinois (Ray et al. 1986).
Mosquitofish now also occur throughout much of the world as a result of
intentional and non-intentional introductions beginning approximately 100
years ago. Intentional introductions have largely been for purposes of
mosquito control, although G. affinis is not considered to be any
more effective against mosquitoes than most native mosquito-eating species
Gambusia affinis is a common inhabitant of lower salinity portions
of the lagoon and associated upland freshwater systems. Salt marshes,
mosquito impoundments, and mesohaline seagrass beds are among the habitats
utilized by this species.
LIFE HISTORY AND POPULATION BIOLOGY
Age, Size, Lifespan
Female mosquitofish reach a maximum total length of around 6-7 cm while
males may reach around 4 cm (FishBase 2004, GLAVCD undated), although most
individuals are somewhat smaller.
The maximum age reported for the species is 3 years (FishBase 2004).
While population densities in some portions of the IRL may be limited by
salinity, Minckley et al. (1991) suggest that Gambusia affinis may
be "possibly the single most abundant freshwater fish in the world".
Accomplished through the use of the male gonopodium. An early description
of mating behavior is that of Collier (1936). The male faces forward and
swings the gonopodium forward, initiating brief contact between the
gonopodial tip and the oviductal opening and transferring sperm to the
Viable sperm can be stored by females for several months, and the stored
sperm in any female is likely derived from multiple matings with a variety
of males (Medlen 1951).
Male mosquitofish mature at approximately 1 month/21 mm SL, and females at
28 mm/6 weeks (McDowall 2000).
Females typically brood around 60 young, but large individuals may carry
300 or more (McDowall 2000).
The gestation period has been experimentally determined to average around
24 days, a timeframe that is in close agreement with field observations of
pond-raised animals (Krumholz 1948). Most fry in the experimental
investigation had absorbed the maternal yolk before being liberated, but
some broods contained animals that had not completely absorbed the yolk
Mosquitofish are subtropical to warm-temperate, and typically occur at
temperatures between 12-29°C (FishBase 2004). Individuals inhabiting ponds
have been observed to reduce activity and "hibernate" in deeper water
during winter months (GLAVCD undated).
The livebearer family (Poecilidae) is principally a freshwater family, and
only mosquitofish and sailfin mollies invade marine habitats in the
southeastern US. Gambusia affini is euryhaline in habit, and is
common in salt marshes and other low-salinity coastal habitats. It is
generally found at lower salinities than the sailfin molly (Hoese and Moore
Laboratory experiments by Cech et al. (1985) revealed that mosquitofish
utilized aquatic surface breathing (i.e., expoiting the oxygen-saturated
surface layer of water) under hypoxic conditions, commencing such behavior
in a facultative manner at oxygen levels of 20-65 torr and in an obligatory
manner below 20 torr at 20°C.
Mosquitofish feed primarily on zooplankton, small insects and insect
larvae, and detrital material (ISSG 2006). As the common name suggests,
they are voracious consumers of mosquito larvae. All sizes and ages of
mosquitofish feed on mosquito larvae, and a large female mosquitofish can
consume hundreds of larvae a day (GLAVCD undated). Rajasekharan and
Chowdaiah (1972) demonstrated that G. affinis could discern between
different species of simultaneously presented mosquito larvae and
preferentially consumed certain species based on a number of factors
facilitating prey capture, including size, vertical position in the water
column, and the tendency of larvae to clump in groups.
Baber and Babbitt (2004) indicate Gambusia () are
capable of effectively consuming tadpoles of two Florida amphibians,
significantly impacting prey density. In addition, G. affinis is
capable of foraging effectively in densely vegetated areas that would
likely provide cover from larger predatory fish.
Mosquitofish, particularly where it occurs as a non-native, also prey
heavily on the eggs and young of co-occurring fish species.
Britton and Moser (1988) report that G. affinis represents a
significant portion of the diets of four species of Camargue herons they
studied. The authors also indicate that female fish are preferentially
consumed over the smaller males, in apparent concordance with optimal
Gambusia affinis has been discovered to be one of several primarily
freshwater fishes that serve as intermediate hosts of nematodes of genus
Falcaustra, adults of whom typically infest reptile or amphibian
hosts (Moravec et al. 1995).
Gambusia affinis occurs in a variety of freshwater and in protected
brackish environments. It preferentially occupies vegetated habitats,
including salt marsh and seagrass beds (Ray 1986). It is benthic and
non-migratory in habit and is most often encountered in standing or
slow-flowing waters (FishBase 2004, IGGS 2006).
Preference experiments by Casterlin and Reynolds (1977) revealed that
mosquitofish selectively occupied areas with subsurface vegetation but
avoided floating cover that restricted access to the water surface.
Lined sole are typically active in the evening hours, spending much of the daytime hours buried in shallow sand.
Gambusia affinis is among the most widely introduced fish species
worldwide, and a number of countries have reported negative environmental
consequences in the wake of mosquitofish introduction (Fishbase 2004).
Although effective as a mosquito control agent and widely introduced for
this purpose, mosquitofish have been found to compete with or displace
indigenous fish and to otherwise disrupt habitats. For example, McDowall
(1990) reports that selective predation by mosquitofish can alter
zooplankton, insect and crustacean communities.
G. affinis has been nominated as among the 100 "World's Worst"
invaders by the Invasive Species Specialist Group (ISSG 2006).
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predator-prey interactions between the fish (Gambusia holbrooki) and
Tadpoles of Hyla squirella and Gastrophryne carolinensis.
Britton RH and ME. 1982. Size specific predation by herons and its effect
on the sex-ratio of natural populations of the mosquito fish Gambusia
affinis Baird and Girard. Oecologia 53:146-151.
Casterlin ME and WW Reynolds. 1977. Aspects of habitat selection in the
mosquitofish Gambusia affinis . Hydrobiologia 55:125-127.
Collier A. 1936. The mechanism of internal fertilization in
Gambusia. Copeia 1936:45-53.
Cech JJ Jr., Massingill MJ, Vondracek B and AL. Linden. 1985. Respiratory
metabolism of mosquitofish, Gambusia affinis : effects of
temperature, dissolved oxygen, and sex difference. Environmental Biology of
FishBase. 2004. Species profile: Gambusia affinis Mosquito fish.
Greater Los Angeles County Vector Control District (GLAVCD). Undated.
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Hoese HD and RH Moore. 1977. Fishes of the Gulf of Mexico. Texas,
Louisiana, and Adjacent Waters. Texas A&M University Press, College Station
TX. 327 p.
Invasive Species Specialist Group (ISSG). 2006. Ecology of Gambusia
affinis . Global Invasive Species database. Available online.
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affinis affinis (Baird & Girard), and Its use in mosquito control.
Ecological Monographs 18:1-43.
McDowall RM. 1990. New Zealand Freshwater Fishes, A Natural History and
Guide. Heinemann Reed. 238 p.
McDowall, R. M. 2000. The Reed Field Guide to New Zealand Freshwater
Fishes. Reed Publishing. 224 p.
Medlen AB. 1951. Preliminary observations on the effects of temperature and
light upon Gambusia affinis . Copeia 1951:148-152.
Minckley WL., Meffe GK, and DL Soltz. 1991. Conservation and management of
short-lived fishes: the cyprinodontoids. Pages 247-82 in: Minckley WL and
JE Deacon (eds.). Battle Against Extinction: Native Fish Management in the
American West. University of Arizona Press, Tucson, Arizona.
Moravec F, Huffman DJ, and DJ Swim. 1995. The first record of fish as
paratenic hosts of Falcaustra spp. (Nematoda: Kathlaniidae). Journal
of Parasitology 81:809-812.
Rajasekharan PT and BN Chowdaiah. 1972. Selective Feeding Behaviour of
Gambusia affinis . Oecologia 11:79-81.
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