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.

Neoergasilus japonicus
Neoergasilus japonicus
(a parasitic copepod)

Copyright Info
Neoergasilus japonicus Harada, 1930

Common name: a parasitic copepod

Taxonomy: available through www.itis.govITIS logo

Identification: There is great morphological variability amongst different populations of N. japonicus. Generally, introduced North American populations have a long body with a subtriangularly-shaped dorsal cephalic shield. The thorax consists of four free segments and a fifth reduced segment, while the abdomen is comprised of three segments. There are recurved and sharp second antennae, as well as rasping two-segmented mandibles. The first antenna consists of six segments and the second antenna of four. The first maxilla displays three spines while the first leg’s endopod displays two spine; there are four setae on the third segment. The first four sets of legs are biramous, while the firth set is uniramous and reduced (Abdelhalim et al. 1993, Baud et al. 2004, Hayden and Rogers 1998, Hudson and Bowen 2002).

Those collected from Saginaw Bay, Lake Huron ranges in size from 0.6– 0.76 mm but in other regions it can reach 1 mm in length (Hudson and Bowen 2002).

Size: to 1mm

Native Range: Neoergasilus japonicus is native to eastern Asia (Hudson and Bowen 2002).

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

Nonindigenous Occurrences: Neoergasilus japonicus was recorded for the first time in North America in 1993 from aquaculture ponds at Auburn University in Alabama (Hayden and Rogers 1998). The following year they were discovered in Saginaw Bay (Michigan), Lake Huron and again in the bay in 2001 (Hudson and Bowen 2002). Specimens were found in the Salt River drainage west of Grand Junction, Colorado, attached to the fins of black bullheads (P. Walker, pers. comm.). In 2011, several specimens of N. japonicus were found on green sunfish and bluegill in an Ottawa National Wildlife Refuge wetland of Crane Creek, adjacent to Lake Erie and east of Toledo, Ohio (P. Hudson, pers.comm.).

Table 1. States with nonindigenous occurrences, the earliest and latest observations in each state, and the tally and names of HUCs with observations†. Names and dates are hyperlinked to their relevant specimen records. The list of references for all nonindigenous occurrences of Neoergasilus japonicus are found here.

StateFirst ObservedLast ObservedTotal HUCs with observations†HUCs with observations†
AL199319931Lower Tallapoosa
CO200920091Colorado Headwaters-Plateau
MI199420021Lake Huron

Table last updated 7/13/2024

† Populations may not be currently present.

Ecology: Neoergasilus japonicus is a freshwater species typical of eutrophic, polluted aquatic habitats. In Lake Huron, it usually attaches to adults more frequently than YOY the fish hosts. It also attaches most frequently to the dorsal fin, followed in frequency by the anal, caudal, pelvic, and pectoral fins, and prefers the region where the fin attaches to the body (Hayden and Rogers 1998, Hudson and Bowen 2002). Host fishes include Largemouth bass, Smallmouth bass, Bluegill, Redear sunfish, Pumpkinseed, Yellow perch, Green sunfish, Rock bass, Channel catfish, Common carp, Goldfish, and Fathead minnows (Hayden and Rogers 1998; Hudson and Bowen II 2002; P. Hudson, pers. comm.).        

Neoergasilus japonicus, once established, can survive on many different host fish species. It is capable of swimming and transferring from one individual fish to another. In fact, larvae, males, and immature females do not live as parasites, but are free-living. Only ovigerous females require a host, while those that are non-ovigerous can detach and reattach to hosts. When attached to hosts, it is likely that feeding on fish tissue contributes to the diet of N. japonicus. In contrast, gut analyses indicate that free-living individuals most likely derive their nutrition from blue-green algae and small invertebrates (Baud et al. 2004, Hudson and Bowen 2002).

The reproductive cycle of N. japonicus is rapid and output can be very high. Females and males probably mate once before females attach to their fish hosts. Spermatozoids can likely be retained for a period of diapause, for example over winter, so that egg development can occur later, for example in the spring. In a life time, females can produce a total of 1500–2000 eggs in 14–29 pairs of egg sacs. Sexual maturity is attained more quickly at temperatures of 30ºC than at 20ºC. Population levels are low during the cold winter months, but increase in the spring. After hatching, N. japonicus passes through 6 nauplius stages and around 5 copepodid stages before reaching the adult stage (Abdelhalim et al. 1993; Baud et al. 2004; Hudson and Bowen 2002; Urawa et al. 1980, 1990).

Means of Introduction: Unknown. However, it is most likely that the introduction was associated with fish culture. It could have occurred via the aquarium trade, aquaculture, bait release, or ballast water introduction (Hudson and Bowen 2002).

Emerald shiner (Notropis atherinoides) and Spottail shiner (Notropis hudsonius) are subject to a wild-capture baitfish fishery in MI, and Bluntnose minnow (Pimephales notatus) is approved for capture for personal use by anglers in MI. All three species can host N. japonicus, and capture and transport of them greatly increases the possibility of spreading the parasite to other water bodies, streams, and rivers (Hudson et al. 2011).

Status: Established where recorded.

Impact of Introduction: A) Realized: In Saginaw Bay, Lake Huron, N. japonicus has most commonly been found infecting pumpkinseed sunfish (Lepomis gibbosus), followed in frequency by yellow perch (Perca flavescens), rock bass (Ambloplites rupestris), bluegill (Lepomis macrochirus), carp (Cyprinus carpio), channel catfish (Ictalurus punctatus), goldfish (Carassius auratus), green sunfish (Lepomis cyanellus), smallmouth bass (Micropterus dolomieu), largemouth bass (Micropterus salmoides), and fathead minnow (Pimephales promelas) (Hudson and Bowen 2002).  

B) Potential: N. japonicus could disperse outside Saginaw Bay and has the potential to do so relatively quickly. It probably moved across Europe in 20 years, where it has been introduced to such countries as the U.K., Finland, France, and Hungary. It has also been introduced to Alabama in the United States. It is widely distributed and often associated with aquaculture. Within the Palearctic region, fish hosts are all cyprinids and percids. One of these is the rudd (Scardinius erythrophthalmus), which has been introduced to and is established in Lake Ontario. On the other hand, N. japonicus is not limited to cyprinids and percids within North America, as it has already been found infecting centrarchids and ictalurids as well (Ponyi and Molnar 1969; Lescher-Moutoue 1979; Mugridge et al. 1982; Jansson and Karppinen 1988; Tuuha et al. 1992; Abdelhalim et al. 1993; Hayden and Rogers 1998; Hudson and Bowen 2002; Knopf and Hoelker 2005).

Remarks: Neoergasilus japonicus is small and relatively easy to miss in field surveys, even by trained biologists (Hudson and Bowen 2002).


References: (click for full references)

Abdelhalim, A.I., J. W. Lewis, and G.A. Boxshall. 1993. The external morphology of adult female ergasilid copepods (Copepoda: Poecilostomatoida): a comparison between Ergasilus and Neoergasilus. Systematic Parasitology 24: 45-52.

Baud, A., C. Cuoc, J. Grey, R. Chappaz, and V. Alekseev. 2004. Seasonal variability in the gut ultrastructure of the parasitic copepod Neoergasilus japonicus (Copepoda, Poecilostomatoida). Canadian Journal of Zoology 82: 1655-1666.

GLMRIS. 2012. Appendix C: Inventory of Available Controls for Aquatic Nuisance Species of Concern, Chicago Area Waterway System. U.S. Army Corps of Engineers.

Hayden, K.J., and W. A. Rogers. 1998. Neoergasilus japonicus (Poecilostomatoida: Ergasilidae), a parasitic copepod new to North America. Journal of Parasitology 84(1): 88-93.

Hudson, P.L. and C.A. Bowen II. 2002. First record of Neoergasilus japonicus (Poecilostomatoida: Ergasilidae), a parasitic copepod new to the Laurentian Great Lakes. Journal of Parasitology 88(4):657-663.

Hudson, P.L., L.T. Lesko, C.A. Bowen II, W.J. Poly, and M.A. Chriscinske.  2011.  Parasitic copepods and branchiurans of the Laurentian Great Lakes.  Ann Arbor, MI: Great Lakes Science Center Home Page. Available: http://www.glsc.usgs.gov/greatlakescopepods/ Accessed 16 December 2011.

Hudson, P. - United States Geological Survey, Great Lakes Science Center, Ann Arbor, MI.

Jansson, A., and E. Karppinen. 1988. Accession to the Zoological Museum, University of Helsinki-Helsingfors, Finland in 1987. Memoranda Societatis prof Fauna et Flora Fennica 64(4): 178-179.

Knopf, K., and F. Hoelker. 2005. First report of Philometra obturans (Nematoda) and Neoergasilus japonicus (Copepoda) in Germany. Acta Parasitologica 50(3): 261-262.

Lescher-Moutoue, F. 1979. Presence of an Ergasilidae copepod Neoergasilus japonicus in France. Crustaceana (Leiden) 37(1): 109-112.

Mugridge, R.E.R., H.G. Stallybrass, and A. Hollman. 1982. Neoergasilus japonicus (Crustacea Ergasilidae). A parasitic copepod new to Britain. Journal of Zoology 197: 551-557.

Ponyi, J., and K. Molnar. 1969. Studies on the parasite fauna of fish in Hungary, part 5, parasitic copepods. Parasitologia Hungarica 2(2): 137-148.

Suárez-Morales, E., A. Paredes-Trujillo, and D. González-Solís. 2010. The introduced Asian parasitic copepod Neoergasilus japonicas (Harada) (Cyclopoida: Ergasilidae) from endangered cichlid teleosts in Mexico. Zoological Science 27(11): 851-855.

Tuuha, H., E.T. Valtonen, and J. Taskinen. 1992. Ergasilid copepods as parasites of perch Perca fluviatilis and roach Rutilus rutilus in central Finland: seasonality, maturity and environmental influence. Journal of Zoology (London) 228(3): 405-422.

Urawa, S., K. Muroga, and S. Kasahara. 1980. Naupliar development of Neoergasilus japonicus (Copepoda: Ergasilidae). Nippon Suisan Gakkaishi 46(8): 941-948.

Urawa, S., K. Muroga, and S. Kasahara. 1990. Growth and fecundity of the parasitic copepod Neoergasilus japonicus (Ergasilidae). Bulletin of the Plankton Society of Japan (Special Volume): 619-625.

Walker, P.G. – Aquatic Animal Health Lab, Colorado Division of Wildlife, Brush, CO.

Other Resources:
Great Lakes Water Life

Author: Kipp, R.M., A.J. Benson, J. Larson, T.H. Makled, A. Fusaro and R. Sturtevant

Revision Date: 9/12/2019

Citation Information:
Kipp, R.M., A.J. Benson, J. Larson, T.H. Makled, A. Fusaro and R. Sturtevant, 2024, Neoergasilus japonicus Harada, 1930: U.S. Geological Survey, Nonindigenous Aquatic Species Database, Gainesville, FL, https://nas.er.usgs.gov/queries/FactSheet.aspx?SpeciesID=2595, Revision Date: 9/12/2019, Access Date: 7/13/2024

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.


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Citation information: U.S. Geological Survey. [2024]. Nonindigenous Aquatic Species Database. Gainesville, Florida. Accessed [7/13/2024].

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