Not Quite Two of a Kind

Mention invasive bivalves, and the zebra mussel (Dreissena polymorpha) is usually the first name that comes up. But their close cousins in the same genus, quagga mussels (Dreissena rostriformis bugensis), are equally troublesome, and their populations are overtaking zebra mussels in some cases. In Hyde’s Quarry in Westminster, Maryland, nearly 90 percent of the mussels collected as part of the eradication effort there were quagga mussels (See “ Pulling the Mussels ,” Chesapeake Quarterly, Vol. 20, No. 2). 

Because zebra mussels have garnered all the attention—and a vast majority of the research funding—quagga mussels are less well known; some resource managers simply use the term “zebra mussel” to cover both species, contributing to their indistinction in the vernacular. But the US Geological Survey (USGS) notes that quagga mussels “have displaced zebra mussels in all offshore areas” of Lakes Michigan, Huron, and Ontario, and “have a more extensive distribution in the Great Lakes, and their abundance far exceeds that of the zebra mussel peak.”

“Quaggas are able to colonize both hard and soft substrata, so their negative impacts on native freshwater mussels, invertebrates, industries, and recreation are unclear,” the USGS says.

In a 2015 paper published in Hydrobiologia, The International Journal of Aquatic Sciences, Alexander Y. Karatayev and Lyubov E. Burlakova, both of the Great Lakes Center, SUNY Buffalo State, and Dianna K. Padilla of Stony Brook University, examined both species in terms of their population dynamics, ability to spread, and potential ecosystem effects. From the outset, they acknowledge that research on zebra mussels far exceeds that of quaggas (“87% of all Dreissena-related papers published since 1989 are on D. polymorpha”) and that the thin, though growing, body of research on quagga mussels is “limiting our ability to predict the spread and ecological impacts of this important freshwater invader.”

Zebra mussels can live up to nine years and can survive out of the water for up to seven days; quaggas live up to five years, and can withstand up to five days out of the water.

Much is made of the mussels’ effects on human infrastructure, but their impact on other species and ecosystems is also profound and complex. They are what’s known as ecosystem engineers—they physically and chemically alter habitats and affect resources used by other species, disrupting and altering local food webs.

As “highly efficient suspension feeders,” they can dramatically improve water clarity—which would seem to be a good thing. But this comes with costs: Their feeding depletes phytoplankton that other key species need for nourishment, and greater clarity allows more light into the water column, contributing to algae blooms and the growth of unwanted benthic algae.

“They can increase habitat complexity for other benthic invertebrates and affect the planktonic community, trophic relationships, and nutrient cycling via their feeding activities,” says Karatayev.

Where these two mussels differ affects their population spread and potential ecological effects.

Likewise, zebra mussels are better equipped to withstand the water motion in the shallow areas, or littoral zones, of water bodies. Quaggas, which can also settle in soft sediment (unlike zebras, which require hard substrate) “prefer quiet areas of deep lakes and reservoirs,” and here they can be widely distributed, rarely forming druses, Karatayev says.

But while quaggas are slower to establish, over the long term they become dominant in most water bodies. “After 9 years or more of coexistence, the density of quagga mussels in both shallow and deep areas of a lake can far outstrip those of zebra mussels,” Karatayev says. Quaggas gain a demographic advantage, he says, because they can reproduce at lower temperatures and in the deeper, darker parts of a lake. “In deep lakes with both species, the quagga mussels will be much more abundant and will impact the ecosystem much stronger, eventually out competing zebra mussels,” Karatayev says.

Karatayev and colleagues predict that these differences in distribution and druse-forming means that “these two species of Dreissena can have very different effects on benthic invertebrate communities. "

According to the website of the  Invasive Mussel Collaborative  , which includes 36 federal, state, provincial, and tribal agencies, non-government groups, research institutions, and private industries working to coordinate research and communications around management and control of invasive mussels, “The feeding of quagga mussels has dramatically reduced primary production in Lake Michigan and Lake Huron and has contributed to basin-wide population crashes of species which are fundamental to food webs and fisheries such as Diporeia, an invertebrate species. Some studies indicate that invasive mussels may even facilitate the establishment of other harmful invasive species like the rusty crayfish.”

Karatayev and colleagues note that dreissenids’ impacts and their system-wide effects in open-body water systems will be determined by “water mixing rates, lake morphology, and turnover rates.” But regardless of those variables, quaggas may be more potent invaders than zebra mussels long-term.

“Because quagga mussels are found in all regions of a lake, and form larger populations, they may filter larger volumes of water and may have greater system-wide effects, especially in deep lakes, than zebra mussels, which are restricted to shallower portions of lakes.”