Cousin to the quagga: The zebra mussel’s heyday in the Great Lakes
By Colleen Svancara
The quagga mussel is fairly new to its role as an invasive in Arizona freshwater lakes. Its not-so-distant cousin the zebra mussel has been wreaking havoc in the Great Lakes area since the 1980s. Arizona ecologists are hoping to prevent problematic situations such as this in their own waters by closely studying the effects of zebra mussels on the Great Lakes.
In a 2005 paper published in the journal Environmental Health Perspectives, Scott Fields stated that this fingernail-sized invader has been seeking out every last river, lake, and creek in the Great Lakes area. The zebra mussel (Dreissena polymorhpa) has been found at densities of up to roughly 21,000 mussels per square foot, which is similar to the densities of quagga mussels (Dreissena bugensis).
Boaters have pulled up their anchors to find them completely covered in layers of the mussels. They reproduce at high rates – laying up to a million eggs a year – by attaching to a substrate and reproducing outward. As Fields noted, researchers have measured as many as 10,000 mussels linked to the shell of one mussel.
Both quagga and zebra mussels, native to Europe, are bivalve filter feeders. Bivalve means they have two symmetrical halves adjoined by a ligament. As with all mollusks, the mantle encloses all its organs and secretes the calcium carbonate, which makes up the animal’s shell.
As with the quagga mussel, the feeding mechanism of the zebra mussel poses serious problems for the ecology of the lakes where the mussels are found.
Zebra mussels drastically change the composition of water. They feed heavily on phytoplankton and protozoans, but also filter small particles like bacteria and silt through their water column. These tiny particles suspended in water are what usually give lake water its murky, opaque appearance. Zebra and quagga mussels filter the water, making it more clear, which directly affects the growth of native plant life.
In Lake Michigan in 2005, newly transparent water allowed sunlight to reach greater depths in the lake. The population of a seaweed-like algae species, Cladophora, suddenly exploded. The plant is sunlight-dependent so with the sudden influx, its numbers grew. The problems arose when the weed died and floated to the surface, or washed up on shore. The thick accumulation of dead biotic mulch gave off a rotting stench of biological decay. The septic smell encompassed the lake.
But the issue goes beyond simple human inconvenience. The increasing amount of dead plant life allowed for new species of bacteria to fester. When mussels filter water, they pick up toxins and industrial pollutants as well. They “bioaccumulate,” or further concentrate the toxins in their fleshy bodies, essentially becoming biohazards. Then fish that consume the mussel also ingest the concentrated toxin. The toxin moves up the food chain, potentially unleashing a botulism epidemic of disease among local birds and fish.
Cornell ecologist Edward Mills has studied birds affected by botulism outbreaks around Lake Erie. Botulism is a paralytic illness caused by the ingestion of the toxin botulinum. The toxin is produced by the bacteria Clostridium botulinum, which favors a low-oxygen environment. Mills linked several outbreaks to a sudden increase in dead plant matter in Lake Erie, which he attributed to mainly zebra and quagga mussels. Researchers believe that as the dead plant matter started to decay, it stripped the water of its oxygen, triggering the growth of Clostridium botulinum in the lake.
Once the bacteria were in the lake, it wasn’t long before the mussels filtered and concentrated it in their soft tissue body. The goby fish, a common predator native to the Great Lakes, would then ingest mussels and further concentrate the toxin. The goby would either die from the disease, or spread the disease to the next predator.
The zebra mussel has proven to be very sensitive to its environment. It can reflect the toxicity of a body of water. Therefore, research efforts have begun regarding using the mussels as bio-monitors for measuring toxin levels. There may be some good to come from these high sea travelers after all.