Zooplankton of the Great Lakes


Mesocyclops edax


Kingdom - Animalia
Phylum - Arthropoda
Subphylum - Crustacea
Class - Maxillopoda
Subclass - Copepoda
Order - Cyclopoida
Family - Cyclopidae
Genus – Mesocyclops
Specific epithet - edax

The name Mesocyclops edax was originally given to this copepod by Coker (1943).  Initially described as Cyclops edax by Forbes (1891), Marsh (1910) reviewed C. edax and found that it was not sufficiently different from C. leuckarti to comprise a distinct species and it was lumped back in with C. leuckarti.   Sars (1918) re-listed the genus of Cyclops leuckarti to Mesocyclops and Kiefer (1929) again divided out distinct individuals which he called M. leukarti edax.  Finally, Coker (1943) was able to provide enough sufficient evidence to declare M. edax as a separate species.  Because of the shifts in taxonomy Balcer et al (1984) suggest that C. leuckarti, C. leuckarti edax and M. leuckarti reported in the great lakes before 1960 may include what we now know to be M. edax.


The largest cyclopoid copepod in the Lake Michigan (Wells 1970), adult CVI females (Figure 1) range in size between 1.3 -1.7 mm while male CVI (Figure 2) are a bit smaller ranging between 0.8 – 1.0 (Balcer et al. 1984).  According to Balcer 1984, the best defining characteristics of M. edax are the “long, palmately spread, terminal setae on the caudal rami” (Figure 1), while Pennak 1978, uses the presence of fine hairs on the inside of the caudal rami (Figure 3) to distinguish them from other closely related species.   Adult females will often be seen with two egg sacs on either side of their urosome (Figure 1) while adult males have a geniculated first antenna (Figure 2).


M. edax is commonly found throughout North America.  It is found in all five great lakes as well as many other smaller lakes and ponds.  Local abundance of these animals can reach as high as 22,000 per cubic meter (Balcer et al. 1984) however other authors have found much lower densities (Selgeby 1975).  Presence of M. edax may be seasonal.  It has been reported that they prefer warmer waters > 8ºC.  Samples collected before water warms to this temperature and after it begins to cool bellow 13ºC may lack M. edax (Andrews 1953).      


Mesocyclops edax is planktonic during the active parts of its seasonal cycle.  Distribution of M. edax is dependant upon water temperature.  It can be found in warmer waters of the littoral zone and pelagic epilimnion of large bodies of water as well as small lakes and ponds (reviewed by Baler et al 1984).  Diapausing M. edax will stay in or near bottom sediments (Balcer et al. 1984). 

Feeding Ecology

It appears that M. edax is omnivorous despite an apparent preference for zooplankton prey.  M. edax will prey on a wide variety of invertebrates and may even prey on vertebrates (Davis 1959b) Documented zooplankton prey includes: Diaphanosoma, Daphnia, other copepods, rotifers, and protazoans (Fryer 1957, Smyly 1961).    It, as well as the closely related M. leuckartii, have also been observed to take phytoplankton (Smyly 1961, Fryer 1957) and may actively filter them from the water (reviewed by Balcer et al). 

 Like other zooplankters M. edax is likely prey for many planktivorous fish species.  Wells (1970) noticed that abundances dropped dramatically after the alewife population explosion in Lake Michigan.  He theorized that alewives were selectively preying on it because of its large size.

Life History

Reproduction in M. edax is sexual.  Males use their geniculate first antennae to hold onto females while attaching a spermatophore or “sperm sac” to the urosome of the female.  Females carry two egg sacks with clutch sizes typically averaging 19-17 eggs (Selgeby 1975).  M. edax undergoes development similar to many other copepods.  Eggs will hatch into nauplii (Figure 4) and molt 4 times while in the naupliar stage until they reach NV (nauplius 5).  At this point nauplii will molt into CI (copepodid 1).  There are 4 more successive molts resulting in a copepodid V.  On the final molt M. edax will become mature adults, CVI.  M. edax is bivoltine producing two generations per year.  Individuals will often diapause during winter as CV and come out in early spring, at which point they will molt into CVI and reproduce. Young from these adults will mature over the summer, reproduce, and their offspring will over winter as CV (Selgeby 1975).   The entire life cycle, from egg to adult, may only take 70 days however this time may be dependant upon environmental conditions food availability (Smyly 1961).   

Literature cited

Andrews, T. F. 1953.  Seasonal variations in relative abundance of Cyclops vernalis, Cyclops bicuspidatus, and Mesocyclops leuckartii in western Lake Erie from July, 1946 to may, 1948.  Ohio Journal of Science 53:91-100

Balcer, M. D., N. L. Korda, and S. I. Dodson. 1984. Zooplankton of the Great Lakes. University of Wisconsin Press, Madison, WI.

Davis, C. C.  1959.  Damage to fish fry by cyclopoid copepods.  Ohio Journal of Science 59(2) 101-102.

Fryer, G. 1957.  The food of some freshwater cyclopoid copepods-and its ecological significance.  Journal of Animal Ecology 26 263-286.

Pennak, R. W. 1978.  Fresh-Water Invertebrates of the United States, Second Edition.  John Willey and Sons, Nex York, New York.

Selgeby, J. H.  1975.  Life histories and abundances of crustacean zooplankton in the outlet of Lake Superior, 1971-1972.  Journal of the Fisheries Research Board of Canada 32 461-470.

Smyly, W. J. P.  1961 The life cycle of the freshwater copepod Cyclops leuckarti Claus in Esthwaite Water.  Journal of Animal Ecology 30 153-169.

Wells, L. 1970. Effects of alewife predation on zooplankton populations in Lake Michigan.  Limnology and Oceanography 15 556-565.



Figure 1.Ventral view of CVI female with egg sacs.  Note the "palmately spread antennae on the caudal rami".


Figure 2. Dorsal view of adult male CVI.  Note the dissected geniculate antennae.

Figure 3. Fine hairs on inside of caudal rami at 400x.

Figure 4. Typical cyclopoid  nauplius.