A quarter of a century has passed since zebra mussel larvae were picked up in the Ponto-Caspian region of the Black Sea and dumped in the waters of the Great Lakes, left to fend for themselves in an unfamiliar habitat. Several years later quagga mussels, a close relative, joined them in a now-familiar invasion saga that has caused billions of dollars in damage and a still-unquantified amount of destruction to the aquatic food web. By the mid-1990’s, the Dreissenid mussels had become widespread in all the Great Lakes and the majority of inland waters of Michigan. Reed’s Lake in East Grand Rapids was not spared from attack; the invasive species have been present there since about the same time.
The City of East Grand Rapids does not monitor the zebra mussel population of the lake, so unfortunately data on first occurrence and population growth are not available. This means that when boat owners pull their crafts from the water this fall and find that mussels have coated the hull, unfortunately all we really know is that the fingernail-sized organisms have succeeded in making the lake their makeshift home, and will continue to do so unless measures are taken to stop them.
The economic cost of the Dreissenid mussel invasion has indeed been huge, but there are ecological costs that are just as important—if not more so, in the long run—that have been mostly overlooked. There are an estimated 281 species of freshwater mussels native to the United States, making it a global “hotspot” for mussel biodiversity. 45 of these are native to Michigan. To put this in perspective, Michigan is home to only 43 species of amphibians and reptiles combined, so 45 species of mussels is pretty impressive. Unfortunately, an estimated one third of these species are threatened, endangered or already extinct, which spells bad news for our aquatic ecosystems.
In freshwater lakes, mussels are generally found living in groups called beds. The presence of a mussel community is felt by multiple trophic levels within the ecosystem. From their anchorage at the bottom of the lake, they filter nutrients from the water that flows past their siphon-like mouths. Consumers of algae, microzooplankton and silt, they keep water clear and levels of dissolved organic material low. In turn, many animals eat mussels: otters, raccoons, muskrat, herons, egrets, catfish, and carp. Clawed mammals are able to pry apart the shells to get to the animal inside, while some fish species simply swallow the mussel whole.
Dreissenids, however, play a slightly different role in the ecosystem. First, each animal is capable of filtering hundreds of gallons of water per day which means a population of Dreissenids could easily clear the entire water column of phytoplankton and microzooplankton in a year, turning the aquatic food chain on its head. Secondly, Dreissenids method of reproduction puts them at an advantage over most native mussels. Dreissenids go through a larval stage in which they are free-swimming organisms (called veligers); however most mussels native to the U.S. require a fish host during their larval stage. This relationship is generally harmless to the fish; the tiny larvae simply attach to the outer gills of a fish and siphon nutrients from the water that is passed over the gills during breathing.
It’s too late to keep zebra and quagga mussels out of the Great Lakes (unlike another potential invader) but wildlife officials and scientists are developing some promising methods of controlling the spread of these damaging species.
A recent study by a group at Cambridge University showed that a combination of toxins had a greater-than-additive effect when used together. David Aldridge, one of the co-authors, used the findings of the study to develop a product now known as BioBullets. The combination of KCl and polyDADMAC is packaged into 10-micron pellets encased in a fat product that is lethal when ingested by zebra mussels. This method of dispersing chemicals uses an estimated 1000x less chemical than traditional methods of pesticide dispersal, and has been approved for use in drinking water treatment in the UK, where it was developed.
The product was shown to be non-lethal to many aquatic species in laboratory tests, and in August two London waste water treatment facilities began 6-month trials using the treatment to clear zebra mussel buildup in pipes.
An alternative to poison, which makes use of a naturally-occurring bacterium, is also gaining traction. Zequanox is a product containing heat-killed Psuedomonas flourescens cells, which were found to be toxic to Dreissenids even at low concentrations. The pesticide was tested on many aquatic species, including Daphnids, fathead minnows, young brown trout, sunfishes, blue mussels, and 6 species of native Unionid mussels.
Unfortunately, both treatment methods still lack data regarding effects of the poisons on other native mussel species, ones more closely related to Dreissenids than Unionidae. It would be expected that a chemical lethal to Dreissenids would affect closely-related species in a similar way. As mentioned, Michigan is home to 45 species of native mussels, so we should be sure that any poisons released into the water will not have detrimental effects on these species.
While long-term control efforts are still being studied, the Wisconsin DNR has been making use of a mechanistic approach to controlling the spread of zebra mussels and other aquatic invasives on a short-term basis. With their mobile decontamination machine, officers spray 140°F water over the hull and through the bilge lines to kill all organisms that might be there, before a boat is moved to a different body of water. This technique would work on other species of exotic animals as well, as few organisms can survive through those temperatures.
We have yet to find the elusive, cure-all treatment to completely eliminate zebra and quagga mussels, and likely never will. However, these new weapons will aid in the fight to limit the spread of zebra and quagga mussels and keep our native mussels healthy and thriving.