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.

Thermocyclops crassus
Thermocyclops crassus
(a copepod)

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Thermocyclops crassus (Fischer, 1853)

Common name: a copepod

Synonyms and Other Names: Thermocyclops hyalinusMesocyclops hyalinus Rehberg 1880

Taxonomy: available through www.itis.govITIS logo

Identification: Cyclopoid copepod.  Body short and stout and its furca is about twice as long as wide.  1:3 ratio between the outer and inner setae attached to the furca.  Females 0.8-1.1mm, Males 0.7mm (Fischer 1853).  Seminal receptacle with a clear “T” shape, with lateral arms relatively short, wide, and weakly recurved posteriorly, female genital somite length/width ratio exceeding 1.2, furcal length/width ratio below 2.5, and the ornamentation on the connecting lamellae of legs 1 and 4 (Gutierrez-Aguirre and Suarez-Morales 2000). 

Similar to the native cyclopoid copepod Mesocyclops edax, T. crassus is slightly smaller and can be distinguished by the lack of hairs on the medial surfaces of the caudal rami. Thermocyclops crassus’ longest caudal setae (medial median terminal caudal seta) are usually recurved ventrally at the tips in adult individuals, whereas M. edax has relatively straight setae. Thermocyclops crassus has a very narrow and smoothly edged hyaline membrane on antennule segment 17, while in M. edax this membrane is wider and coarsely serrate. The coupler of swimming leg 4 in T. crassus has  two large protrusions ornamented with 4-6 spines while in M. edax the fourth leg coupler margin has two small triangular, unornamented protrusions. In Thermocyclops crassus, leg 4 endopodite article 3 with its lateral terminal spine is less than half the length of the medial terminal spine. In contrast the fourth leg, third endopodite segment of M. edax is characterized by a terminal spine similar in length to its lateral spine. Leg five (P5) of T. crassus has spines that are approximately equal in length and extend beyond the midpoint of the genital double somite. The inner spine of T. crassus is inserted distally, which differs from M. edax whose inner spine is inserted toward the middle of the distal segment (Connolly et al. 2017).

Size: 0.7-1.1 mm

Native Range: Thermocyclops crassus is present throughout Europe, Asia, Africa and Australia. Generally considered Eurasian in origin (Ueda and Reid 2003).

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

Nonindigenous Occurrences: In 1991, it was detected in Lake Champlain (Vermont) but remains rare (Duchovnay et al. 1992).  Found in the western basin of Lake Erie in samples collected from 2014 through 2016 (Connolly et al. 2017).  Reported as introduced in Costa Rica (Collado et al. 1984) and Mexico in 1998 (Gutierrez-Aguirre 2000).

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 Thermocyclops crassus are found here.

StateFirst ObservedLast ObservedTotal HUCs with observations†HUCs with observations†
IN201720171Lake Michigan
OH201420182Cedar-Portage; Lake Erie
VT199119911Lake Champlain
WI201820181Lake Superior

Table last updated 2/27/2023

† Populations may not be currently present.

Ecology: Thermocyclops crassus is a thermophilic cyclopoid with a preference for eutrophic waters (Duchovnay et al. 1992). In its native range, T. crassus can be found in a broad range of habitats including rice fields, lakes, rivers, and marshes (Fernando and Ponyi 1981). It is primarily pelagic, but it also inhabits littoral zones with dense stands of immersed macrophytes (Duchovnay et al. 1992). It is able to tolerate salinities up to 7.2 % and has been collected from waters of pH 5.9-8.4, but the optimum pH is 7-8 (Duchovnay et al. 1992).

Thermocyclops crassus is generally regarded to be omnivorous with a preference for herbaceous prey, although evidence suggests that diets may vary among separate populations in different environments. T. crassus has a fine-mesh food collection grid formed by setae and setules that allow this species to feed effectively enough on nanoplankton to subsist on a herbivorous diet (Hopp et al. 1997; Hopp and Maier 2005). Moriarty et al. (1973) described a population in Lake George, Uganda that is primarily herbivorous throughout its life and other studies have observed T. crassus to feed mainly on diatoms, cryptomonads, and cyanophyceans (Duchovnay et al. 1992). This species typically thrives in mesotrophic and eutrophic waters (Duchovnay et al. 1992) and is capable of feeding on cyanobacteria—particularly Microcystis, which is a substantial food source for T. crassus in Lake George (Moriarty et al. 1973; Haney 1987). The flexibility in this species feeding behavior might help to explain its broad distribution in temperate and tropical regions throughout the world.

This species’ life cycle appears to be dictated predominantly by water temperature. Thermocyclops crassus bury themselves in the mud and enter diapause during or before winter when water temperature is 15-17 °C and day length is around 14-15 hours (Maier 1989a, 1990).  Emergence typically occurs in April when water temperature is 9.5-14 °C and day length is 11-13 hours (Maier 1989a). Egg production occurs when water temperature is >10 °C, which typically is in April or May and lasts until October in temperate regions (Maier 1988, 1989b; Kobari and Ban 1998). In Turkey, egg production reached a maximum of 26 eggs in June (Bozkurt and Can 2014), but Maier (1989a) reported that females in the Gronne, a shallow eutrophic lake in Southern Germany, carried 18 to 32 eggs on average and produced 3 generations per year between April and October. At tropical temperatures, maturation time can be as short as a few weeks with multiple generations per year, although in temperate waters, T. crassus may have as few as two generations per year (Duchovnay et al. 1992). At 12 °C development time between copepodite stages ranges from 4-8 days. At temperatures greater than 12 °C and up to 18 °C, development times ranges from 3-6 days (Maier 1989a).

Means of Introduction: Unknown.  First US population in Lake Champlain, 1991.  Potential source populations include Scandinavia, Mexico, or South America. The species may have reached Lake Champlain via the Seaway, canals, ballast water or overland with recreational vessels.   A study of transoceanic ships entering the Great Lakes in 2001 through 2002 -- before the current standards on flushing ballast water went into effect -- found one Thermocyclops crassus in the sediment of a ballast water tank on one ship.  If this species invaded via ballast water, it may have done so prior to regulatory changes in 2006 but remained undetected for a decade or more.  

Status: Established in Lake Erie and Lake Champlain.

Impact of Introduction: To date, there is no evidence of adverse impact on the Lake Champlain ecosystem.

Risk assessment conducted by the USFWS concluded that the risk posed by this species was 'uncertain' (USFWS 2016)



Remarks: This species can be difficult to discriminate from similar congeners, which has led to taxonomic confusion and erroneous published reports. This species has been referred to in scientific literature as Thermocyclops (or Mesocyclops) hyalinus (Moriarty et al. 1973), which has been identified as the most common synonym of Thermocyclops crassus (Rylov 1963; Kiefer 1978). Thermocyclops was formerly included in Mesocyclops spp., which also has added to the confusion (Duchovnay et al. 1992). Additionally, multiple reports of T. crassus in the Americas were found to actually refer to T. decipiens (Reid 1989). This species is also similar in appearance to the native Great Lakes cyclopoid Mesocyclops edax, which could complicate future identifications (see identification for distinguishing characteristics).

Thermocyclops crassus was initially found in the western basin of Lake Erie in August 2014 during the U.S. EPA Great Lakes National Program Office (GLNPO)’s Great Lakes long-term biological monitoring program. From 2014 to 2016, basin-wide densities of T. crassus increased in the western basin from 0.6 individuals/m3  to 23.7 individuals/m3. The highest density was observed in the southeastern portion of the western basin in 2016 where T. crassus were detected at 70.8 individuals/m3. While the western basin population of T. crassus is growing, this species exists in low densities relative to the more common copepods M. edax and Leptodiaptomus sicilioides, which averaged 2000 individuals/m3. However, increasing densities in the eastern part of the western basin suggests that this species’ range is expanding and approaching the currently uncolonized central basin. Additionally, females with egg sacs or spermatophores were found in 2014 and 2016 indicating that this is an established breeding population (Connolly et al. 2017).

References: (click for full references)

Bozkurt, A., and M.F. Can. 2014. Seasonal variations in body length and fecundity of 2 copepod species: Thermocyclops crassus (Fischer, 1853) and Eudiaptomus drieschi (Poppe & Mrázek, 1895). Turkish Journal of Zoology 38(2):222-228. dx.doi.org/10.3906/zoo-1007-7.

Connolly, J.K., J.M. Watkins, E.K. Hinchey, L.G. Rudstam, and J.W. Reid. 2017. New cyclopoid copepod (Thermocyclops crassus) reported in the Laurentian Great Lakes. Journal of Great Lakes Research 43(3):198-203. https://doi.org/10.1016/j.jglr.2017.03.020.

Collado, C., D. Defaye, B.H. Dussart, and C.H. Fernando. 1984. The freshwater Copepoda (Crustacea) of Costa Rica with notes on some species. Hydrobiologia 119(2):89-99. dx.doi.org/10.1007/BF00011948.

Duchovnay, A., J.W. Reid, and A. McIntosh. 1992. Thermocyclops crassus (Crustacea: Copepoda) present in North America: a new record from Lake Champlain. Journal of Great Lakes Research 18(3):415-419.

Dumont, H.J. 1965. On five cyclopoids and a new harpacticide for the fauna of Belgium, and on the evolution of the fauna of Lake Overmere. Biologisch Jaarboek Dodonaea 33:365-382.

Fernando, C.H., and J.E. Ponyi. 1981. The freeliving freshwater cyclopoid copepoda (Crustacea) of Malaysia and Singapore. Hydrobiologia 78:113-123. dx.doi.org/10.1007/BF00007584.

Gutierrez-Aguirre, M., and E. Suarez-Morales. 2000. The Eurasian Thermocyclops crassus (Fischer, 1853) (Copepoda, Cyclopoida) Found in Southeastern Mexico. Crustaceana 73(6):705-713. http://www.jstor.org/stable/20106333.

Haney, J.F. 1987. Field studies on zooplankton-cyanobacteria interactions. New Zealand Journal of Marine and Freshwater Research 21(3):467-475. http://dx.doi.org/10.1080/00288330.1987.9516242.

Hopp, U., G. Maier, and R. Bleher. 1997. Reproduction and adult longevity of five species of planktonic cyclopoid copepods reared on different diets: a comparative study. Freshwater Biology 38:289-300. dx.doi.org/10.1046/j.1365-2427.1997.00214.x.

Hopp, U., and G. Maier. 2005. Implication of the feeding limb morphology for herbivorous feeding in some freshwater cyclopoid copepods. Freshwater Biology 50(5):742-747. dx.doi.org/10.1111/j.1365-2427.2005.01362.x.

Kiefer, F. 1978. Freilebende Copepoda. Zooplankton der Binnengewasser 2:343 pp.

Kobari, T., and S. Ban. 1998. Life cycles of two limnetic cyclopoid copepods, Cyclops vicinus and Thermocyclops crassus, in two different habitats. Journal of Plankton Research 20(6):1073-1086. dx.doi.org/10.1093/plankt/20.6.1073.

Kramer, A.M., O. Sarnelle, and R.A. Knapp. 2008. Allee effect limits colonization success of sexually reproducing zooplankton. Ecology 89(10):2760-2769. http://www.jstor.org/stable/27650821.

Link, J. 1996. Capture probabilities of Lake Superior zooplankton by an obligate planktivorous fish — the lake herring. Transactions of the American Fisheries Society 125:139-142. http://dx.doi.org/10.1577/1548-8659(1996)125<0139:CPOLSZ>2.3.CO;2.

Mair, G. 1989a. The seasonal cycle of Thermocyclops crassus (Fischer, 1853) (Copepoda: Cyclopoida) in a shallow, eutrophic lake. Hydrobiologia 178:43-58. dx.doi.org/10.1007/BF00006112.

Maier, G. 1989b. The effect of temperature on the development times of eggs, naupliar and copepodite stages of five species of cyclopoid copepods. Hydrobiologia 184:79-88. http://link.springer.com/10.1007/BF00014304.

Maier, G. 1990. Spatial distribution of resting stages, rate of emergence from diapause and times to adulthood and to the appearance of the first clutch in 3 species of cyclopoid copepods. Hydrobiologia 206(1):11-18. dx.doi.org/10.1007/BF00018965.

Marten, G.G., E.S. Bordes, and M. Nguyen. 1994. Use of cyclopoid copepods for mosquito control. Hydrobiologia 292/293:491-496. dx.doi.org/10.1007/BF00229976.

Moriarty, D.J.W., J.P.E.C. Darlington, I.G. Dunn, C.M. Moriarty, and M.P. Tevlin. 1973. Feeding and grazing in Lake George, Uganda. Proceedings of the Royal Society of London, Series B, Biological Sciences 184(1076):299-319. http://www.jstor.org/stable/76177.

Nam, V.S., N.T. Yen, B.H. Kay, G.G. Marten, and J.W. Reid. 1998. Eradication of Aedes aegypti from a village in Vietnam, using copepods and community participation. American Journal of Tropical Medicine and Hygiene 59(4):657-660. https://doi.org/10.4269/ajtmh.1998.59.657.

Paerl, H.W., and T.G. Otten. 2013. Harmful cyanobacterial blooms: Causes, consequences, and controls. Microbial Ecology 65:995-1010. http://www.unc.edu/ims/paerllab/research/cyanohabs/me2013.pdf.

Reid, J.W. 1989. The distribution of species of the genus Thermocyclops (Copepoda, Cyclopoida) in the western hemisphere, with description of T. parvus, new species. Hydrobiologia 175:149-174. dx.doi.org/10.1007/BF00765125.

Reid, J.W., and R.M. Pinto-Coelho. 1994. An Afro-Asian Continental Copepod, Mesocyclops ogunnus, found in Brazil; with a new key to the species of Mesocyclops in South America and a review of intercontinental introductions of Copepods. Limnologica 24:359-368.

Rylov, V.M. 1963. Freshwater Cyclopoida. Fauna of USSR. Crustacea. Volume 3. National Science Foundation and Israel Program for Scientific Translations.

Ueda, H., and J.W. Reid, eds. 2003. Copepoda: Cyclopoida, Genera Mesocyclops and Thermocyclops. Guides to the Identification of the Microinvertebrates of the Continental Waters of the World. Volume 20. Backhuys Publishers, Leiden, The Netherlands.

U.S. Environmental Protection Agency (USEPA). 2016. Thermocyclops crassus Frequently Asked Questions. Created on 11/01/2016. Accessed on 12/15/2016.

Vanderploeg, H.A., S.A. Pothoven, G.L. Fahnenstiel, J.F. Cavaletto, J.R. Liebig. 2012. Seasonal zooplankton dynamics in Lake Michigan: Disentangling impacts of resource limitation, ecosystem engineering, and predation during a critical ecosystem transition. Journal of Great Lakes Research 38(2):336-352. dx.doi.org/10.1016/j.jglr.2012.02.005.

Vidhya, K., V. Uthayakumar, S. Muthukumar, S. Munirasu, and V. Ramasubramanian. 2014. The effects of mixed algal diets on population growth, egg productivity and nutritional profiles in cyclopoid copepods (Thermocyclops hyalinus and Mesocyclops aspericornis). The Journal of Basic and Applied Zoology 67(2):58-65. http://www.sciencedirect.com/science/article/pii/S2090989614000174.

USFWS.  2016.  Thermocyclops crassus.  Ecological Risk Screening Summary.   

Author: Sturtevant, R., and P. Alsip

Revision Date: 9/12/2019

Citation Information:
Sturtevant, R., and P. Alsip, 2023, Thermocyclops crassus (Fischer, 1853): U.S. Geological Survey, Nonindigenous Aquatic Species Database, Gainesville, FL, https://nas.er.usgs.gov/queries/FactSheet.aspx?SpeciesID=2793, Revision Date: 9/12/2019, Access Date: 5/27/2023

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