Description: H:\pub_html\zooplankton web\Acanthocyclops\images\cmuwordmark.gifZooplankton of the Great Lakes

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Organism: Acanthocyclops vernalis


Kingdom:     Animalia

   Phylum:    Arthropoda

      Subphylum: Crustacea

         Class:         Maxillopoda

            Subclass:      Copepoda

               Order:          Cyclopoida

                  Family:         Cyclopidae

                     Genus: Acanthocyclops

                        Species:            vernalis

Description: Female%20head%20in%20paint

Fig. A: Cephalic region of a female Acanthocyclops vernalis.



“Quick Reproduction (36 Hrs)”


“Sexually Dimorphic”


“Geniculate in Males”

Taxonomic History:

Acanthocyclops vernalis can be commonly misidentified in a sample as the morphologically similar Cyclops vernalis, C. americanus,  or even C. brevispinosus.  Originally a subgenus under genus Cyclops, Kiefer (1960) noted there are actually several genera that fall under Cyclops, hence the reason Acanthocyclops is now its own genus with over 70 species/subspecies (Dussart and Defaye, 2006). The work of Kiefer (1978) indicated that A. robustus found in the Great Lakes may be a new species (possibly arising due to different environmental pressures), being regularly misidentified as A. vernalis. Though a key does exist to distinguish A. vernalis from A. robustus (Dodson, 1994), the characteristics it uses for separation are often difficult to detect. Dodson et al. (2003) noted that these irregularly present characteristics (Especially: a patch of spines on the anterior face of the P4 coxa and two terminal spines on the terminal segment of the of the P4 endopod.) are due to cryptic speciation – a situation where populations become genetically distinct but maintain morphological similarity. A. robustus is still commonly, and acceptingly, identified as A. vernalis since morphologic characterization is so unpredictable (Balcer et al., 1984).



A. vernalis exhibit sexual dimorphism, with males being smaller – Length: 0.8-1.0 mm. Dry weight: 2.4-2.6g – than females – Length: 1.0-1.4 mm. Dry weight: 4.8-6.4g – (see Fig. 1) (Balcer et al., 1984; Hawkins and Evans, 1979). Figure 2 shows the four terminal setae per caudal ramus. The medial pair of setae are longest and the lateral seta is always found along the caudal ramus within one third of the posterior end (Fig. 2).


Description: male%20and%20female%20in%20Paint

Fig. 1: Acanthocyclops vernalis female (left) and male (right) showing sexual dimorphic size, 1st antennae geniculation in males, and the distinct 5th leg, often useful in distinguishing A. vernalis from Diacyclops thomasi  (Balcer et al., 1984). 


Description: Tail%20in%20Paint

Fig. 2: A. vernalistail showing elongate medial setae and lateral seta located within one third the distance down the posterior end of the caudal ramus.


Like all copepods in subclass Copepoda, A. vernalis lack a compound eye. Both first antenna of the male are geniculate (Fig. 1 and Fig. 3) while the female’s are straight; none usually reaching past the genital segment. Though similar in many ways to Diacyclops thomasi, A. vernalis 5th legs are distinct (Balcer et al., 1984).


Description: Geniculation%20%20in%20paint

Fig. 3: A. vernalis male showing the geniculate 1st antenna present on both antennae.




A. vernalis are found in all five Great Lakes (Balcer et al., 1984). And, though one of the most common species in North America (Yeatman, 1944), Patalas (1972) reports that they only make up less than 1% of the crustacean zooplankton in our Great Lakes. The higher the eutrophic level of the lake, the more likely there will be a healthy population of A. vernalis present (Balcer et al., 1984).



Fryer (1985) states that A. vernalis in Europe are “exclusively benthic organisms”, preferring lightly acidic waters low in calcium and total ion concentration. Evans and Stewart (1977) would agree that A. vernalis are mainly benthic, but they can be found throughout the water column (thus epibenthic), possibly even exhibiting diurnal migration – coming up to the top of the water column at night. Their centralization to nearshore areas in mainly eutrophic lake conditions potentially makes Acanthocyclops vernalis a good indicator of lake ecosystem health. For open water, Lake Erie’s western basin seems to be the only place in the Great Lakes region eutrophic enough for these organisms to live (Patalas, 1972).



These predacious organisms are known to consume Bosmina, Ceriodaphnia reticulata, cladocerans, and even their own nauplii (Balcer et al., 1984). The smallest prey item is not always the one that is chosen for food. Carapace integrity, shape, and escape strategy seem to be the three most important factors to a prey item if it wishes to survive (Li and Li, 1979). Li and Li (1979) list Asplanchna, Diaphanosoma, and Diaptomus as the three preferred species of A. vernalis. For bacteria, there is still a fair amount of debate over whether or not copepods in general are bactivores (Work and Havens, 2003).


Life History:

A. vernalis is able to reproduce throughout the year in some lakes, even under the ice. However, reproduction is highly dependant upon temperature, where extremely high temperatures cause dormancy and low temperatures will slow reproduction. Predictably, when temperatures are favorable (20oC), A. vernalis produce many small offspring (50% mortality, 7-8 days to mature) but when temperatures drop to 7-10oC, the adults produce larger, fewer offspring at a slower rate (92% mortality, 44 days to mature). In Great Lakes studies, few to no A. vernalis  are present in samples taken between December and May. There is an 8-10oC cutoff observed in Lake Erie, below which A. vernalis adults are not commonly seen. And, Lake Superior exhibits the slowest reproduction, producing just one generation each year (Balcer et al., 1984). 


In the absolute optimal conditions, the female will hatch, on average, a brood every 36 hours for up to four weeks. After one mating event, 40-80 eggs are dropped into the two egg sacs. However, in the case of Lake Superior, diapausing copepodids, stuck in the CIV or CV stage, are important for continuing the population after they hatch the next spring in more optimal temperatures (Balcer et al., 1984).


Works Cited:

Balcer, M.D., Korda, N.L., Dodson, S.I. (1984). Zooplankton of the Great Lakes; A guide to the Identification and Ecology of the Common Crustacean Species. The University of Wisconson Press, 93-95.

Dodson, S. (1994). Morphological Analysis of Wisconsin (U.S.A.) Species of the Acanthocyclops vernalis Group (Copepoda: Cyclopoida). Journal of Crustacean Biology, 14(1), 113-131.

Dodson, S.I., Grishanin, A.K., Gross, K, Wyngaard, G.A. (2003). Morphological analysis of some cryptic species in the Acanthocyclops vernalis species complex from North America. Hydrobiologia, 500, 131-143.

Dussart, B.H. and Defaye, D. (2006). World Directory of Crustacea Copepoda. II-Cyclopiformes. Backhuys Publishers, Leiden.

Evans, M.S. and Stewart, J.A. (1977). Epibenthic and Benthic microcrustaceans (copepods, cladocerans, ostracods) from a nearshore area in southeastern Lake Michigan. Limnology and Oceanography, 22(6), 1059-1066.

Fryer, G. (1985). An ecological validation of a taxonomic distinction: the ecology of Acanthocyclops vernalis and A. robustus (Crustacea: Gopepoda). Zoological Journal of the Linnean Society, 84(2), 165-180.

Hawkins, B.E. and Evans, M.S. (1979). Seasonal cycles of zooplankton biomass in southeastern Lake Michigan. Journal of Great Lakes Research, 5(3-4), 256-263.

Kiefer, F. (1960). Ruderfuskrebse (Copepoda). Kosmos-Verlag, Stuttgart.

Kiefer, F. (1978). Freilebende Copepoda. Binnengewasser, 26(2): 1-343.

Li, J.L. and Li, H.W. (1979). Species-Specific Factors Affecting Predator-Prey Interactions of the Copepod Acanthocyclops vernalis with its Natural Prey. Limnology and Oceanography, 24(4), 613-626.

Patalas, K. (1972). Crustacean plankton and the eutrophication of St. Lawrence Great Lakes. Journal of the Fisheries Research Board Canada, 29(10), 1451-1462.

Work, K.A. and Havens, K.E. (2003). Short Communication; Zooplankton grazing on bacteria and cyanobacteria in a eutrophic lake. Journal of Plankton Research, 25(10), 1301-1307.

Yeatman, H.C. (1944). American cyclopoid copepods of the viridis-vernalis group (including a description of Cyclops carolinianus n.sp.). American Midland Naturalist, 32(1), 1-90.