They aren't wrapped like mummies or built from parts scavenged from graveyards; they don't have curved fangs or ripping claws; they didn't come to earth from a distant galaxy to pop unexpectedly from people's bellies, but we've still got good reason to be scared, for a minuscule but monstrous mussel is invading Missouri.
You may never have heard of zebra mussels, but you soon will. As sure as beesbuzz and birds chirp, these dime-sized invaders are going to affect every one of us.
Zebra mussels (Dreissena polymorpha) are relatively new to North America, much less Missouri. The striped mollusks (the alternating, zebra like stripes suggested their name) traveled to this continent from Europe in the late 1980s, hitching a ride in the ballast tanks of ocean-going ships, according to most reports.
They colonized in the Upper Midwest and were first discovered in Lake St. Clair, near Detroit, in 1988. The zebra mussels continued to spread throughout the Great Lakes and, inevitably, found their way into major waterways, including the Hudson, the Susquehanna, the Illinois and, of course, the Mississippi.
The first mussels (10 of them) were discovered in Missouri at the Melvin Price Locks and Dam, north of St. Louis, in September 1991. In November of the same year, four mussels were found at river mile 47, near Cape Girardeau. The mussels were attached to concrete blocks that were anchoring devices meant to sample for zebra mussels.
The mussels are now common in the Mississippi River. Bob Hrabik, fisheries program coordinator at the Conservation Department's LTRM station in Cape Girardeau, estimates that in February each square meter of underwater hard surface in the lower river - wing dams, rip rap, natural rocks - was covered with about 1,500 zebra mussels.
"We're no longer monitoring densities, because we know they are here and we know they are high," Hrabik said.
The mussels are of several different sizes and, therefore, likely contain several different year classes. "We're now looking at how they transfer themselves through the system. Are they coming from the Illinois River or down the river from Minnesota or are they reproducing here?" This latest study involves counting veliger numbers throughout the river system.
Veliger refers to a larval stage of zebra mussels. Each adult female zebra mussel may release millions of eggs during the warm water months. The eggs mature into tiny free-swimming veligers, which float among the plankton and can be dispersed by currents. About 2-4 weeks after hatching, they settle and will attach to any hard surface, including rock, wood, concrete, plastic, crayfish shells and other mussel shells.
If the tiny - about the size of a grain of sand, at this point - zebra mussels don't land on a suitable surface, they have a single foot to help them swim or crawl in search of one.
Zebra mussels attach themselves to objects with small elastic threads, called byssal threads. Because they can also attach to one another they can quickly form thick living layers.
At a Detroit Edison power plant off Lake Erie, for example, zebra mussels built up like heavy cholesterol in a cooling water intake pipe. According to a plant official they went from nondetectable levels in 1988 to 700,000 per square meter in 1989.
A simultaneous infestation of the water intake pipe of a nearby municipal water treatment plant virtually shut off the faucets of the entire town of Monroe, Mich.
Water pipes are an ideal habitat for zebra mussels because they provide a hard surface and a steady stream of food. Zebra mussels feed on plankton, which they filter from the water. They pump water in, strain it of algae and microscopic animals, then pump it out, along with their wastes.
Billions of these flow-through filter pumps working full -time can have a dramatic effect on the water. Lake Erie, just over a decade ago labeled a eutrophic dead sea, now is known for water clear as a gin bottle.
Filtering sounds like a good thing, except that it threatens established ecosystems.
In any water, big fish feed on smaller fish that have eaten smaller organisms. The chain goes all the way down to the microscopic plankton. Uncountable numbers of zebra mussels successfully competing for this plankton, however, could collapse the chain, devastating sport and commercial fisheries.
The mussels affect ecosystems in other ways. For example, in the Illinois River, the respiration of huge accumulations of zebra mussels is being blamed for low oxygen levels that have caused fish kills. Zebra mussel filtering also allows the sun to penetrate deeper into the water, allowing weeds to multiply.
Zebra mussels directly threaten native mussel populations by blanketing them and preventing them from getting nutrients. In 1986, Lake St. Clair contained 18 different species of mussels. Researchers estimate that in a few years there will be no more native species of mussels living there.
We hesitate to make value judgments about any species, but one thing is sure: if biodiversity is good, zebra mussels are bad.
The mussels also play havoc with our recreational and economic uses of waterways. Beaches littered with dead zebra mussels shells are unfit for bare feet, and the decomposing mussels stink. Oh, do they stink! "They make rotting fish smell like Chanel #5!" according to an often-quoted water company official.
The mussels also encrust docks and bumpers, and large numbers of them can sink buoys or rafts. Layers of them attach like barnacles to the hulls of boats, slowing them down. They can "creep" into and plug the water cooling channels of marine engines, resulting in engine overheating.
Recreational divers may see more as waters clear, but everything of interest on the bottom will be coated with zebra mussels. A Chevrolet Camaro Canadian police retrieved out of northwestern Lake Erie provides an example of how thoroughly they dominate the underwater environment. After only eight months underwater, every part of the car - cloth, glass, metal, plastic - was coated with up to three inches of zebra mussels.
Some fear that such accumulations of mussels on lock chambers and on dams and water control structures could disrupt shipping and power generation on waterways and reservoirs. But, although prodigious numbers of zebra mussels have been found on every lock and dam in the Mississippi River, they haven't as yet caused serious problems.
"We're frequently having to clean or replace underwater probes to computerized water level monitors at Lock and Dam 26 at Alton," research limnologist Andrew Miller, of the U.S. Army Corps of Engineers Waterways Experimentation Station in Vicksburg said. "But the gears and chamber doors at lock chambers are fairly self-cleaning, and the larger machinery at reservoirs is probably not vulnerable."
Given the predilection zebra mussels have for water pipes, municipal water companies, power plants and other industries that draw water from major rivers or reservoirs may have to take special measures, such as burying their intake pipes or introducing chemicals, to combat zebra mussels.
Both the Corps of Engineers and Union Electric, which own or operate several power generating facilities on Missouri waters, continually monitor their facilities for zebra mussel infestation.
"We're a big water user," Union Electric biologist Frank Putz said, "but if we catch them early, we can control them inside our plants by manipulating water levels, mechanical cleaning, adding chemicals or heating the water. But zebra mussels are going to be uncontrollable in the outside environment."
Experts disagree about the potential geographic limits of the zebra mussel. Already they have penetrated far north into Canada and have been found near the mouth of the Mississippi. The genetic plasticity of the mussels allows them t o adapt to a variety of environments - even salt water.
Researchers now know that zebra mussels seem to prefer hard water, where there is plenty of calcium to build their shells. They can't reproduce in waters that are highly acidic, which may limit their distribution. Because of their preference for solid surfaces, they are seldom numerous in lakes or rivers with muddy or mucky bottoms, however they will attach themselves to plants growing from a soft bottom.
No one doubts that zebra mussels will continue to colonize new waters. Distribution maps can't keep pace with their spread. So far they have tended to move from the Great Lakes along major river systems to the south and east, but they have also been found in numerous landlocked lakes. Although their progress is likely to be slower, zebra mussels will inevitably find their way west. They have already established themselves in Oklahoma, via the Arkansas River.
Zebra mussels have a tremendous and, sometimes, insidious potential to get around. Adults can latch onto barges and boat hulls and be transported upriver. Currents carry billions of their veligers downstream. They may be transported by birds and animals, or they might be swimming in bilge water, minnow buckets or livewells or be attached to boats, motors or trailers. Even when out of the water they can remain alive for several days.
At a check station in the state of California, for example, boats with live zebra mussels attached were intercepted. Those boats had been trailered across the country from the Great Lakes, about 40 hours travel time.
Bob Hrabik told how zebra mussels showed up in a small strip pit in Illinois that was closed to fishing and boating. Investigators concluded that the mussels had been inadvertently introduced by scuba divers, after a check of divers who dove in infested waters revealed as many as 200 zebra mussel veligers on each of their suits.
How hospitable Missouri waters will be to the zebra mussels is still in question. Miller said the fact that they have not yet been found in the Missouri River may indicate that they may not be able to tolerate its strong current and muddy water. However, other turbid rivers, including the Cumberland and the Tennessee have self-sustaining populations.
As of the spring of this year, zebra mussels in Missouri have been limited to the Mississippi River, but that's only the vanguard of the invasion. "Zebra mussels are capable of colonizing everywhere in Missouri," said Al Buchanan, an environmental services biologist with the Conservation Department. "I'm almost certain they'll be in Lake of the Ozarks this year; there's so much boat traffic, so many accesses."
Zebra mussels are likely to thrive in the Lake of the Ozarks, Buchanan said, and the population will provide a constant feed of larvae through the dam and downstream. They are also likely to be spread by boats to other reservoirs.
"Predicting what will happen to fish populations is sheer speculation,"Buchanan said. "High numbers of zebra mussels could affect shad, which also feed on plankton, and we don't know what will happen with predator species or plankton-eating paddlefish."
Lots of other questions remain unanswerable. Will zebra mussels establish themselves in the Current, Meramec, Jack's Fork, Eleven Point or North Fork rivers?
What about fish hatcheries? "It would be impossible to stock fish from an infested hatchery," Cindy Borgwordt, Fisheries Division information officer said, "because of the danger of contamination of new waters."
Already, the zebra mussel threat has forced the Conservation Department to close Hunnewell Lake, source of the Hunnewell hatchery's water, to privately owned boats to reduce the threat of zebra mussel contamination from craft that have recently been in the Mississippi River.
To keep Lake Sherwood, near St. Louis, free of zebra mussels, the lake association has plans to install a wash rack at the lake's single access and will require all boats to be washed before being launched.
Other states have taken more stringent measures. Minnesota, for example, has set up highway check stations that stop boaters and inspect their boats. People found transporting zebra mussels, water milfoil and other aquatic troublemakers are being fined.
Because the invasion is just happening now and no one can predict its course, management options in Missouri are limited. This summer, the Conservation Department posted zebra mussel alert posters at all water access sites and has produced id cards and brochures about zebra mussels. Posters, id cards and brochures are available free by writing to "Zebra Mussel," Missouri Conservation Department, P.O. Box 180, Jefferson City, 65102-0180.
What else might be done? A few fish species - drum, suckers, redear sunfish -will eat zebra mussels, as will some diving ducks, but these native species are unlikely to make much of a dent in the population.
A new exotic species, the round goby, has followed the zebra mussels into the Great Lakes and appears to have taken hold in southern Lake Michigan. The goby's large mouth and peg-like teeth suit it well for feeding on zebra mussels. The bottom dwelling fish seldom reach 8 inches in length, but they may provide food for gamefish.
Researchers are also testing to see whether zebra mussels mixed with wood chips and other materials will compost. No other prominent economic uses for zebra mussels loom on the horizon, but if you can figure out a good use for them, it will likely enrich you more than could the lottery.
According to Borgwordt, the best approach for Missourians right now is to conscientiously work to stop the spread of zebra mussels in order to buy time for a control method to be developed.
With our moderate temperatures and fertile waters, Missouri seems primed for an explosion of zebra mussels. It could be in 3 years, 5 years, 10 years or next year.
What effect zebra mussels will have on our ecosystems and environment is anybody's guess. They may change the aquascape entirely, or their populations might explode, crash and then settle into a more moderate persence among Missouri's biota.
Not knowing makes facing the threat even harder.
It's like waiting for a hurricane that's coming, but you don't know how severe it will be or where it will hit the hardest. After you've done what you can to protect yourself, all that's left is to hunker down and try to ride it out
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