Illinois Natural History Survey - University of Illinois

New Predator Invades the Great Lakes

One of the most recent additions to the Great Lakes is another tiny stowaway from Eurasia. Cercopagis pengoi is a small (< 2 mm body length) predatory crustacean that was first discovered in Lake Ontario in 1998. Researchers from INHS found it in plankton samples from southwest Lake Michigan in the fall of 1999. Cercopagis, which has become known as the "fishhook flea," possesses a tail spine that can be up to five times as long as its body. In Cercopagis, the tail spine includes a predominate curve, which separates it from the closely related Bythotrephes cederstroemi that invaded the Great Lakes in the 1980s. Mature individuals possess three pairs of lateral barbs on the tail spine of Cercopagis. Because of this long, spiny, appendage, Cercopagis and Bythotrephes are often found fouling fishing lines in the Great Lakes. The clogging of reels and fouling of nets makes these exotic species potential nuisances in these systems.

Both Cercopagis and Bythotrephes possess life history traits that make them good invaders. Like many other species of zooplankton, they reproduce by means of cyclic parthenogenesis, which means there is an alternation of reproductive mode. For most of the year, only females are present in the water. These females produce eggs asexually, which in a few days are released as newborn daughters. With this mode of reproduction, a single introduced individual could potentially colonize an entire lake!

When living conditions begin to deteriorate (not enough food, too many predators), something in the environment signals the females to begin producing sons instead of daughters. Once there are both males and females in the water, a different type of egg is produced. These eggs are known as "resting eggs" or "diapausing eggs" because instead of hatching right away they are capable of remaining dormant during the time when environmental conditions are bad. Once favorable conditions return, some of these eggs hatch and start a new population.

Cercopagis pengoi, a new exotic microinvertebrate that has invaded Illinois waters. (Permission to reproduce this drawing was kindly granted by Backhuys Publishers, Leiden, the Netherlands.)

Live animals require water to be transported from lake to lake. Therefore, the control measures used for other exotic aquatic species (emptying of bait buckets and live wells, washing and drying of all gear) will also help to reduce the spread of Cercopagis. The diapausing eggs are a different story. These eggs can hatch after they have dried up or frozen, even if it is several years later. Moreover, since females carrying these eggs can "stick" to fishing gear and other recreational equipment, care must be taking to thoroughly clean all equipment after leaving a lake (instead of just allowing things to dry).

It is not known what effect this new species will have on the Great Lakes ecosystem. When Bythotrephes invaded Lake Michigan, its entrance to the lake coincided with marked changes in the food web. Both Bythotrephes and Cercopagis are predators, eating other species of zooplankton that are also the primary food for many species of fish. The presence of another invertebrate zooplankton predator may alter the existing zooplankton assemblage, with implications for the growth and survival of important fish species like yellow perch and alewife. However, Cercopagis is considerably smaller than Bythotrephes, so it may fall victim to predation by the larger predatory species. In addition, Cercopagis and Bythotrephesmay provide additional food for fish. Bythotrephes is commonly eaten by yellow perch and alewife, although the benefit of eating this spiny species for fish is unclear due to problems they experience digesting and passing the spiny leftovers. Cercopagis also was found in alewife guts last fall, but the potential impact of these and other fish on Cercopagis is not clear. Future research will help us understand the role of this new species in North American Lakes.

Carla Caceres, John Dettmers, and Patrice M. Charlebois, Center for Aquatic Ecology

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