Our Lake Has Fleas
Leptodora kindtii - representative of the 35 native species identified in Lake Superior’s waterflea community. Graphic courtesy of Key to Common Great Lakes Crustacean Zooplankton
Lake Superior is swarming with fleas. At least 36 species of cladocerans, or waterfleas, reproduce in the lake and it looks like a 37th is on its way. Unlike Fido’s fleas, waterfleas are usually not alarming. In most cases they are diminutive but pivotal members of aquatic communities. However, two rouge species of waterfleas in the Great Lakes region are giving the entire order a bad name. Adding to waterflea concerns, the grotesque tumors forming on up to 34 percent of some waterflea species in parts of Lake Michigan are scaring public health experts and ecologists.
Like the Daphnia you might remember studying in biology class, cladocerans are benign one-eyed creatures that move by vigorous jerks of their antennae. Several predatory species snatch other zooplankton from the water but the majority of cladocerans channel algae, phytoplankton, and organic detritus into a food stream flowing towards their mouth parts. Small fish and fish fry eagerly swallow most waterfleas that can be as little as the dot on this “i” or as big as this “0.”
Spiny waterflea, Bythotrephes cederstroemi - invaded Lake Superior in 1987. They have spread to inland lakes in Minnesota, Michigan, and Ontario and are found throughout all of the Great Lakes. Near Duluth they live in Island Lake and Boulder Lake. Graphic courtesy of the Ontario Federation of Anglers and Hunters
Fishhook waterflea, Cercopagis pengoi - not found in Lake Superior, but watch out in 2000. This relative of the spiny waterflea is spreading rapidly. If you find waterfleas in Lake Superior or any of Minnesota’s inland lakes that look like the “bad” one pictured below, please preserve the specimen in alcohol and report your sighting to Minnesota Sea Grant at 218-726-8712. Graphic courtesy of the Ontario Federation of Anglers and Hunters
As summer progresses, waterflea populations can respond to the altering water conditions with bizarre changes in individual shape and by restructuring their population. In a presumed anti-predator adaption, some Daphnia species exhibit cyclomorphosis, or a seasonal change in shape. Their rounded heads become pointed helmets. Their single eyes shrink and their tails may elongate. Research suggests that these changes are at least partially driven by increasing concentrations of kairomone in the water. Kairomone is a hormone released from predators like midges (gnatlike insects). These changes can also occur when the tiny animals are exposed to high concentrations of certain insecticides during the first stages of life. Kairomone and low levels of insecticides can act together in water to markedly reduce the population growth rate of cladocerans.
Waterfleas change their reproductive strategy as the warm season wears on. For most of the year, waterflea populations are almost exclusively female. These females reproduce parthenogeneticly - unfertilized eggs develop into young. Although the mechanism is not totally understood, unfertilized eggs produce female offspring until crowding, food availability, water temperature, and light intensity cause chromosomal changes leading to parthenogenic male eggs. The same conditions, if continued, appear to generate females that can copulate with the males and produce larger opaque “resting” eggs. These “super eggs” can survive conditions that harm adult waterfleas and help ensure survival of the species.
And The Ugly!
Polyphemus pediculus- a cladoceran from Lake Michigan that developed a tumor. Graphic courtesy of Key to Common Great Lakes Crustacean Zooplankton
During a 1998 study, the U.S. Environmental Protection Agency found three major predatory (zooplankton-eating) cladocerans in the Great Lakes: the native Leptodora kindtii, the invasive spiny waterflea (Bythotrephes cederstroemi), and another aquatic nuisance species, the fishhook waterflea (Cercopagis pengoi). Experts presume both the spiny and fishhook waterfleas survived the trip across the Atlantic in the freshwater ballast of cargo ships. In Lake Superior, spiny waterfleas have clogged fishing gear with their sticky little bodies for over a decade. Doug Jensen, Minnesota Sea Grant’s Exotic Species Information Center coordinator, predicts that fishhook waterfleas, which were found in Lake Ontario in late July of 1998 and began fouling fishing nets by August, will show up in Lake Superior this year. The long spiny tales on both of these invasive waterfleas deter potential predators.
Fishhook waterfleas reached North America despite the 1993 mandatory ballast water management exchange program that bans ships from discharging freshwater from foreign sources into the Great Lakes. These invaders that originated from Northern Europe and the Caspian Sea areas, spread from Lake Ontario to southern Lake Michigan in a single year. Charter boat captain Burt Atkinson first noticed masses of these waterfleas (that can look and feel like wet cotton batten) in the open waters of southern Lake Michigan. Lakers, sea-going ships, and fishing boats can spread fishhook waterfleas, which have been found at densities of 1,000 individuals per cubic meter (264 gallons) of water in Lake Ontario.
Often, anglers report new sightings. During later summer, these fleas are commonly caught on fishing lines and down-rigger cables where they look like gelatinous blobs. According to Jensen, spiny waterfleas spread to 30 inland lakes in Minnesota, Michigan, and Ontario by sticking to tackle and lines. He suggests that fishhhook waterfleas traveled to the Finger Lakes in New York on lines and tackle, too. Both fleas can be spread when bait buckets or live wells become contaminated and are then used on other waters. Boaters and anglers can help to stop the invading fishhook waterflea by observing the same procedures used to prevent the spread of other exotic species:
- Remove plants and animals from boats and equipment
- Drain the boat of all water (including the bait bucket)
- Never dump bait in the water
- Rinse the boat with a high-pressure sprayer or 40°C (104°F) water or allow the boat to dry for at least five days before launching it into another waterbody
- Remove suspicious material from fishing lines and downriggers, especially where the line meets the swivel, lure, and downrigger ball connection
Along with fishhook waterfleas, which compete with young and small fish for food, Lake Michigan is producing waterfleas that develop tumors. Despite a life span of only weeks, waterfleas in Lake Michigan have grown tumors half as big their bodies. Tumor-like abnormalities on zooplankton are rare but have been reported from polluted seas. Although the tumors don’t appear to be cancerous, they certainly affect the waterfleas’ ability to swim. They also hit ecological alarm buttons - especially because yellow perch are simultaneously developing tumors and the shrimp-like diporeia are disappearing. Researchers are investigating the zebra mussel’s impact on the ecosystem, potential pollutants, possible bacterial or viral infections, and ultra-violet radiation levels in an attempt to identify the potential cause(s) for the tumors appearing on zooplankton and yellow perch in Lake Michigan.
By Sharon Moen