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The Nonindigenous Occurrences section of the NAS species profiles has a new structure. The section is now dynamically updated from the NAS database to ensure that it contains the most current and accurate information. Occurrences are summarized in Table 1, alphabetically by state, with years of earliest and most recent observations, and the tally and names of drainages where the species was observed. The table contains hyperlinks to collections tables of specimens based on the states, years, and drainages selected. References to specimens that were not obtained through sighting reports and personal communications are found through the hyperlink in the Table 1 caption or through the individual specimens linked in the collections tables.

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Common name: Asian Swamp Eel

Synonyms and Other Names: Fluta alba, Monopterus alba, M. javanenesis; rice eel, ricefield eel, belut, rice paddy eel, ta-unagi

Taxonomy: available through www.itis.gov

Identification: Asian Swamp Eel is morphologically similar to two North American native fishes: American Eel (Anguilla rostrata) and lampreys (Ichthyomyzon, Lampetra, Lethenteron, or Petromyzon spp.). Asian Swamp Eels can be distinguished from American Eels by the presence/absence of pectoral fins (present in A. rostrata; absent in Monopterus sp.). Asian Swamp Eels may be distinguished from lampreys by the morphology of the gill opening (a single V-shaped opening in swamp eels; 7 small, pore-like gill openings in lampreys) and mouth/teeth (lampreys lack jaws and have an ovoid oral disc with embedded teeth). Asian Swamp Eels are also morphologically similar to two native salamanders (Siren and Amphiuma spp.), but can be distinguished by the presence/absence of legs/limbs (front and hind legs present in Amphiuma, front legs only in Siren, no legs/limbs in Monopterus). An identification guide highlighting to distinguishing Asian Swamp Eels from the taxa outlined above can be downloaded here.

Identifying characteristics and illustrations or photographs were given by Day (1958), Sterba (1973), Rosen and Greenwood (1976), Masuda et al. (1984), Talwar and Jhingran (1992), and Kottelat et al. (1993); characteristics were also provided by Smith (1945) and Merrick and Schmida (1984). This eel was included in keys of Rosen and Greenwood (1976), Talwar and Jhingran (1992), and Kottelat et al. (1993).

Most individuals in Florida are olive-drab brown in color with yellow-orange bellies (as shown above). However, some specimens are brightly colored with variations of orange, pink, and a calico pattern (Shafland et al. 2010).

Asian Swamp Eel is very morphologically similar to another introduced synbranchid, Cuchia (M. cuchia), and can be distinguished by the following characters (Nico et al. 2019):

	M. cuchia	M. albus/javanensis
Paired, lateral suprapharyngeal air chambers	present	absent
Head shape during aerial respiration	laterally expands on sides of head	throat expands ventrally
Body scales	very small, limited to tail region	absent
Palatine teeth	large, single row	small, two rows
Body coloration	upper body brown, with small (2-3+ mm diameter) circular/oval black spots	variable, but generally olive or brown, with irregular dark flecks (<1 mm diameter)

 

Size: Up to 100 cm (39 inches)

Native Range: Asia, from northern India and Burma to China, perhaps Asiatic Russia, Japan, and the Indo-Malayan Archipelago (Bailey and Gans 1998); possibly northeastern Australia (Merrick and Schmida 1984). Because of taxonomic confusion and the probability of crytic undescribed taxa included within the nomen M. albus (Collins et al. 2002), this range description for the species M. albus is probably overly broad.

Genetic analysis indicates that there have been multiple introductions from different geographic areas (Collins et al. 2002). The Atlanta population is from Japan or Korea; Florida populations in Tampa and North Miami are from Southern China while the population in Homestead is from Indo-China, the Malay Peninsula, or the East Indies (Collins et al. 2002).

Alaska	Hawaii	Puerto Rico & Virgin Islands	Guam Saipan

Hydrologic Unit Codes (HUCs) Explained
Interactive maps: Point Distribution Maps

Ecology: Swamp eels are generally found in slowly moving freshwater regions. They are noctural, and will often burrow into soft sediments or occupy crevices and small spaces (Shafland et al. 2010). In their native range, Asian Swamp Eel consumes a wide variety of invertebrate and vertebrate prey including fish (Yang et al. 1997; Cheng et al. 2003; Hill and Watson 2007). Hill and Watson's (2007) investigation into the diet of an introduced population near Tampa, Florida, revealed prey items such as amphipods, crayfish, fish, fish eggs, insects, oligochaetes, organic material, plant material and a tadpole. Shafland et al. (2010) found primarily fish (in 56% of stomachs), crustaceans (32%) (mostly crayfish), and insects (27%). Fish species included Swamp Darter (Etheostoma fusiforme), Bluefin Killifish (Lucania goodei), Eastern Mosquitofish (Gambusia holbrooki), other swamp eels, Fat Sleeper (Dormitator maculatus), Largemouth Bass (Micropterus salmoides), Mayan Cichlid (Cichlasoma urophthalmus), Tadpole Madtom (Noturus gyrinus), Bluegill (Lepomis macrochirus), Jaguar Guapote (Parachromis managuensis), African Jewelfish (Hemichromis letourneuxi), Black Acara (Cichasoma bimaculatum), and Spotted Sunfish (Lepomis punctatus). Swamp eels had also eaten mollusks, frogs, a turtle, fish eggs, and a snake's head. Shafland et al. (2010) gives further details on size and frequency of prey items, as well as comparison of day and night feeding.

This species is a sequential hermaphrodite: all individuals are born and mature as females and some later transform into males (Liem 1963; Shafland et al. 2010). Populations studied in south Florida tend to be heavily skewed towards females (90-98%; Shafland et al. 2010). In Snake Creek, individuals transitioning ranged in size from 694-782 mm TL, whereas the smallest male observed at another location was 434 mm TL. The smallest mature female was 318 mm TL and the smallest mature male was 434 mm TL (Shafland et al. 2010). A mature female averaged 439 (range 268-642) eggs (Shafland et al. 2010).

Laboratory studies show that swamp eels stopped feeding at 14-16ºC and died at 8-9ºC (Shafland et al. 2010). This temperature coincides with a range that would not extend farther north than Jacksonville, Florida (Shafland and Pestrak 1982). However, the Georgia population has survived air temperatures below freezing and ice cover over their pond habitat (Starnes et al. 1998) for many years indicating that it is different genetically (as determined by Collins et al. 2002). Collins et al. (2002) predicted these ecological differences based on genetics.

This species can breathe air, using atmospheric oxygen absorbed via a vascularized breathing apparatus at the rear of their mouths (Shafland et al. 2010).

Long and LaFleur (2011) used otoliths to estimate average daily growth rates and hatching dates for juvenile swamp eels in the Chattahoochee River, Georgia population, with an estimated growth rate of 0.2 cm/day.

Means of Introduction: Its introduction into Florida was probably the result of either an aquarium release, a fish farm escape or release, or release of specimens from the live food-fish market. The Tampa population is near a former fish farm. Colorful specimens may indicate selective breeding for the aquarium trade. They are a popular ethnic food item and may have been released by that trade. The eel was probably introduced into Georgia as an aquarium release. In Georgia, adults were first collected in 1994, although they were likely present since 1990 or before (Starnes et al. 1998; Turkewitz 2006). It was presumably brought to Hawaii by Asian immigrants as a food fish (Devick 1991b). Brock (1960) stated that it was established in Hawaii prior to 1900. Devick (1991b) listed it as one of six fish species that were successfully introduced into Hawaii in the 19th century. However, Cobb (1902) and Jordan and Evermann (1902, 1905) made no mention of finding the species in their turn-of-the-century surveys of Hawaiian fishes.

Status: Established in Florida (Shafland et al. 2010), Georgia (Starnes et al. 1998), and Hawaii (Devick 1991b). Status in New Jersey is unknown.

Impact of Introduction:

Summary of species impacts derived from literature review. Click on an icon to find out more...

Ecological	Economic	Human Health

Using a 26 year dataset in the Florida Everglades, Pintar et al. (2023) found that Monopterus albus/javenensis reduced the population of the crayfishes Procambarus alleni and P. fallax by >99%, and of the fishes Jordanella floridae by >99%, Fundulus confluentus by >90%, and F. chrysotus and Gambusia holbrooki by >50%. In both Georgia and Florida there is concern that the species will spread to adjacent water bodies as has occurred in the Everglades region of Florida (Kline et al. 2013). The Georgia impoundments are connected to the Chattahoochee River. Shafland et al. (2010) studied swamp eel populations established in south Florida and reported no deleterious ecological effects, although it should be noted that the investigators focused on populations inhabiting canals and associated waterways, habitats already highly disturbed. In addition, their study was largely interested in possible harm caused by swamp eels to sport fishes. Nico et al. (2011) examined the occurrence of internal parasites in both imported, wild-caught swamp eels from a U.S. retail food market and from an introduced population in Florida, finding parasites in nearly all specimens and highlighting the potential of Monopterus as a vector for introduction of macroparasites. Asian Swamp Eels are a known host of multiple Gnathostoma spp. nematodes, and are a potenial source of gnathostomiasis in humans (Cole et al. 2014).

Remarks: Although there are reports of this species moving over land, Shafland et al. (2010) found no evidence of that after studying them for many years. They are also reputed to be able to live out of water for a considerable length of time (Day 1958). Swamp eel populations in Florida show significant extended tolerance to moderate salinity levels (up to 14-16 ppt), suggesting that coastal and estuarine areas are potential pathways for dispersal (Schofield and Nico 2009).

The taxonomy of the genus Monopterus is in need of systematic review. Monopterus albus was initially thought to be a single species, but subsequent study has shown four known populations of this "species" (in Florida and Georgia) may actually be three genetically distinct (yet morphologically similar/identical) species or taxa, each from a different area of Asia (Collins et al. 2002).

Georgia specimens were identified by W.C. Starnes. Voucher specimens: Florida (NCSM 27827; UF 175761; USNM 361067.5271753); Hawaii (BPBM 3448, 3546, 5542, 5970, 28361, 30601, 30604, 33499; UF 98934); Georgia (NCSM 29671, 29709, 29710, 29714; UF 99740, 99872; USNM 339154.5241497, 339155.5241498).

In March of 2021, Monopterus albus were found in a live food market in Jackson, Mississippi, and the animals were seized (pers comm D. Riecke, 2021).

References: (click for full references)

Brock, V.E. 1960. The introduction of aquatic animals into Hawaiian waters. International Revue der Gesamten Hydrobiologie 45(4):463-480.

Bryant, R. - Georgia Game and Fish, Atlanta, GA. 1994.

Bailey, R.M., and C. Gans. 1998. Two new synbranchid fishes, Monopterus roseni from peninsular India and M. desilvai from Sri Lanka. Occasional Papers of the Museum of Zoology, No. 726, University of Michigan, Ann Arbor, Michigan, 18 pp.

Cheng, H., Y. Guo, Q. Yu, and R. Zhou. 2003. The rice field eel as a model system for vertebrate sexual development. Cytogenetic and Genome Research 101:274–277.

Collins, T.M., J.C. Trexler, L.G. Nico, and T.A. Rawlings. 2002. Genetic diversity in a morphologically conservative invasive taxon: Multiple introductions of swamp eels to the southeastern United States. Conservation Biology 16:1024-1035.

Cole, R.A., A. Choudhury, L.G. Nico, and K.M. Griffin. 2014. Gnathostoma spp. in live Asian Swamp Eels (Monopterus spp.) from food markets and wild populations, United States. Emerging Infectious Diseases 20(4):634-642. https://wwwnc.cdc.gov/eid/article/20/4/13-1566_article

Day, F. 1958. The fishes of India; being a natural history of the fishes known to inhabit the seas and fresh waters of India, Burma, and Ceylon, volume I. William Dawson and Sons Ltd., London, England.

Devick, W.S. 1991a. Disturbances and fluctuations in the Wahiawa Reservoir ecosystem. Project F-14-R-15, Job 4, Study I. Division of Aquatic Resources, Hawaii Department of Land and Natural Resources. 21 pp.

Devick, W.S. 1991b. Patterns of introductions of aquatic organisms to Hawaiian freshwater habitats. 189-213 in New directions in research, management and conservation of Hawaiian freshwater stream ecosystems. Proceedings of the 1990 symposium on freshwater stream biology and fisheries management, Division of Aquatic Resources, Hawaii Department of Land and Natural Resources.

Hill, J.E. and C.A. Watson. 2007. Diet of the nonindigenous swamp eel in tropical ornamental aquaculture ponds in West-Central Florida. North American Journal of Aquaculture 69:139–146

Kline, J.L., W.F. Loftus, K. Kotun, J.C. Trexler, J.S. Rehage, J.J. Lorenz, and M. Robinson. 2013. Recent fish introductions into Everglades National Park: an unforeseen consequence of water-management? Wetlands. http://link.springer.com/article/10.1007/s13157-012-0362-0.

Kottelat, M., A. J. Whitten, S.N. Kartikasari, and S. Wirjoatmodjo. 1993. Freshwater fishes of Western Indonesia and Sulawesi. Periplus Editions, Ltd., Republic of Indonesia.

Liem, K.F. 1963. Sex reversal as a natural process in the synbranchiform fish Monopterus albus. Copeia 1963(2):303-312. http://www.jstor.org/stable/1441348.

Long, J.M., and C. LaFleur. 2011. Estimation of daily age and timing of hatching of exotic Asian swamp eels Monopterus albus (Zuiew, 1793) in a backwater marsh of the Chattahoochee River, Georgia, USA. Journal of Applied Ichthyology 27:1019-1022.

Maciolek, J.A. 1984. Exotic fishes in Hawaii and other islands of Oceania. 131-161 in W.R. Courtenay, Jr., and J.R. Stauffer, Jr., eds. Distribution, biology, and management of exotic fishes. The Johns Hopkins University Press, Baltimore, MD.

Masuda, H., K. Amaoka, C. Araga, T. Uyeno, and T. Yoshino, editors. 1984. The fishes of the Japanese Archipelago. Tokai University Press.

Merrick, J.R., and G.E. Schmida. 1984. Australian freshwater fishes: biology and management. Griffin Press, Netley, South Australia.

Nico, L.G., P. Sharp, and T.M. Collins. 2011. Imported Asian swamp eels (Synbranchidae: Monopterus) in North American live food markets: potential vectors of non-native parasites. Aquatic Invasions 6:69-76.

Nico, L.G., A.J. Ropicki, J.V. Kilian, and M. Harper. 2019. Asian swamp eels in North America linked to the live-food trade and prayer-release rituals. Aquatic Invasions 14(4):775-814. https://www.reabic.net/aquaticinvasions/2019/AI_2019_Nico_etal.pdf

Pintar, M.R., N.J. Dorn, J.L. Kline, and J.C. Trexler. 2023. Hydrology-mediated ecological function of a large wetland threatened by an invasive predator. Science of the Total Environment 857(159245):1-8.

Roberts, T.R. 1989. The freshwater fishes of Western Borneo (Kalimantan Barat, Indonesia). Memoirs of the California Academy of Science 14. California Academy of Sciences, San Francisco, CA. 210 pp.

Schofield, P.J. and L.G. Nico. 2007. Toxicity of 5% rotenone to swamp eels. North American Journal of Fisheries Management 27:435-459.

Schofield, P.J. and L.G. Nico. 2009. Salinity tolerance of non-native Asian swamp eels (Teleostei: Synbranchidae) in Florida, USA: comparison of three populations and implications for dispersal. Environmental Biology of Fishes 85:51-59.

Shafland, P.L., K.B. Gestring, and M.S. Sanford. 2010. An assessment of the Asian swamp eel (Monopterus albus) in Florida.  Reviews in Fisheries Science 18(1):25-39.

Smith, C. 2008. (pers. comm.) New Jersey Division of Fish and Wildlife.

Smith, H.M. 1945. The fresh-water fishes of Siam, or Thailand. Bulletin of the U.S. National Museum (Smithsonian Institution) 188:1-622.

Starnes, W. - North Carolina State Museum of Natural History, Raleigh, NC.

Starnes, W.C., R.T. Brayand, and G.C. Greer. 1998. Perilous experiment: the Asian rice eel in Georgia. Georgia Wildlife: Urban Wildlife 7: 60-70. http://www.sherpaguides.com/georgia/atlanta_urban_wildlife/rice_eel/index.html

Sterba, G. 1973. Freshwater fishes of the world. English translation and revision from German. Two volumes. Tropical Fish Hobbyist Publications, Inc., Neptune City, NJ.

Talwar, P.K., and A.G. Jhingran, editors. 1992. Inland fishes of India and adjacent countries. A. A. Balkema, Rotterdam, The Netherlands. Two volumes.

Tilmant, J.T. 1999. Management of nonindigenous aquatic fish in the U.S. National Park System. National Park Service. 50 pp.

Turkewitz, J. 2006. Alien eels slither close to Hooch: Researchers fear creatures could disrupt ecosystem. The Atlanta-Journal Constitution 7(25): 4 pp.

Yang, D., F. Chen, D. Li, and B. Liu. 1997. [Preliminary study on the food composition of mud eel, Monopterus albus.] Acta Hydrobiologica Sinica 21:24–30. (In Chinese with English abstract.)

Other Resources:
Monopterus albus (Global Invasive Species Database)

US Fish and Wildlife Service Ecological Risk Screening Summary for Monopterus albus

Author: Fuller, P.L., Nico, L.G., Cannister, M., and Neilson, M.E.

Peer Review Date: 10/18/2013

This information is preliminary or provisional and is subject to revision. It is being provided to meet the need for timely best science. The information has not received final approval by the U.S. Geological Survey (USGS) and is provided on the condition that neither the USGS nor the U.S. Government shall be held liable for any damages resulting from the authorized or unauthorized use of the information.