WATERFLEAS (ORDER CLADOCERA)

SPINY (Bythotrephes cederstroemi)
FISHHOOK (Cercopagis pengoi)

Cladocerans are commonly called waterfleas due to their small size and jerky swimming motion. Like other crustacea, they shed their shells as they grow. About 400 species of water fleas occur worldwide with different species native to different countries.

ORIGIN
The spiny waterflea probably hitchhiked to the Great Lakes aboard a ship that traveled from the port of St. Petersburg, Russia. In North America the spiny waterflea has invaded all the Great Lakes and inland lakes of Ontario, Michigan, and Minnesota. Belgium, the Netherlands, and parts of Germany have also been invaded. Although the spiny waterflea is a native inhabitant of many European lakes, it has been such a successful invader it’s hard to know its exact native range; today it is found throughout the Palearctic from Great Britain to the Bering Sea. The fishhook waterflea’s native range includes the Caspian, Azov, and Aral Seas, and brackish areas of the Black Sea; the fishhook waterflea has also been a successful invader of the Baltic Sea and the Great Lakes.

HABITAT
Waterfleas are extremely abundant in most freshwater habitats; some species are part of the plankton, living in the open water of lakes and ponds, while others live on the bottom or on aquatic plants. While some waterfleas are herbivores, the Spiny and Fishhook waterfleas are both planktonic predators. The fishhook flea can inhabit fresh and brackish water.

IDENTIFICATION
Full grown waterfleas are barely visible to the naked eye, about 1cm (0.4 inches) in length. The spiny and fishhook waterfleas are easy to tell from the smaller native water fleas by their exceptionally long, barbed tail spine that comprises more than half the animal's total length. Fishhook waterfleas can be distinguished from spiny waterfleas by the unique loop at the end of their tails and by a brood pouch that is more pointed than the spiny waterfleas. Native waterfleas tend to have shorter tail spines.

Fishhook Waterflea

Spiny Waterflea

SPREAD
Fishhook and spiny waterfleas have three competitive advantages over our native waterfleas. First, they’re slightly larger than native waterfleas they prey upon. In fact, these waterfleas are not very choosey and are able to eat a wide variety of plankton. Their barbed tail spines discourages fish and insects from eating them. When predators avoid eating invading cladocerans, they dominate the zooplankton community, depriving small fish and other plankton feeders of food. This, in turn, also reduces food for larger fish and predators. And because waterfleas can produce offspring parthenogenically with many generations in a season, they are capable of reaching astounding densities very quickly! Now that these invasive waterfleas have been introduced to North America they are easily carried to new locations on fishing equipment or in live wells of boats.

2003 Distribution of fishhook waterfleas

Asexual and Sexual Generations in Water Fleas
The interesting way in which waterfleas reproduce makes them very adaptable to environmental change. Most cladocerans can reproduce by parthenogenesis, with females producing asexual eggs that hatch into genetically identical daughters. As a result, males are not present in the population for much of the year. As long as environmental conditions remain favorable, females will continue to produce female offspring. If the environment starts to deteriorate due to overcrowding, cooling weather, lack of food or oxygen depletion, eggs are produced that develop into males and females, capable of sexual reproduction. In this generation, females mate with males to produce resting eggs that can survive through winter and periods of drought. In the spring or when environmental conditions are again favorable, the resting eggs all hatch into parthenogenic females.

IMPACT

• The spiny waterflea's introduction has probably caused the disappearance of two native waterfleas that couldn’t compete along with the invader.
• Invading waterfleas are inedible for small fish; as spiny and fishhook waterfleas populations grow, they monopolize sources of food for other animals.
• Waterflea introductions have decreased growth or survival rates for many fish, particularly young plankton feeders.
• Accumulation on fishing line can fowl fishing lines and commercial fishing nets; the resulting drop in angling effort can reduce the income for many fishing related businesses.

Waterfleas tangled on fishing line

PREVENTION AND CONTROL
Thoroughly clean all fishing tackle, diving gear, nets, boats, and anything else that may hold water before moving from one body of water to another. Drain water from boat motors, livewells, bilges, and transom wells before leaving an access area. Always empty bait buckets on land; do not release live bait into the water or transfer live animals from one body of water into another. Discard contaminated fishing line and nets that can not be cleaned.

FOR MORE INFORMATION ON WATERFLEAS

SPINY WATERFLEA

• Spiny Waterflea Life History and Effects on the Great Lakes
• Invasive Species: Spiny Waterflea
• Spiny and Fishhook Waterfleas
• Harmful Aquatic Hitchhikers
• Spiny Waterflea: Another unwelcome newcomer to the Great Lakes

FISHHOOK FLEA

• New Flea Imperils Fish
• Invasive Species: Fishhook Waterflea
• Cercopagis pengoi Invades Lake Ontario: October 1999

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