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.

Argulus japonicus
Argulus japonicus
(Japanese fishlouse)

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Argulus japonicus Thiele, 1900

Common name: Japanese fishlouse

Taxonomy: available through www.itis.govITIS logo

Identification: This small, brown ectoparasitic branchiuran exhibits great morphological variation within and between populations. In general, individuals are shaped like shields and are golden brown with stumpy tails. It ranges in size from 4-9 mm long and 3-6 mm wide (Pilgrim, 1967). Its ribbed suckers, with rods exhibiting 5–9 imbricate plates, emerge from the maxillules. The long mouth tube juts out ventrally from the head and can be fully retracted into the body. There is a preoral spine on the ventral midline and there are post-antennal spines. The first antenna displays a knob at the anterior edge and the second antenna comprises two knobbed sections in the basal part and three sections in the distal part. Swimming legs are flagellated (Pilgrim 1967; Amin 1981; Swanepoel and Avenant-Oldewage 1992; Gresty et al. 1993; Bunkley-Williams and Williams 1994).

Size: 4–8 mm long and 3–6 mm wide (Pilgrim 1967)

Native Range: Argulus japonicus is native to Asia where its common hosts, Goldfish (Carassius auratus) and Common carp (Cyprinus carpio), are also native species (Mills et al. 1993; Bunkley-Williams and Williams 1994).

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

Nonindigenous Occurrences: Established in Chesapeake Bay, Maryland (Ruiz et al. 2000).

Recorded for the first time in the Great Lakes drainage from Lake Michigan, part of the Fox River system in Wisconsin that drains into Lake Michigan, in the mid to late 1970s. It was recorded in 1989 from the Fox River itself (LaMarre and Cochran 1992). Considering that this parasitic branchiuran is found on exotic C. auratus, a much less recent invader, it was likely introduced many decades ago and remained undetected for years (Amin 1981; Galarowicz and Cochran 1991; Lammarre and Cochran 1992; Mills et al. 1993). It has been collected from gizzard shad (Dorosoma cepedianum) in Illinois around 1980 (Poly 1998). Listed as being present in unspecified locations in Florida, Georgia, and Louisiana in the southeast United States as well as in California, Hawaii, Illinois, Maryland, Washington, and Wisconsin (McLaughlin et al., 2005).

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 Argulus japonicus are found here.

StateFirst ObservedLast ObservedTotal HUCs with observations†HUCs with observations†
CA200320031California Region
FL200320031South Atlantic-Gulf Region
GA200320031South Atlantic-Gulf Region
IL198019801Big Muddy
MD200020001Upper Chesapeake Bay
MI200220032Great Lakes Region; Lake Huron
WA200320193Lower Snake; Lower Snake-Tucannon; Pacific Northwest Region
WI198919891Lower Fox

Table last updated 7/18/2024

† Populations may not be currently present.

* HUCs are not listed for states where the observation(s) cannot be approximated to a HUC (e.g. state centroids or Canadian provinces).

Ecology: Reproduction in dioecious A. japonicus is sexual and output is high. Although mating occurs on the body of the host fish, females lay 1–9 strings of 5–226 eggs in 1–6 rows on hard substrate, which they cover in gelatinous excretions. Eggs hatch best at 20–30ºC, and hatch in 15 days at 35ºC and in 61 days at 10ºC. Newly hatched larvae survive for one or two days on nutrition from their yolk sacs but soon require nutrition from the host’s epithelial cells and mucus. After hatching, the rest of the life cycle (excluding egg-laying), takes place on the body of the host fish in 30–100 days. Argulus japonicus lays eggs continuously, although in winter eggs may stay dormant until warmer temperatures conducive to hatching occur in spring. As this species develops it passes through around 6 different larval instar stages before metamorphosing into an adult. Adults can survive without a host for around 2 weeks (Shafir and Van As 1986; Avenant-Oldewage and Swanepoel 1993; Ikuta and Makioka 1997; Hill and McClain 2004; Schram et al. 2005; Tam and Avenant-Oldewage 2006).            

Argulus japonicus has been recorded in mesohaline conditions in Chesapeake Bay, although most records in other parts of the world are from freshwater environments. This parasite is typical of eutrophic habitats (Ruiz et al. 2000; Hill and McClain 2004).

Means of Introduction: Argulus japonicus was very likely introduced to North America with its host C. auratus through the aquarium industry (Mills et al. 1993).

Status: Established where recorded.

Impact of Introduction:

A) Realized: A. japonicus parasitizes introduced Channel catfish (Ictalurus punctatus) in Lake Michigan. However, it is most typically found on C. auratus and C. carpio (Amin 1981).            

B) Potential: A. japonicus can cause severe damage to the integument of its hosts, sometimes resulting in death. It can influence the appetite of a fish, subsequently affecting growth rates. It attaches to the skin, fins, or gills by way of suckers, and then feeds. It may also aid in the transmission of Rhabdovirus carpio, spring viraemia rhabdovirus, larval nematodes, and the fungal disease caused by Saprolegnia in some parts of the world (Gresty et al. 1993; Avenant-Oldewage 2001). This species has the potential to cause increased stress in Rainbow trout (Oncorhynchus mykiss), when it occurs in high densities.  Experiments indicate that it does not prefer its common host C. auratus over native North American species such as the Fathead minnow (Pimephales promelas), nor is it more likely to detach from the latter as compared to the former (Lamarre and Cochran 1992).

A. japonicus is now widely distributed around the world and parasitizes many species of fish. In fact, it is probably capable of infecting almost all species of freshwater fish. It has been recorded on Labeo rohita, Catla catla, and Cirrhina mrigala in Pakistan; various cyprinids in Malaysia; tilapias, barbs, I. punctatus, Ctenopharyngodon idella, and other species in Puerto Rico and Cuba; and many species in South Africa, New Zealand, Britain, and other parts of Europe. In South Africa it has been known to reach near epidemic levels in all fish species present in some water bodies, and has the potential to cause drastic effects in local fish populations (Pilgrim 1967; Kruger et al. 1983; Seng 1986; Jafri and Ahmed 1991; Rushton-Mellor 1992; Bunkley-Williams and Williams 1994; Avenant-Oldewage 2001; Haond et al. 2003).            


References: (click for full references)

Amin, O.M. 1981. On the Crustacean ectoparasites of fishes from southeast Wisconsin. Transactions of the American Microscopical Society 100(2):142-150.

Avenant-Oldewage, A. 2001. Argulus japonicus in the Olifants River system: possible conservation threat? South African Journal of Wildlife Research 31(1-2):59-63.

Avenant-Oldewage, A., and J.H. Swanepoel. 1993. The male reproductive system and mechanisms of sperm transfer in Argulus japonicus (Crustacea: Branchiura). Journal of Morphology 215(1):51-63.

Bunkley-Williams, L., and E.H. Williams, Jr. 1994. Parasites of Puerto Rican Freshwater Sport Fishes. Puerto Rico Department of Natural and Environmental Resources, San Juan, Puerto Rico and Department of Marine Sciences, University of Puerto Rico, Mayaguez, Puerto Rico. 168 pp.

Galarowicz, T., and P.A. Cochran. 1991. Response by the parasitic crustacean Argulus japonicus to host chemical cues. Journal of Freshwater Ecology 6:455-456.

Gault, N.F.S., D.J. Kilpatrick, and M.T. Stewart. 2002. Biological control of the fish louse in a rainbow trout fishery. Journal of Fish Biology 60(1):226-237.

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

Gresty, K.A., G.A. Boxshall, and K. Nagasawa. 1993. The fine structure and function of the cephalic appendages of the branchiuran parasite, Argulus japonicus Thiele. Philosophical Transactions of the Royal Society of London B Biological Sciences 339(1287):119-135.

Haond, C., D.T. Nolan, N.M. Ruane, J. Rotllant, and S.E.W. Bonga. 2003. Cortisol influences the host-parasite interaction between the rainbow trout (Oncorhynchus mykiss) and the crustacean ectoparasite Argulus japonicus. Parasitology 127(6):551-560.

Hill, T.D., and J.R. McClain, eds. 2004. 2002 Activities of the Central Great Lakes Bi-national Lake Sturgeon Group. U.S. Fish & Wildlife Service, Alpena Fishery Resources Office Station Report, Alpena, Michigan. 60 pp.

Hudson, P.L., and L.T. Lesko. 2003. Free-living and Parasitic Copepods of the Laurentian Great Lakes: Keys and Details on Individual Species. Ann Arbor, Michigan: USGS Great Lakes Science Center Home Page. Available from: http://www.glsc.usgs.gov/greatlakescopepods/

Ikuta, K., and T. Makioka. 1997. Structure of the adult ovary and oogenesis in Argulus japonicus Thiele (Crustacea: Branchiura). Journal of Morphology 231(1):29-39.

Jafri, S.I.H., and S.S. Ahmed. 1991. A new record of ectoparasitic crustaceans (Branchiura: Argulidae) from major carps in Sindh, Pakistan. Pakistan Journal of Zoology 23(1):12-14.

Kruger, I., J.G. Van As, and J.E. Saayman. 1983. Observations on the occurrence of the fish louse Argulus japonicus Thiele, 1900 in the western Transvaal. South African Journal of Zoology 18(4):408-410.

Lamarre, E., and P.A. Cochran. 1992. Lack of host species selection by the exotic parasitic crustacean Argulus japonicus. Journal of Freshwater Ecology 7(1):77-80.

McLaughlin, P.L., and 37 others. 2005. Common and scientific names of aquatic invertebrates from the United States and Canada: Crustaceans. American Fisheries Society Special Publication 31, Bethesda, MD.

Mills, E.L., J.H. Leach, J.T. Carlton, and C.L. Secor. 1993. Exotic species in the Great Lakes: a history of biotic crises and anthropogenic introductions. Journal of Great Lakes Research 19(1):1-54.

Pilgrim, R.L.C. 1967. Argulus japonicus Thiele, 1900 (Crustacea: Branchiura) – a new record for New Zealand. New Zealand Journal of Marine and Freshwater Research 1:395-398.

Poly, W.J. 1998. New state, host, and distribution records of the fish ectoparasite, Argulus (Branchiura), from Illinois (USA). Crustaceana (Leiden) 71(1):1-8.

Ruiz, G.M., P. Fofonoff, J.T. Carlton, M.J. Wonham, and A.H. Hines. 2000. Invasion of coastal marine communities in North America: apparent patterns, processes, and biases. Annual Review in Ecology and Systematics 2000:481-531.

Rushton-Mellor, S.K. 1992. Discovery of the fish louse, Argulus japonicus Thiele (Crustacea: Branchiura), in Britain. Aquaculture and Fisheries Management 23:269-271.

Schram, T.A., L. Iversen, P.A. Heuch, and E. Sterud. 2005. Argulus sp. (Crustacea: Branchiura) on cod, Gadus morhua from Finnmark, northern Norway. Journal of the Marine Biological Association of the U.K. 85:81-86.

Seng, L.T. 1986. Two ectoparasitic crustaceans belonging to the family Argulidae (Crustacea: Branchiura) in Malaysian freshwater fishes. Malayan Nature Journal 39(3):157-164.

Singhal, R.N., S. Jeet, and R.W. Davies. 1986. Chemotherapy of six ectoparasitic diseases of cultured fish. Aquaculture 54(3):165-171.

Shafir, A., and J.G. Van As. 1986. Laying, development of hatching of eggs of the fish ectoparasite Argulus japonicus (Crustacea: Branchiura). Journal of Zoology (London) 210:401-414.

Swanepoel, J.H., and A. Avenant-Oldewage. 1992. Comments on the morphology of the pre-oral spine in Argulus (Crustacea: Branchiura). Journal of Morphology 212(2):155-162.

Tam, Q., and A. Avenant-Oldewage. 2006. The digestive system of larval Argulus japonicus (Branchiura). Journal of Crustacean Biology 26(4):447-454.

Taylor, N.G.H., C. Sommerville, and R. Wootten. 2006. The epidemiology of Argulus spp. (Crustacea: Branchiura) infections in stillwater trout fisheries. Journal of Fish Diseases 29(4):193-200.

Tonguthai, K. 1997. Control of freshwater fish parasites: a Southeast Asian perspective. International Journal for Parasitology 27(10):1185-1191.

Other Resources:
Great Lakes Water Life

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

Revision Date: 9/12/2019

Citation Information:
Kipp, R.M., J. Larson, A. Fusaro, T.H. Makled, A.J. Benson and R. Sturtevant, 2024, Argulus japonicus Thiele, 1900: U.S. Geological Survey, Nonindigenous Aquatic Species Database, Gainesville, FL, https://nas.er.usgs.gov/queries/FactSheet.aspx?speciesID=166, Revision Date: 9/12/2019, Access Date: 7/18/2024

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 [7/18/2024].

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