Mosquitofish

Description

Mosquitofish are small and of a dull grey coloring, with a large abdomen, and have rounded dorsal and caudal fins and an upturned mouth. Sexual dimorphism is pronounced; mature females reach a maximum overall length of 7 cm (2.8 in), while males reach only 4 cm (1.6 in). Sexual dimorphism is also seen in the physiological structures of the body. The anal fins on adult females resemble the dorsal fins, while the anal fins of adult males are pointed. This pointed fin, referred to as a gonopodium, is used to deposit milt inside the female. Adult female mosquitofish can be identified by a gravid spot they possess on the posterior of their abdomens. Other species considered similar to G. affinis include Poecilia latipinna, Poecilia reticulata, and Xiphophorus maculatus; they are is commonly misidentified as mosquitofish.

Naming and taxonomy

The mosquitofish is a member of the family Poeciliidae of order Cyprinodontiformes. The genus name Gambusia is derived from the Cuban Spanish term gambusino, meaning "useless". The common name, mosquitofish, is derived from their use for biological control of mosquitoes, which itself was based on early observations that, in certain circumstances, they can reduce mosquito abundances. Classification of the western mosquitofish has been difficult due to their similarity to the eastern mosquitofish, and according to ITIS (Integrated Taxonomic Information System), G. holbrooki (eastern mosquitofish) may be an invalid taxonomic name, and could be considered a subspecies of G. affinis.

Diet

Mosquitofish are diet generalists, but they are considered "larvivorous" because they consume the larvae of mosquitoes and other aquatic insects. Their diet consists of zooplankton, small insects and insect larvae, and detritus material. Mosquitofish feed on mosquito larvae at all stages of life, if mosquito larvae are available in the environment. Adult females can consume up to hundreds of mosquito larvae in one day. Maximum consumption rate in a day by one mosquitofish has been observed to be from 42%–167% of its own body weight. However, they can suffer mortality if fed only mosquito larvae, and survivors of this diet shows poor growth and maturation. As generalists, mosquitofish also shown cannibalistic behavior on the young of their own species.

Habitat

The native range of the mosquitofish is from southern parts of Illinois and Indiana, throughout the Mississippi River and its tributary waters, to as far south as the Gulf Coast in the northeastern parts of Mexico. They are found most abundantly in shallow water protected from larger fish. Mosquitofish can survive relatively inhospitable environments, and are resilient to low oxygen concentrations, high salt concentrations (up to twice that of sea water), and temperatures up to 42 °C (108 °F) for short periods. Because of their notable adaptability to harsh conditions and their global introduction into many habitats for mosquito control, they have been described as the most widespread freshwater fish in the world.

Environmental impact

Mosquitofish were intentionally introduced in many areas with large mosquito populations to decrease the population of mosquitoes by eating the mosquito larvae. However, retrospectively, many introductions could be considered ill-advised; in most cases native fishes supplied control of mosquito populations, and introducing mosquitofish has been harmful to indigenous aquatic life. Mosquitofish introduction outside of their native range can also be harmful to ecosystems. Mosquitofish can consume or injure other small fish or otherwise harm them through competition. The ecological impacts of mosquitofish are partly dictated by their sex ratio, which can vary dramatically across their introduced range. Mosquitofish in Australia are considered noxious pests where they pose a threat to native fish and frog populations and little evidence indicates they have controlled mosquito populations or mosquito-borne diseases.

However, from the 1920s to the 1950s, mosquitofish were considered by some to be a significant factor in eradicating malaria in South America, southern Russia, and Ukraine. A somewhat famous example of mosquitofish eradicating malaria is on the coast of the Black Sea near Sochi in Russia. In Sochi, the mosquitofish is commemorated for eradicating malaria by a monument of the fish. Mosquitofish are still employed for biological control of mosquitoes in some places. In 2008, in some parts of California and in Clark County, Nevada, mosquitofish were bred in aquariums so people could stock stagnant pools of water with the mosquitofish to reduce the number of West Nile virus cases.

Reproduction

Reproduction of the mosquitofish starts with the male arranging the rays of the gonopodium (modified anal fin) into a slight tube. The male mosquitofish uses this tubular fin to secrete milt into the female's genital aperture in the process of internal fertilization. The female's genital aperture is located just behind the anal fin and is an opening for the milt to fertilize the ova within the ovary. Mosquitofish are within the infraclass Teleostei and as all teleosts, mosquitofish lack a uterus, so production of oocytes and gestation occur within the ovary of a female mosquitofish. Inside the female, sperm from multiple males can be stored to later fertilize ova. Based on laboratory experiments, female mosquitofish become vitellogenic when springtime temperatures reach 14 °C (57 °F), and then the oocytes mature when the average temperature reaches about 18 °C (64 °F). Then late in the summer when the photoperiod is less than 12.5 hours long, the next clutch of oocytes develops. In one reproductive season, a female may fertilize, with stored milt, two to six broods of embryos, with the size of the brood decreasing as the season progresses. Reproduction rates are highly dependent on temperature and ration level. As temperature increases from 20 to 30 °C, mean age at first reproduction decreases from 191 to 56 days, and brood size and mass of offspring increase significantly. Interbrood interval estimates at 25 and 30 °C are 23 and 19 days, respectively.

Embryology

Mosquitofish have a 16- to 28-day gestation period. They are lecithotrophic, which means during gestation, nutrients are provided to the embryos by a yolk sac. If the gestation period is shorter, each newborn will at birth still have a yolk sac connected through a slit located on the ventral side of the body wall. Brood size of females depends on the size of the given female; larger females are more capable of a larger brood quantity than smaller females. Many females have a brood quantity of up to or more than 60 young. Mosquitofish are viviparous, which means after the gestation of a brood, the female will have live birth. In most cases, the newborn brood will have an equal male to female sex ratio.

Growth

After birth, newborn mosquitofish are about 8 to 9 mm (0.31 to 0.35 in) in length. As juveniles, they grow at a rate of about 0.2 mm (0.0079 in) per day, but growth is highly temperature-dependent. Growth rates of juvenile mosquitofish reach their peak when the water temperature is within a range of 24 to 30 °C (75 to 86 °F), depending on resource availability. As temperatures rise above or dip below this range, growth rates decrease. Consistent temperatures at or above 35 °C (95 °F) are typically lethal, while growth stops when temperatures are at or below 10 °C (50 °F). For male mosquitofish, sexual maturity is reached in about 43 to 62 days, but maturation age is also dependent on temperature and resources. Female mosquitofish reach sexual maturity in about 21 to 28 days if born early within the reproductive season. The lifespan of a mosquitofish averages less than a year and the maximum is about 1.5 years. However, mosquitofish kept as pets can live much longer, with owners reporting lifespans of over three years. Male mosquitofish lifespans are considerably shorter than females.