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Monopterus albusAsian swamp eel(Also: Rice eel; Swamp eel; white ricefield eel)

By Wyatt Erzen

Geographic Range

Asian swamp eels, Monopterus albus, are native to the Oriental region, from southeast to east Asia. Asian swamp eels stretch as far west as east India that includes the Greater Sunda Islands, the Malay Peninsula, and the Indochinese Peninsula. Their range continues as far east as Shanghai, Taiwan, and the Miyako Islands of Japan. This species extends as far north as Beijing, China then southward to Malaysia, the Philippines, and Indonesia.

Matsumoto et al. (2010) suggest that genetically, Asian swamp eels consist of three geographical clades: the China-Japan, Ryukyu Island, and Southeast Asia clades. The authors argued that these clades should be separate species.

This species has been introduced to the Neartic region. They are found on the southeast coast of the United States, as far north as the Chattahoochee National Forest in Georgia and stretches downward through Atlanta to Columbus. They have also been observed in a stretch of waterways in Florida that reaches as far north as Tampa and southward to Port Charlotte. These eels also have been found in central Miami. (Matsumoto, et al., 2010; Shafland, et al., 2010)

Habitat

Asian swamp eels typically are found in diverse aquatic habitats in their native and introduced regions. This includes lakes, rivers, ponds, marshes, swamps, and waste drains or sewers. These eels tolerate fresh and brackish waters. They thrive in muddy environments because of their cryptically-colored skin, and adaptation to low-oxygen levels. Asian swamp eels burrow underneath rocks or soft mud during the day. They can survive in conditions with up to 80% mud and 20% water areas. Their prime habitat temperature ranges from 25-31 degrees Celsius.

They also have a unique adaptation that allows them to travel across land for short distances that can help in moving among suitable habitats.

In their introduced region of Miami and Florida, Asian swamp eels are found in rivers and canals that run east-southeastward connecting the interior waters to the Atlantic Ocean. Sow et al. (2018) reported that eels inhabit large canals in Miami that average 40 meters in width and run 3-4 meters in maximum depth. The canals also possess dense submerged vegetation for hiding. (Sow, et al., 2018)

  • Aquatic Biomes
  • lakes and ponds
  • rivers and streams
  • temporary pools
  • brackish water
  • Range depth
    0 to 3 m
    0.00 to 9.84 ft

Physical Description

Asian swamp eels are cryptically-colored, usually dark brown or green color. Typically, they have a single v-shaped gill opening underneath their throats. Individuals have two rows of mandibular teeth and have reduced eyes with a thin covering of skin. They typically don’t have any scales and are finless. Glands on their skin releases a protective mucous along their whole body to help them navigate muddy waters. Despite their gills, these eels have the ability to obtain as much as a quarter of their oxygen needs via cutaneous respiration.

Asian swamp eels in their native land average a total length of 240-375 mm, and a maximum length of 1000 mm. This species has a mouth heights, widths, and gape area of 1.5 mm, 1.5 mm, and 8 mm^2, respectively. They have terminal mouths orientation and laterally-located eyes.

Khanh and Ngan (2010) reported that in their native region the male individuals had a mean body weight of 250g and were greater than 500 mm in total length. Females were reported to have a mean body weight of 40-100g and a total length of 300-400 mm. This size difference is due to the species being a sequential, protogynous hermaphrodite. Average masses are about 100 g. The average birth mass of Asian swamp eels is 2.04 grams.

This species is typically confused with snakes, other fish, or leeches depending on their size. (Khanh and Ngan, 2010; Sow, et al., 2018; Ward-Campbell, et al., 2005)

  • Sexual Dimorphism
  • male larger
  • Range mass
    40 to 250 g
    1.41 to 8.81 oz
  • Average mass
    100 g
    3.52 oz
  • Range length
    30 to 1000 mm
    1.18 to 39.37 in

Development

Asian swamp eels eggs incubate in a nest, hatching after 7-8 months. he average birth mass of Asian swamp eels is 2.04 grams. After 7-10 days post-hatching, the yolk-sac is completely absorbed. These eels are sequential, protogynous hermaphrodites; all of the hatchlings are born as females, and can later naturally transform into males to balance the sex ratio in the population.

The change in sex from female to male is completed in 8-30 weeks. Chan and Phillips (1967) reported that there exists an "intersex" period during the transition, when individuals contain both male and female gonads. This transformation generally occurs when individuals are 35-45 cm long and at least 1-2 years old (after the females' first spawning event). The authors found that by the time the individuals reach 55 cm or greater, only 8.3% of them are still female.

These eels presumably exhibit indeterminate growth. (Chan and Phillips, 1967; Chan, et al., 1972; Khanh and Ngan, 2010)

Reproduction

Asian swamp eels are a size-advantage based species. Typically, males with greater size in the population build a nest to attract females. Their nests are free-floating bubble-nests that are located near the shoreline and in narrow passageways among underwater vegetation in shallow water. The narrow passageways allows for only one nest to be located for optimal reproduction. The females will pick the most favorable nest depending on the size of the nest and male, and move from nest to nest until they don't have any more eggs to lay. This results in larger males mating more frequently than smaller males.

Males are highly territorial of their nests and protect them by biting attacks on the other males, and can result in injury or death. (Baensch and Riehl, 1985; Matsumoto, et al., 2010)

Asian swamp eels typically breed twice from March to September in two separate spawning periods. Khanh and Ngan (2010) found that females typically spawn 38 to 625 eggs and average 295 eggs per spawning period. Fertilization is external.

This species is oviparous, and incubate as eggs for 7-8 months. The average birth mass of Asian swamp eels is 2.04 grams. Xiaojuan et al. (2014) reported that in captivity females took 1 year to reach sexual maturity. Chan et al. (1972) stated that sexual transformation takes 8-30 weeks to complete, thus male sexual maturation is 60-82 weeks. (Chan and Phillips, 1967; Chan, et al., 1972; Khanh and Ngan, 2010; Xiaojuan, et al., 2014)

  • Breeding interval
    Asian swamp eels breed one to two times yearly during the warmer months from March to September
  • Breeding season
    March and September (Two times for spawning)
  • Range number of offspring
    38 to 625
  • Range time to hatching
    7 to 8 months
  • Average time to independence
    0 months
  • Average age at sexual or reproductive maturity (female)
    1 years
  • Range age at sexual or reproductive maturity (male)
    60 to 82 weeks

The Asian swamp eel is a hermaphroditic species in which females transition to males. After fertilization, males tend to protect the nest of fertilized eggs with force because its a size dominated species. The males protect the nest until the eggs are ready to hatch after 7-8 months. The males provide no parental care after the young hatch. The females provide no parental care beyond the act of spawning. (Khanh and Ngan, 2010; Matsumoto, et al., 2010)

  • Parental Investment
  • male parental care
  • pre-hatching/birth
    • provisioning
      • male
    • protecting
      • male

Lifespan/Longevity

There is nothing published reporting longevity in the wild for genus, family, order, or class. This species has no longevity records for individuals in captive because they are used for food, and medicinal exporting.

Behavior

Asian swamp eels are nocturnal predators. Throughout the day, they spend their time burrowed deep into mud or thick vegetation hidden from other animals. Individuals are very aggressive and voracious when they encounter prey. The only interspecific social encounters that have been recorded have been during mating times, and the males have been seen to be very aggressive to other Asian swamp eels. Males can make bubble nests in shallow areas, and females deposit their eggs in them.

These eels can breathe using gills or via cutaneous respiration if they are in low-oxygen conditions or out of the water. They have the ability to cross land to reach other suitable waterways. Rainboth (1996) suggests that they can live out of water for extended periods so long as the skin remains moist (for cutaneous respiration). They can burrow as deep as 1.5 m into muddy substrates and wait out dry seasons. ("Fishes of the Cambodian Mekong", 1996; Matsumoto, et al., 2010)

Home Range

Male Asian swamp eels defend a territory when they protect their fertilized eggs during the mating season. They usually protect a nested area where they try to hide the eggs in a narrow passageway with their bodies in between the entrance and the eggs. Home ranges and territory sizes have not been quantified for these males. (Matsumoto, et al., 2010)

Communication and Perception

There is nothing published reporting on communication and perception for Asian swamp eels.

There are reports of a close relative, the swamp eel, Monopterus digressus. Reported by Vincent and Thomas (2011), this related swamp eel exhibits visually-directed movements when foraging. This species was shown to favor using olfactory cues to locate a prey and tactile stimuli on the oral region to catch the prey. It is assumed that Asian swamp eels share similar aspects of perception. (Vincent and Thomas, 2011)

Food Habits

Asian swamp eels are opportunistic omnivores. In their native lands, they commonly consumer mollusks and crustaceans. Hill and Watson (2007) reported that in their introduced region, Asian swamp eels prey is wide-ranging including: amphipods, crayfish, fish, fish eggs, insects, oligochaetes, organic material, plant material, and tadpoles. Asian swamp eels also feed on snails, insect larvae and frogs. Sow et al. (2018) found that in captivity, Asian swamp eels are fed guppy fish (Ikan cere), and redworms (Lumbricus rubellus, and Eisenia fetida). In rearing activities cow's blood is also used as a food item. ("Fishes of the Cambodian Mekong", 1996; Hill and Watson, 2007; Sow, et al., 2018)

  • Animal Foods
  • amphibians
  • fish
  • eggs
  • blood
  • insects
  • mollusks
  • terrestrial worms
  • aquatic or marine worms
  • aquatic crustaceans
  • other marine invertebrates

Predation

Asian swamp eels have few natural predators because of their cryptic coloration and nocturnal, burrowing lifestyle. This species is difficult for predators to locate. Their main predator is humans (Homo sapiens) who consume them.

Asian swamp eels are also vulnerable to competition within their species. They can be killed during mating periods because of their size-based advantage relationship. (Bricking, 2002; Matsumoto, et al., 2011; Sow, et al., 2018)

  • Anti-predator Adaptations
  • cryptic
  • Known Predators
    • Humans (Homo sapiens)
    • Other Asian swamp eels (Monopterus albus)

Ecosystem Roles

Asian swamp eels are known as primary predators in their native region.

The United States Department of Agriculture (2018) states that Asian swamp eels are an invasive species, but due to lack of evidence there is no known ecosystem effect in their introduced region. Boping and Wenbin (2007) and Frantisek and Wang (2002) reported that Asian swamp eels are hosts to a few parasitic species including spiny-headed worms (Pallisentis celatus), roundworms (Eustrongylides sp., and Dentiphilometra monopteri). Cole et al. (2014) reported that 5 roundworms Gnathostoma turgidum AL3, and 1 roundworm Gnathostoma lamothei AL3 were found in the muscle and kidney of three wild- caught Asian swamp eels, respectively. (Boping and Wenbin, 2007; Cole, et al., 2014; František and Wang, 2002; United States Department of Agriculture, 2018)

Commensal/Parasitic Species
  • Spiny-headed worm, Pallisentis celatus
  • Roundworm, Dentiphilometra monopteri
  • Roundworm, Eustrongylides sp.
  • Roundworm, Gnathostoma turgidum
  • Roundworm, Gnathostoma lamothei

Economic Importance for Humans: Positive

Asian swamp eels are prized in their native region because they are a high demand delicacy with many uses. They are consumed in Japan, China, Taiwan, and South Korea, and each country exports the species to Holland, France, Spain, and Australia. Asian swamp eels are raised in captivity for traditional medicine in Asia.

Traditional medicine suggests to consumers that their blood, and slime can help treat cataracts. Their bone is used to supposedly postpone menopause in women and is also used in traditional cancer medication for stage 1 patients with any cancer. Ruslan Roy (2011) documented that 60,000 kg are used in traditional medicine each day in Asia. Norhafiza (2013) reported that the 2011 market price for Asian swamp eels ranged from RM (Malaysian Ringgit) 5.2-7.0/kg (1.25-1.68 USD/1kg). (Norhafiza, 2013; ; Ruslan Roy, 2011; Sow, et al., 2018)

Economic Importance for Humans: Negative

The United States Department of Agriculture (2018) stated that Asian swamp eels are an invasive species in North America. However, Shafland et al. (2010) reported no evidence that Asian swamp eels have a negative economic impact on humans. The U.S. Army(2005) reported that Asian swamp eels may displace native species and contribute to accelerated drying of shallow water bodies by their extensive burrowing systems which interferes with USACE operations. ("Invasive marine and estuarine animals of the South Atlantic and Puerto Rico", 2005; Shafland, et al., 2010; United States Department of Agriculture, 2018)

Conservation Status

Asian swamp eels are indicated as a species of "Least Concern" by the IUCN Red List. They have no special status on the US Federal list, CITES, and the State of Michigan list.

Asian swamp eels have no widespread threats from any predators. The species is stated as invasive in their introduced region but there are no reported threats that they have imposed on other species. In Florida, Shafland et al. (2010) reported no evidence that Asian swamp eels are having impacts on the ecosystem indicative of invasive species.

This species has no known reports of population management in their native or introduced region. They are harvested for food and traditional medicine in their native lands, but there are no reported caps for harvests per year. They can reportedly be captured via a variety of trapping methods, including by hand. (Dahanukar, 2010; "Fishes of the Cambodian Mekong", 1996; Shafland, et al., 2010)

Contributors

Wyatt Erzen (author), Radford University, Layne DiBuono (editor), Radford University, Lindsey Lee (editor), Radford University, Kioshi Lettsome (editor), Radford University, Karen Powers (editor), Radford University, Tanya Dewey (editor), University of Michigan-Ann Arbor.

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

agricultural

living in landscapes dominated by human agriculture.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

brackish water

areas with salty water, usually in coastal marshes and estuaries.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

cryptic

having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.

detritus

particles of organic material from dead and decomposing organisms. Detritus is the result of the activity of decomposers (organisms that decompose organic material).

drug

a substance used for the diagnosis, cure, mitigation, treatment, or prevention of disease

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

estuarine

an area where a freshwater river meets the ocean and tidal influences result in fluctuations in salinity.

external fertilization

fertilization takes place outside the female's body

fertilization

union of egg and spermatozoan

food

A substance that provides both nutrients and energy to a living thing.

freshwater

mainly lives in water that is not salty.

indeterminate growth

Animals with indeterminate growth continue to grow throughout their lives.

insectivore

An animal that eats mainly insects or spiders.

introduced

referring to animal species that have been transported to and established populations in regions outside of their natural range, usually through human action.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

macroalgae

seaweed. Algae that are large and photosynthetic.

male parental care

parental care is carried out by males

marsh

marshes are wetland areas often dominated by grasses and reeds.

molluscivore

eats mollusks, members of Phylum Mollusca

motile

having the capacity to move from one place to another.

natatorial

specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.

nocturnal

active during the night

omnivore

an animal that mainly eats all kinds of things, including plants and animals

oriental

found in the oriental region of the world. In other words, India and southeast Asia.

World Map

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

piscivore

an animal that mainly eats fish

polygynandrous

the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

protogynous

condition of hermaphroditic animals (and plants) in which the female organs and their products appear before the male organs and their products

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

sanguivore

an animal that mainly eats blood

seasonal breeding

breeding is confined to a particular season

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

solitary

lives alone

swamp

a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.

tactile

uses touch to communicate

territorial

defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement

visual

uses sight to communicate

References

U.S. Fish & Wildlife Service. Asian swamp eel Monopterus albus. None. U.S. Department of the Interior, 1849 C Street NW, Washington, DC 20240: U.S. Fish & Wildlife Service. 2018.

Rainboth, W.J. and FAO. Fishes of the Cambodian Mekong. FAO species identification field guide for fishery purposes. Rome, Italy: FAO. 1996. Accessed November 19, 2018 at http://www.fao.org/docrep/010/v8731e/v8731e00.htm.

U.S. Army Corps of Engineers. Invasive marine and estuarine animals of the South Atlantic and Puerto Rico. ERDC/TN ANSRP-05-5. Vicksburg, Mississippi: U.S. Army Engineer Research and Development Center. 2005.

Baensch, H., R. Riehl. 1985. Aquarien Atlas. Melle, Germany: Mergus.

Boping, Z., W. Wenbin. 2007. Seasonal population dynamics of Pallisentis (Neosentis) celatus (Acanthocephala: Quadrigyridae) in the intestine of the rice-field eel Monopterus albus in China. Journal of Helminthology, 81/4: 415-420.

Bricking, E. 2002. "Asian swamp eel (Monopterus albus)" (On-line). Introduced Species Summary Project. Accessed September 15, 2018 at http://www.columbia.edu/itc/cerc/danoff-burg/invasion_bio/inv_spp_summ/Monopterus_albus.html.

Chan, S., F. Tang, B. Lofts. 1972. The role of sex steroids on natural sex reversal in Monopterus albus. Fourth International Congress of Endocrinology, 256: 348.

Chan, S., J. Phillips. 1967. The structure of the gonad during natural sex reversal in Monopterus albus. Journal of Zoology, 151: 129-141.

Cole, R., A. Choudhury, L. Nico, K. Griffin. 2014. Gnathostoma spinigerum in live Asain swamp eels (Monopterus spp.) from food markets and wild populations, United States. Emerging Infectious Diseases, 20/4: 634-642.

Dahanukar, N. 2010. "Monopterus albus" (On-line). The IUCN Red List of Threatened Species 2010: e.T166148A6183570. Accessed September 05, 2018 at http://dx.doi.org/10.2305/IUCN.UK.2010-4.RLTS.T166148A6183570.en.

Davidson, A. 1975. Fish and Fish Dishes of Laos. Vientiane, Laos: Imprimerie Nationale Vientiane.

František, F., G. Wang. 2002. Dentiphilometra monopteri n. gen., n. sp. (Nematoda: Philometridae) from the abdominal cavity of the ricefield eel Monopterus albus in China. Journal of Parasitology, 88/5: 961-966.

Halwart, M. 2008. Biodiversity, nutrition and livelihoods in aquatic rice-based ecosystems. Biodiversity, 9/1-2: 36-40.

Herrawati, V., R. Nugroho, J. Hutabarat, B. Prayitno, O. Karnaradjasa. 2018. The growth performance and nutrient quality of Asian swamp eel Monopterus albus in Central Java Indonesia in a freshwater aquaculture system with different feeds. Journal of Aquatic Food Product Technology, 27/6: 658-666.

Hill, J., C. Watson. 2007. Diet of the nonindigenous Asian swamp eel in tropical ornamental aquaculture ponds in west-central Florida. North American Journal of Aquaculture, 69/2: 139-146.

Khanh, N., H. Ngan. 2010. Current practices of rice field eel Monopterus albus culture in Vietnam. Aquaculture Asia Magazine, 15/3: 26-29.

Liem, K. 1967. Functional morphology of the integumentary, respiratory, and digestive systems of the synbranchoid fish Monopterus albus. Copeia, 1967/2: 375-388.

Matsumoto, S., K. Takeshi, Y. Motoomi, T. Hirohiko, Y. Yuji, M. Takahiko, K. Kaoru, K. Masanori, N. Mutsumi. 2010. Cryptic diversification of the swamp eel Monopterus albus in east and southeast Asia, with special reference to the Ryukyuan populations. Ichthyological Research, 57/1: 71-77.

Matsumoto, S., T. Takeyama, N. Ohnishi, M. Kohda. 2011. Mating system and size advantage of male mating in the protogynous swamp eel Monopterus albus with paternal care. Zoological Science, 28/5: 360-367.

Norhafiza, M. 2013. "Belut, menu popular masa kini" (On-line). Accessed November 03, 2018 at http://www.sinarharian.com.my/edisi/selangor-kl/belut-menu-popular-masa-kini-1.206653.

Ruslan Roy, M. 2011. Pembudidayaan Belut. Kuala Lumpur, Malaysia: SynergyMedia.

Sayer, J., J. Davenport. 1991. Amphibious fish: Why do they leave water?. Reviews in Fish Biology and Fisheries, 1/2: 159-181.

Shafland, P., K. Gestring, M. Stanford. 2010. An assessment of the Asian swamp eel Monopterus albus in Florida. Reviews in Fisheries Science, 18/1: 25-39.

Shapiro, D. 1979. Social behavior, group structure, and the control of sex reversal in hermaphroditic fish. Advances in the Study of Behavior, 10: 43-102.

Sow, Y., A. Ismail, S. Zulkifli, M. Azmai, K. Hambali. 2018. Ecology and biology of the commercially valuable freshwater Asian swamp eel, Monopterus albus. Malayan Nature Journal, 70/1: 47-53.

United States Department of Agriculture, 2018. "Asian swamp eel" (On-line). National Invasive Species Information Center. Accessed November 03, 2018 at https://www.invasivespeciesinfo.gov/aquatics/swampeel.shtml.

Vincent, M., J. Thomas. 2011. Observations on the foraging behaviour of a subterranean fish Monopterus digressus (Synbranchiformes: Synbranchidae). Ichthyological Research, 58/1: 95-98.

Ward-Campbell, B., F. Beamish, C. Kongchaiya. 2005. Morphological characteristics in relation to diet in five coexisting Thai fish species. Journal of Fish Biology, 67/5: 1266-1279.

Xiaojuan, S., G. Shiyuan, Y. Hanwen. 2014. Age of sexual maturity of Monopterus albus cultured in cage. Agricultural Science and Technology, 31/5: 629-633.

Yang, D., F. Chen, Y. Su, L. Yang. 2008. Study on parasitic infection of Monopterus albus. Hubei Agricultural Sciences, 11: Abstract Only.

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