Cercopagis pengoi is the latest exotic
crustacean to invade the Great Lakes. This predatory cladoceran was first
identified by Canadian scientists in early August of 1998, and was reported via Internet
by Dr. Hugh
MacIsaac at the end of August . Cercopagis is indigenous to the Caspian, Azov,
and Aral seas (Rivier 1998), and was reported to have invaded the Baltic Sea in
1992 (Ojaveer & Lumberg 1995,
Ojaveer
1997 ). Cercopagis is similar to another recent
invader in the Great Lakes, Bythotrephes cederstroemi. Both Cercopagis
and Bythotrephes belong to the family Cercopagididae, and have long caudal
processes with up to three pairs of barbs along the proximal end of the process. Both
species occur in brackish and pure freshwater environments. In addition to sexual
reproduction, Cercopagidids most commonly reproduce parthenogenically, which allows them
to quickly establish new populations with a relatively small seed population.
The Great Lakes National Program Office monitors biological
and chemical data across all five Laurentian Great Lakes during two annual
surveys, one in spring and one in summer. Currently the zooplankton program
takes vertical tows from depths of 20 and 100 meters, using 63 and 153
micron mesh nets, respectively. In 1998 the summer survey included a total
of 72 sites, with 8 sites sampled in Lake Ontario between August 5-7.
Cercopagis was found in 4 of these 8 sites, all occurring in the
central basin (Figure 1). Lake Erie was sampled August 2-4,
and no Cercopagis were found at any of the 20 sites sampled.
Densities in Figure 1 were calculated from the 20 meter tow, since it is
believed that Cercopagis generally reside in the warmer, upper waters. However,
because of their large eye and brood sac, they are highly vulnerable to predation by
larger planktivorous fishes (Ojaveer & Lumberg 1995). To avoid predation and possibly
to follow migrating prey, Cercopagis do migrate below 20 meters during the day, as
shown in Figure 2. In
the early afternoon (site 49), high densities of Cercopagis were found below 20
meters. In contrast, all Cercopagis were above 20 meters at sunrise (site 55).
Body length of Cercopagis from site 55 was measured to the
nearest 0.1mm. Only animals with all 3 barb pairs present were measured, and when the
entire caudal process was present, the full length of the animal was measured. Figure 3 presents body lengths (including caudal process)
of the three categories of individuals encountered: sexual females with resting eggs,
parthenogenic females with embryos in the brood sac, and males, and
Figure 4 presents body lengths without the caudal process. The gray shaded
boxes are the ranges for Cercapagis from the Caspian-Azov Sea area (Rivier 1998).
Both the females and males from Lake Ontario exceed the size range of those found in the
Caspian-Azov Sea.
It is unknown at this point how long Cercopagis has inhabited the
Great Lakes before first being reported, and what future impacts it may have. Given the
linkages between Lake Ontario and the other lakes, it is likely that this animal will
spread throughout the lakes in time. Given the high densities observed during the summer
1998 cruise, it is possible that predation pressure by Cercopagis on smaller
cladocerans can affect both the size and composition of phytoplankton communities. In
addition, Cercopagis may impact fish populations by competing with 0-year fishes
for small prey items, or conversely by becoming prey itself for fishes beyond the 0-year
stage.
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CITATIONS
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Mordukhai-Boltovskoi, F.D. & Rivier, I.K. 1987.
Predatory cladocerans
of the world fauna. Nauka, Leningrad.184 pp.
Ojaveer, H. & Lumberg, A. 1995. On the role of Cercopagis
(Cercopagis) pengoi (Ostroumov) in Pärnu Bay and the
NE part of the Gulf of
Riga ecosystem.
Proceedings of the Estonian Academy of Sciences,Ecology, 5: 20-25.
Rivier, I.K. 1998. The Predatory Cladocera (Onychopoda: Podomidae,
Polyphemidae, Cercopagidae) and Leptodorida of the World.
Backhuys, Leiden. 214 pp.
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ADDENDUM
The
Finnish Institute of Marine Research's English language page on invaders in the Gulf of
Finland and the Gulf of Bothnia.
The
Heureka Science Centre, Finland, page on Cercopagis resting eggs.
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