To date, my research on exotic zooplankton species has focused on two exotic predatory zooplankton species, Bythotrephes cederstroemi, commonly known as the spiny water flea, and Cercopagis pengoi, known as the fish hook flea. Bythotrephes cederstroemi was introduced to the Laurentian Great Lakes from Northern Europe in the mid-1980's, probably in the ballast water of ocean-going ships. After Bythotrephes invaded Lake Michigan, many of the native herbivorous zooplankton species declined dramatically, and some species that were favorite prey for young fish essentially vanished from the lake. Little was known about Bythotrephes in its native habitat, and much of my research on this topic has centered on determining what the role of this predator is in lake food webs. Specifically, we tried to answer the questions: What does Bythotrephes eat? How much does Bythotrephes eat? Could Bythotrephes be responsible for the huge changes in Lake Michigan zooplankton communities after its invasion? Might large populations of Bythotrephes have a negative effect on small fish?
This is a photograph of an adult female Bythotrephes cederstroemi
hatched from a resting egg and raised in the laboratory by Peder Yurista.
The large oval on the back of the organism is a broad sack containing embryos
(you can make out the black eye-spots if you squint). On the right side
of the image is the eye spot of the adult, and on the left side of the
image is the long tailspine with four pairs of barbs (adult females not
hatched from resting eggs have only three pairs of barbs). Individuals
hatched from resting eggs are smaller than those that develop parthenogenetically
in the brood sack. This individual was about 0.9 cm in length. Bythotrephes
cederstroemi collected from Lake Michigan may reach well over 1 cm
in length and weigh more than 1 mg (dry weight). This is BIG for
a zooplankter. For comparison, this organism eats a relatively large
herbivorous zooplankton, Daphnia, the (non-spiny) waterflea, that
only reaches 3 mm in length.
This is a photograph of the same Bythotrephes cederstroemi shown above, only now it is consuming a Daphnia pulicaria. The blob beneath the predatory Bythotrephes is the Daphnia. The Daphnia's head is covered by the feeding appendages of the Bythotrephes. If you look closely, you can see a vertical line on the right side of the body of the Daphnia. This line is the gut of the Daphnia; the gut is visible because it contains the green phytoplankton (algae) food of the Daphnia (if your web browser and computer monitor are good, and if you look at the vertical gut line of the Daphnia long enough, you may be able to convince yourself that you can see the green pigment).
To determine the effects of Bythotrephes cederstroemi on the Great Lakes ecosystems, I collaborated with another limnologist, Peder Yurista, to measure energetic expenditures by Bythotrephes cederstroemi and we used these data to construct a bioenergetic model. This model predicts minimum prey consumption by the invader's populations. Our model indicates that Bythotrephes could have been responsible for dramatic changes in the zooplankton community of Lake Michigan that followed its arrival. We subsequently confirmed the predictions of this model experimentally. We also developed a method employing allozyme electrophoresis to assess predator diet composition. Because Bythotrephes ingests no identifiable parts of its prey (it ingests only soft tissues), a simple gut content analysis (looking at what items are in its digestive tract) to determine diet composition was impossible. We analyzed allozyme signals from the gut contents of field-collected Bythotrephes and compared them to the signals from potential prey items in Lake Michigan's offshore zooplankton assemblage to determine which prey were being consumed. Additional studies on the feeding ecology of Bythotrephes also led to the prediction that this invader may be severely decreasing trophic transfer efficiency of energy from herbivorous zooplankton to fish in Lake Michigan.
At SUNY ESF we are continuing to work on the problems posed by exotic species. These questions are becoming increasingly important, as exotics such as zebra mussels (Dreissena polymorpha), purple loosestrife, and millfoil spread rapidly across large regions of the country and disrupt native fauna and flora. Recently, Cercopagis, a relative of Bythotrephes, invaded Lake Ontario. This predatory zooplankton is much smaller than Bythotrephes (see photos below), and may consume smaller food items. Current work in my laboratory on Cercopagis involves determining the diet of Cercopagis and constructing an energetic model for the species (Mike Snyder's M.S. thesis). We are also looking at hatching cues for the resting eggs and trying to determine ways to prevent the further spread of Cercopagis to other lakes. We are comparing the behavior of Cercopagis with that of its early-arriving relative Bythotrephes. Cercopagis appears to be spreading beyond the Great Lakes faster than did Bythotrephes, so it may soon become a permanent resident of a lake near you. As with Bythotrephes, little work has been published on Cercopagis in its native habitat, making it difficult to predict the effects of its invasion on North American lakes.
Recently we have begun work on a multi-investigator project funded by NOAA Sea Grant to determine the role of exotic invertebrate predators in the Great Lakes. We are determining how native and exotic invertebrate predators interact, and we are examining whether the exotics are changing the flow of energy and essential materials from plants (algae) to fish. Investigators at the University of Illinois and the Illinois Natural History Survey are sampling Lake Michigan, where both Cercopagis and Bythotrephes coexist. At SUNY ESF we are sampling Lake Erie, where Bythotrephes is abundant, Lake Ontario, where Cercopagis is abundant, and Lake Champlain, which has yet to be invaded by these exotics, but still has our native invertebrate predators. We are also running predation and fish assimilation experiments in conjunction with the field sampling. Check back here next year to see what we're learning!
Cercopagis (top) and Bythotrephes (bottom)
Leptodora (top), a native invertebrate predator, Cercopagis, the fish hook flea (second from the top), Bythotrephes, the spiny water flea (third from top), and a larval herring. Note the large size differences between the two exotic species and that they both possess long spiny tailspines. The native invertebrate predator, Leptodora, is much less spiny and may be more edible for fish. Note also that Bythotrephes is much larger than the larval fish. One of the objectives of my lab's current research is to quantify any predatory or competitive interactions between these species, and to determine the importance of these interactions for food webs of the Great Lakes.
Research on exotic invertebrate predators in the Schulz lab is funded by NOAA Sea Grant and by the Great Lakes Reseach Consortium.
Publications by K.L. Schulz on Bythotrephes cederstroemi:
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