Gambusia Affinis)

Pacific Northwest Aquatic Invasive Species Profile: Western mosquitofish (Gambusia affinis) Laura Johnson FISH 423 December 3, 2008 Figure 1. Western mosquitofish G. affinis (photo source: www.usgs.gov). Diagnostic information eleven short spines on ray 3 (Page and Burr 1991). Order: Cyprinodontiformes Until 1988, both the western mosquitofish (G. Family: Poeciliidae affinis) and eastern mosquitofish (G. holbrooki) Genus: Gambusia were classified as subspecies of G. affinis. The Species: affinis classification of each fish as a separate species is important since they are native to different Common names: Western mosquitofish, portions of the eastern United States (Wooten et mosquitofish al. 1988). G. affinis can be distinguished from G. holbrooki by having six dorsal rays instead of The western mosquitofish, Gambusia seven, and a lack of prominent teeth on affinis, is a small (maximum 6.5 cm) gray or gonopodial ray three (Page and Burr 1991). brown fish with a rounded tail and upturned mouth (Figures 1 and 2). It may have a large Life-history and basic ecology dusky to black teardrop marking beneath its eye (as in Figure 1), but this marking is sometimes Life cycle reduced (as in Figure 2). G. affinis has a dark G. affinis are ovoviviparous, meaning stripe along its back to the dorsal fin, yellow and that the young develop within eggs inside the blue iridescence on transparent silver-gray body mother’s body and are then born live and do not sides, and six dorsal rays. G. affinis can be receive additional nourishment from the mother further distinguished from other members of its (Wydoski and Whitney 2003). Newborns are genus by a gonopodium with an elbow on ray 4a small, and weigh only about 1.2 to 1.3 mg. composed of two or more segments, and eight to Maturation can occur in as little as 3 to 4 weeks, although individuals born at the end of the reproductive season may delay maturity for 6 to 7 months, until the beginning of the following season. The lifespan of G. affinis differs for males and females, with females living 6 months to 1.5 years, and males averaging a much shorter lifespan, although an accurate estimate is unavailable (Haynes and Cashner 1995). Figure 2. G. affinis in a human hand to demonstrate approximate size (photo source: www.aqua-fish.net) Feeding habits anything but a mosquito larvae specialist (Gido G. affinis is omnivorous and utilizes a and Franssen 2007; Goodsell and Kats 1999). variety of food sources, which may give it an The predatory feeding behavior of G. affinis advantage in colonizing new sites (Lockwood et causes serious ecosystem alterations, as shown al. 2007). It is an aggressive predator and by Hurlbert et al. (1972) and Meffe (1985). commonly preys on the eggs, juveniles, and Hurlbert et al. (1972) conducted a feeding study small adults of other fish species. Like many involving G. affinis and found that it reduced predatory fishes, they have strong, conical teeth crustaceans, insects, and rotifer populations and short guts, and they consume terrestrial and within experimental pools, which subsequently aquatic vertebrates, detritus, algae, and vascular caused an increase in phytoplankton plants (Meffe and Snelson 1989). Grubb (1972) populations. Additionally, Meffe (1985) also found that G. affinis prey heavily on anuran documented the impact of G. affinis on the amphibian eggs when they are available (Grubb population of the Sonoran topminnow 1972). Similarly, G. affinis preys heavily on the (Poeciliopsis occidentalis), an endangered fish larvae of California newts and the tadpoles of native to the southwestern United States. G. Pacific treefrogs, and may negatively affect their affinis is implicated in the local extirpation of P. populations (Goodsell and Kats 1999). G. occidentalis in its native habitat, partially due to affinis also engage in cannibalism, which may predation by G. affinis (Meffe 1985). provide growth and reproductive benefits for individuals (Meffe and Crump 1987). Reproductive strategies Although G. affinis received both its name and G. affinis displays high fecundity and reputation for its supposed predation on short gestation periods, both of which may mosquito larvae, studies have demonstrated that confer success in the biological invasion process when other food sources are available, G. affinis (Haynes and Cashner 1995; Lockwood et al. does not necessarily prefer mosquito larvae. For 2007). The reproductive season generally lasts example, Goodsell and Kats (1999) showed that about 7 months, although in warmer climates even when mosquito larvae were provided as such as Hawaii, reproduction may occur year- food for G. affinis in a controlled feeding round. In areas where reproductive seasons are experiment, they still preyed voraciously on shorter, they usually begin in spring and end in amphibian tadpoles. Furthermore, analyses of fall. The gestation period ranges from only one stomach contents of wild-caught G. affinis in to three weeks. Females may produce anywhere two separate studies revealed a variety of prey in from one to seven broods per reproductive their stomachs, proving that G. affinis is season, and each brood may contain up to about 200 embryos. The fecundity of G. affinis is high, but ultimately depends on female size and these conditions are lethal to most freshwater reproductive status as well as geographic fish, G. affinis individuals were able to survive location. Fecundity tends to decrease from north at least a week in this environment (Hubbs to south and east to west (Haynes and Cashner 2000). 1995). In addition, female G. affinis are able to store sperm for extended periods of time, so that Biotic associations one mating may result in multiple broods (Farr The primary biotic associations of G. 1989). affinis are parasitic, and no commensal or pathogenic associations are described in the Environmental optima and tolerances literature. G. affinis individuals commonly The optimal environment of G. affinis is suffer from black spot disease, which is caused warm, shallow, slow-moving waters with dense by a parasitic trematode commonly known as a vegetation, high mineral content, and abundant black grub (Tobler and Schlupp 2008). Black food organisms. Specifically, they thrive in grubs use freshwater fish as an intermediate fresh and brackish water, and are found in host, between their definitive host, the belted ponds, ditches, lakes, creeks, rivers, and springs. kingfisher, and snails. Black grubs enter They are generally limited by cold temperatures, freshwater fish such as G. affinis by penetrating and cannot tolerate temperatures below 4 the skin and becoming enclosed in the fish’s degrees Celsius, which limits the distribution of tissue, where they form pinhead-sized black G. affinis in the northern United States. They spots, hence the name black spot disease (Figure are occasionally found in colder climates due to 3). The trematodes can greatly harm the host annual stocking, but they are not able to fish, but they cannot spread from fish to fish. establish a self-sustaining population. However, The penetration of the parasites into the skin can they can tolerate water temperatures up to 38 cause mechanical damage and hemorrhage of degrees Celsius and thrive in warm climates the host fish, as well as lipid depletion, which (Wydoski and Whitney 2003). can lead to death (Lane and Morris 2000). Hubbs (2000) demonstrated the ability of G. Tobler and Schlupp (2008) observed a decrease affinis to thrive in both high and low in shoaling behavior among G. affinis with black environmental quality environments. During a spot disease. They hypothesized that individuals drought in west Texas in 1988, the Diamond/Y with different markings (black spots) caused Draw drainage experienced extremely poor increased detection and subsequent predation of environmental quality of 41 ppt salinity, total healthy individuals. Additionally, black spot ammonia over 10 ppm, nitrates over 100 ppm, disease is associated with increased energy and dissolved oxygen below 1 ppm. Although demands due to lipid loss, and shoaling with sp., although the effect of this parasite on natural populations of G. affinis is not known (Crandall and Bowser 1981). Current geographic distribution G. affinis has established populations in many states outside of its native range, which is Figure 3. A fish (bass) exhibiting black spot disease located in the southeastern United States (Figure (photo source: Fisheries Division, Michigan Dept. of 4). G. affinis exists throughout the Pacific Natural Resources). Northwest, mostly along river corridors such as the Columbia, Willamette, Snake, Klamath, individuals requiring extra resources may reduce Clark Fork, and Flathead (Figure 5). In food available to other group members. As a Washington State, biologists found G. affinis result, healthy G. affinis preferred to shoal with overwintering in ponds near the confluence of other healthy individuals and excluded those the Snake and Columbia Rivers in the 1970s, with black spot disease (Tobler and Schlupp and it has since spread into other portions of the 2008). Columbia River. In the 1990s, they were Tapeworms may also infect G. affinis, which captured in the John Day River, Lewis River, serve as the definitive host for the tapeworm and Yakima River. Generally, they inhabit Bothriocephalus acheilognathi. Granath and slow-moving backwaters and sloughs in the Esch (1983) found that infection with B. mid- to lower Columbia River and the lower acheilognathi caused decreased survival in G. reaches of the Columbia’s tributaries. However, affinis. Increasing temperatures reduced since they are widely distributed for mosquito survival in infected fish, as higher temperatures control in private ponds, they may occur in caused increased growth of the tapeworm almost any water body in the Pacific Northwest (Granath and Esch 1983). (Wydoski and Whitney 2003). Scientists have found G. affinis populations infected with the microsporidian parasitic History of Invasiveness Glugea sp. in California. Glugea sp. can occupy the majority of the abdominal cavity of G.

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