Zooplankton of the Great Lakes

A predator of small-bodied zooplankton, C. pengoi is characterized by a pronounced compund eye and a pair of raptorial appendages, followed by four pairs of biramous grasping appendages (lacking exopods) along the abdomen and barbs on the tail spine that are either straight (pengoi) or forward-bent (ossani-type) (Ojaveer et al. 2001). Evident is an extended caudal tail spine terminating in a c-hook. The adaptive significance of the hooked tailspine is forward motion and navigation through waters, a defensive mechanism to combat predation by larval and YOY fish, and regulation of bouyancy (Grigovich et al. 2000). Average lengths of C. pengoi range from 0.90 mm to 2.00 mm, with the caudal process length of as much as eight times the body length (Grigorovich et al. 2000). Females are marked by a large, pointed carapace-covered brood pouch with clutch sizes as large as 24 (Makarewicz et al. 2001). In contrast to C. Pengoi, the congener Bythotrephes is a larger and more robust organism, characterized by a comparibly larger compound eye, round brood pouch, and a markedly thicker caudal spine, lacking a hook (Burkhardt 1994).

There are four instar stages marked by a molt and represented by the emergence of paired spines at the base of the caudal appendage for each instar stage. Males are significantly smaller than females of comparable age: they have a lower body mass and length, exhibit a smaller spine with a decreased difference between the span of articular spines, and the segments between the spines are smaller (Grigorovich et al. 2000).

Reproduction and Life Cycle

C. pengoi exhibit overlapping generations and modes of reproduction, however, the organism typically reproduces parthenogenically in the spring and summer and switches to sexual reproduction in the autumn, thereby producing ephippial eggs. The brood pouch shape and size can differ dramatically between instar stages (Stage1 and Comparison), and brood size is larger in newly inhabited areas compared to those of females from its native region, suggestive of rapid perpetuation of the species (Grigorovich et al. 2000, Krylov and Panov 1998). Morphological features that distinguish Bythotrephes from Cercopagis include the tail spine, shape of the brood pouch, and average size. Bythotrephes In Lake Ontario in 1998, two distinct forms of Cercopagis were found, varying in size, tailspine characteristics, claw formation and brood pouch shape. However, upon mitochondrial DNA analysis, Makarewicz and colleagues (2001) found that although Cercopagis ossani and Cercopagis pengoi key as two different species morphologically, these are genetically the same organism, with the first generation of C. pengoi to arise from resting eggs exhibiting the ossani phenotype (Makarewicz et al. 2001, Ojaverr et al., 2001).


The dispersal of C. pengoi is independent of salinity. Unlike most cladocerans, C. pengoi does not undergo diurnal vertical migration (DVM) (Ojaveer et al. 2001). The evolutionary significance for this behavior is thought to be increased reproduction due to the warmer waters in the epilimnion, despite the increased risk of predation by larval and YOY fish. In Lake Ontario, the abundance of C. pengoi reaches levels of nearly 74% of total crustacean zooplankton biomass, primarily dominated by stage III parthogenic females. Males, which are primarily absent in the spring and summer (< 2%), represent approxiamtely 8% of the population in the autumn (Makarewicz 2001, Ojaveer 2001).

Potential Ecological Impact for the Great Lakes

The disruption of the structure and function of the aquatic environment brought on by the invasion of a new species can yield a severe ecological and economic consequences. The fouling of lines by C. pengoi has already caused severe economic loss for sport fishermen in the Great Lakes region. Given the rapid perpetuation of the species in newly invaded territories and the potential to restructure the zooplankton ecosystem, C. pengoi may be an energetic addition as a dietary supplement for adult fish, or may prove to cause dramatic alterations in the diet of YOY and larval fish, consequently depleting the food source and becoming an energetic deficit (Mills 1993, Ojaveer et al. 2001).

Preventing Further Spread of Cercopagis pengoi

The believed method of introduction of C. pengoi is discharged ballast waters from the Ponto-Caspian region into the Great Lakes. There is concern that all inland lakes with connections to seawater may be vulnerable to intense invasion, with sport fishing and fishery and shipping industries exacerbating the conditions. Because inhabitance is independent of salinity, once the restings eggs are present, if not disposed of properly by bleach or an alternate cleansing agent, there exists the potential for the eggs to rehydrate and hatch (MacIsaac and Grigorovich, 1999, Makarewicz et al. 2001).

Literature Cited

Burkhardt, S. 1994. Seasonal size variation in the predatory cladoceran in Lake Michigan. Freshwater Biology. 31: 97-108.

Grigorovich, I.A., MacIsaac, H.J., Rivier, I.K., Aladin, N.V., and V.E. Panov. 2000. Comparative Biology of the Predatory Cladoceran Cercopagis pengoi from Lake Ontario, Baltic Sea, and Caspian Sea. Archives de Hydrobiologie, August, 2000 149(1): 23-50.

Krylov, P.I. and V.E. Panov. 1998. Resting Eggs In the Life Cycle of a Recent Invader of the Baltic Sea. Archives de Hydrobiologie: Special Issues in Advanced Limnology 52: 383-392.

MacIsaac, H.J. and I.A. Grigorovich. 1999. Ponto-Caspian Invaders of the Great Lakes. Journal of Great Lakes Research: International Association of Great Lakes Research 25(1): 1-2.

MacIssac, H.J., Grigorovich, I.A., Hoyle, J.A., Yan, N.D., and V.E. Panov. 1999. Canadian Journal of Fisheries and Aquatic Sciences 56: 1-5.

Makarewicz, J.C., Grigorovich, I.A., Mills, E., Damaske, E., Critescu, M.E., Pearsall, W., LaVoie, M.J., Keats, R., Rudstam, L., Herbert, P., Halbritter, H., Keey, T., Matkovich, C., and H.J. MacIsaac. 2001. Distribution, Fecundity, and Genetics of Ceropagis pengoi (Ostroumov) (Crustacea, Cladocera) in Lake Ontario. Journal of Great Lakes Research 27(1): 19-32.

Mills, E.L., Leach, J.H., Carlton, J.T., 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: 1-54.

Ojaveer, H., Kuhns, L.A., Barbiero, R.P., and M.L. Tuchman. 2001. Distribution and Population Characteristics of Cercopagis pengoi in Lake Ontario. Journal Of Great Lakes Research 27(1): 10-18.