#mosquitofish

You get an Eastern Mosquitofish

For today's Wet Beast Wednesday topic, we're going to examine a small fish with a big impact. Or rather two closely-related fish. The eastern and western mosquitofish are ecologically important species that have unfortunately become a major problem in a lot of places. Dive in to learn how and why.

(image: a female (top) and male (bottom) eastern mosquitofish. They are both small, grey-brown fish with large abdomens, large eyes, upward-facing mouths, and rounded fins. The male is considerably smaller than the female and his anal fin has a long, pointed projection training from it. The female's belly is distended. End ID)

The western mosquitofish (Gambuisa affinis) and eastern mosquitofish (Gambusia holbrooki) are two very similar and closely-related species of livebearers (though it has been suggested that the eastern mosquitofish should be reclassified as a subspecies of the western mosquitofish). There are other fish in Gambusia, but I'm focusing on these two for today. They are small and sexually dimporphic in size. Males reach about 4 cm (western) or 3.8 cm (eastern) while the females grow larger, reaching 7 cm (western) or 6.4 cm (eastern). Their bodies are a grey or brown color, with lighter underbellies and they have upturned mouths. The fins are rounded and have small spots on them. The fins are the best way to tell western and eastern mosquitofish apart, as they are so visually similar to each other. Eastern mosquitofish typically have more fin rays in the dorsal and anal fins than western mosquitofish. Males and females can also be distinguished by the anal fins. In females, the anal fin looks like the dorsal fin. In males, some anal fin rays are modified into a tube-like structure called a gonopodium that is used in reproduction.

(Image: a male (top) and female (bottom) western mosquitofish. They look almost identical to the eastern mosquitofish. Arrows in the picture point to the male's gonopodium and a dark patch on the female's side)

Mosquitofish are freshwater fish found in the southeastern United States. The western mosquitofish has a larger range, being found in the Mississippi River and its tributaries south of middle Illinois and Indiana, east to Louisiana, and west to Texas and northernmost Mexico. The eastern Mosquitofish is found in Atlantic drainage systems to the east of the Western mosquitofish's range from Florida to Maryland. Both species have also been introduced outside of their native ranges. They prefer shallow, slow-moving or still water with vegetation or other places to hide from predators. Mosquitofish can survive water conditions that would be inhospitable to most other species, including high and low temperatures, high pH, high concentrations of chemicals dangerous to other fish, low oxygen levels, and salt levels twice as high as seawater (albeit temporarily). Mosquitofish are generalist omnivores. Both consume algae, organic detritus, invertebrates, and fish larvae. Western mosquitofish tend to be more carnivorous while eastern mosquitofish are more herbivorous, but both species can easily switch up their diets based on availability. Aquatic insect larvae makes up a large portion of their diet. The name mosquitofish comes from their history of being sold as a predator of mosquito larvae. They typically swim near the surface of the water and catch food from below. Mosquitofish are described as being aggressive to other fish and tadpoles and are known to bite fins and prey on fish larvae. Predators of mosquitofish include bass, sunfish, and catfish.

This is an example of a mosquitofish exhibiting a form of melanism. These are often called dalmatian mosquitofish. (Image: a male mosquitofish who is white with black patches instead of its usual coloration)

Mosquitofish are livebearers, meaning they reproduce internally and give live birth. When mating, the male will insert his gonopodium into the female's coaca. The tube-like structure of the gonopodium lets sperm flow through it into the female. Females can store the sperm of several males internally for a while before she fertilizes her eggs. Mosquitofish are ovoviviparous, meaning the eggs develop and hatch internally and receive no no extra nutrition from the mother, making do with yolk. Pregnant females develop distended abdomens. The size of the brood varies based on the mother's age and water quality can can reach up to 100 larvae. Once the larvae hatch, they are released into the water by the mother. Mosquitofish larvae are born more developed than larvae of other fish, giving them a better chance of survival than fish born from external eggs. Western mosquitofish are reported to have two main breeding seasons per year, in spring and late summer. A female born in the spring breeding season can reach sexual maturity in time for the late summer breeding season in good conditions. Eastern mosquitofish mate continuously from mid spring to mid autumn, with midsummer being peak breeding time. Eastern mosquitofish reach sexual maturity in around 9 weeks. A female eastern mosquitofish can have up to 9 broods a year. Male eastern mosquitofish are reportedly highly aggressive during mating season and will pursue and bite females. Females often form shoals to protect themselves from aggressive males.

(image: a pregnant mosquitofish held in had. Its belly is highly distended and white. Small shapes are visible through the belly, which are likely the larvae. End ID)

Both species of mosquitofish are classified as least concern by the IUCN, meaning they are not in danger of extinction. Mosquitofish are commercially important due to being sold as mosquito control. In the USA, they are sold to pond owners as fish that need very little care and will eat mosquito larvae. This has led to mosquitofish being introduced far outside of their range. Thanks to governmental programs and private efforts at biological mosquito control, mosquitofish can now be found on every continent except Antarctica. They are considered an invasive species in many regions as they can outcompete native fish and threaten local insects and other species that have an aquatic larval state. Mosquitofish are considered a noxious pest in Australia, where they were introduced to control mosquitos. Environmentalists now think that the mosquitofish have actually worsened the problem as they don't eat that many mosquito larvae and are outcompeting native fish who were much better at controlling mosquito populations. They also worsen the water quality by overeating herbivorous zooplankton, leading to algal blooms and are attacking tadpoles, leading to population declines in multiple species of frogs. Because mosquitofish are so resilient and adaptable, it is very difficult to develop control measures for them that don't also eradicate native species. One control method that has shown progress may be my favorite method of dealing with invasive species. Some engineers from the University of West Australia built a robotic largemouth bass that has been observed to chase away mosquitofish and alter their behavior.

Image: a robotic largemouth bass built to as population control. It is a fish-shaped robot covered in white material with markings to make it look like a bass. The eyes are red. End ID)

Eastern Mosquitofish Art :D

Florida Wildlife; vol. 9, no. 11. April, 1956. Illustration by Wallace Hughes.

A delightfully chubby Least Killifish female and a very cute "dalmatian" melanistic Mosquitofish male, both caught with a net in the retention pond at work.

AKA Plague Minnow.

A wildly invasive livebearing Poeciliid native to the Eastern and Southern US, then introduced to other parts of the world such as New South Wales, supposedly to control mosquitoes. Despite the fact we already had native fish such as the Empire Gudgeon that already did that. They now are driving many populations of those natives to extinction, through harrassment and eating the fry.

Some other Gambusia are actually quite threatened species - at least two are extinct.