Cyclops strenuus
Fischer, 1851
Common Name:
A copepod
Synonyms and Other Names:
Identification:
Adults of this copepod species are yellow to brown colored. The caudal ramus of females, which displays no dorsal ridge in the Great Lakes, is 5–7 times longer than it is wide and exhibits a hair-covered inner surface. There is a lateral seta located at 73–87% of the distance from the base to the tip of the caudal ramus in females. In the same sex, the 5th leg displays two separate segments and the distal segment is less than 2 times as long as it is wide. Females’ antennules consist of 17 segments (Einsle 1989; Hudson et al. 1998).
Size:
In Canada and Alaska, female C. strenuus are around 1.5 to 1.8 mm long and males are around 1.15 to 1.45 mm long (Reed and McIntyre 1995).
Native Range:
Cyclops strenuus is a circumboreal species native to Europe, Alaska, the Yukon, and the Northwest Territories. There have also been reports from Asia which are not yet confirmed. There is a possibility that populations in the upper Great Lakes drainage are native relicts (Reed and McIntyre 1995; Hudson et al. 1998; Tackx et al. 2004; J. W. Reid and P. L. Hudson, unpublished data).
Great Lakes Nonindigenous Occurrences:
Cyclops strenuus has been recorded from Lake Superior (1972) and a tributary, St. Mary’s River (Selgeby 1975; Reed and McIntyre 1995; Hudson et al. 1998).
Table 1. Great Lakes region nonindigenous occurrences, the earliest and latest observations in each state/province, 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 Cyclops strenuus are found here.
Full list of USGS occurrences
Table last updated 12/17/2025
† Populations may not be currently present.
Ecology:
Cyclops strenuus is capable of producing a resting stage that becomes dormant in the sediments at the 4th or 5th copepodid stage, the pre-fertilized adult female stage, or the fertilized adult female stage. There is no encystment and if fertilized females become dormant, sperm can survive for around 7 months. The diapause period can occur in autumn, winter, or summer. In some permanent waters, C. strenuus can reproduce all year or may produce one or two generations that can undergo diapause.
Cyclops strenuus occurs in all kinds of environments, including ephemeral and permanent waters, subterranean waters, and sphagnum moors (Egmork 1955; Bothar and Oertel 1973; Pesce and Fabrizi 1979; Sarvala 1979; Maier 1990b; Nass and Nilssen 1991a; Frisch 2001, 2002). Cyclops strenuus is typically found in freshwater and sometimes at very low salinities. It can occur in waters with high pH, even slightly over 10. It can survive temperatures of around 0–25ºC, but is particularly abundant at the warmer temperatures. Naupliar mortality increases at lower temperatures.
In one unusual Norwegian population, reproduction can occur at 0–3ºC in an oligotrophic lake. In reproducing females, the number of eggs per sac is around 31–66 (Elgmork and Halvorsen 1976; Maier 1990a; Hansen et al. 1991; Nass and Nilssen 1991a; Frisch 2001; Tackx et al. 2004).
Cyclops strenuus is a host to many different parasites in its native range, some of which include the cestodes Triaenophorus crassus and Diphylobothrium spp.; the eel tapeworm Bothriocephalus claviceps; the eel swimbladder nematodes Anguillicola crassus, A. globiceps, and A. novaezelandiae; the tapeworms Proteocephalus torulosus and P. neglectus; the acanthocephalid worm Pallisentis nagpurensis; and the helminth Traienophorus nodulosus (Sysoev 1982; George and Nadakal 1983; Scholz 1991, 1993, 1997; Moravec et al. 1994a, b; Nagasawa et al. 1994; Dorucu 1999; Pulkkinen et al. 2000).
Cyclops strenuus is a common prey species for perch, Perca fluviatilis, in its native range. Cyclops strenuus itself is a carnivorous species and preys on ciliates, cladocerans, copepod nauplia, and rotifer nauplia. In Lake Glubokoye, in the former USSR, this species’ presence, along with that of the predator Leptodora kindtii, is negatively correlated with diversity and evenness of the prey community it utilizes. Algae are frequently found in the intestine of this species (Gilyarov 1976, 1977; Brabrand et al. 1983; Rossi et al. 1984; Treasurer 1992).
Great Lakes Means of Introduction:
Unclear. Cyclops strenuus could have been introduced in ballast water, transferred with stocking programs, released with bait, discharged from live well water, transferred with recreational gear, transferred with waterfowl, or it may have dispersed via the Long Lac-Ogoki diversion project that connects the Hudson’s Bay drainage to Lake Superior (Hudson et al. 1998; Grigorovich et al. 2003; Holeck et al. 2004; Duggan et al. 2005).
Great Lakes Status:
Cyclops strenuus is reproducing and overwintering in the Great Lakes (U.S. EPA 2008). A recent survey of zooplankton community composition in the Great Lakes reports C. strenuus in the western basin of Lake Erie at a biomass of 5 μg/m3 for 2012-2016 (Barbiero et al. 2019).
Great Lakes Impacts:
Summary of species impacts derived from literature review. Click on an icon to find out more...
| Environmental | Socioeconomic |
|
| | |
Cyclops strenuus has a moderate environmental impact in the Great Lakes. This copepod is an intermediate host for a number of different parasites, e.g. representatives of the genera Proteocephalus, Triaenophorus and Bothriocephalus (Olifirenko and Korniienko 2021). Cyclops strenuus can negatively impact recreationally fish by facilitating the transfer of parasites, such as Bothriocephalus gowkongensis (Nomonov et al. 2023). A carnivorous species that preys on ciliates, cladocerans, copepod nauplia, and rotifer nauplia, presence of C. strenuus is negatively correlated with diversity and evenness of the prey community it utilizes but it also serves as a food resource for predatory fish (Gilyarov 1976, 1977; Sterligova and Ilmast 2017).
There is little or no evidence to support that Cyclops strenuus has significant socio-economic impacts in the Great Lakes.
Cyclops strenuus is an intermediate host for larvae of Diphyllobotrium latum, the broad fish tapeworm. This tapeworm is a common agent of human diphyllobothriasis (Ismailovna and Salavat Uly 2023). Through the consumption of raw or undercooked fish, Diphyllobothrium spp. can also infect humans, causing diphyllobothriasis in the digestive system (USFDA 2012). Although this disease can affect 20 million people annually, it is considered rare in the United States and is not thought to be present in the Great Lakes region currently (Scholz et al. 2009; USFDA 2012).
There is little or no evidence to support that Cyclops strenuus has significant beneficial impacts in the Great Lakes.
While research is insufficient to adequately assess beneficial impacts, Cyclops strenuus does not have any realized beneficial impacts in the region. There is no evidence that this species improves water quality, acts as a biocontrol agent, or provides significant ecosystem services. In addition, C. strenuus is not expected to have commercial, recreational, medicinal, or research value.
Management:
Regulations (pertaining to the Great Lakes)
There are no known regulations for this species.
Note: Check federal, state/provincial, and local regulations for the most up-to-date information.
Control
Biological
Treasurer (1992) found that Eurasian perch Perca fluviatilis larvae selectively prey on Cyclops strenuus abyssorum in the Scottish lochs Kinord and Davan. The total zooplankton reduction observed was minimal, but Treasurer suggested that grazing by larvae is likely to impact copepod populations (1992).
Physical
There are no known physical control methods for this species.
Chemical
The Great Lakes and Mississippi River Interbasin Study (GLMRIS 2012) suggests that alteration of water quality using carbon dioxide, ozone, nitrogen, and/or sodium thiosulfate could be effective in preventing upstream and downstream movement of copepods. It should be noted that the effectiveness of these methods is likely significantly diminished against copepod ephippia.
Note: Check state/provincial and local regulations for the most up-to-date information regarding permits for control methods. Follow all label instructions.
Remarks:
It is possible that in eastern Lake Superior and St. Marys River C. strenuus populations are native relics from northern North America (Hudson et al. 1998).
References
(click for full reference list)
Author:
Kipp, R.M., J. Larson, A. Fusaro, and C. Shelly
Contributing Agencies:
Revision Date:
9/30/2025
Citation for this information:
Kipp, R.M., J. Larson, A. Fusaro, and C. Shelly, 2025, Cyclops strenuus Fischer, 1851: U.S. Geological Survey, Nonindigenous Aquatic Species Database, Gainesville, FL, and NOAA Great Lakes Aquatic Nonindigenous Species Information System, Ann Arbor, MI, https://nas.er.usgs.gov/queries/greatlakes/FactSheet.aspx?Species_ID=2711, Revision Date: 9/30/2025, Access Date: 12/18/2025
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.