Freshwater mussels breathe rivers clean. They are also sensitive to changes in the environment, which puts them at substantial risk. They and the habitat they require are disappearing at a disconcerting rate.

Their presence in lakes and ponds are indicative of high water quality. Their absence tells a different story. The latter is the more-familiar tale.

Rhode Island’s populations of mussels have been degraded by a long history of damming and pollution discharges into rivers. Lake populations have largely been eliminated by basin reconfigurations, pollution, and urban development, according to a 2006 study.

Between 1980 and 2006, Christopher Raithel and Raymond Hartenstine inventoried freshwater mussel populations at 129 sites throughout Rhode Island. They found eight native mussel species: eastern elliptio (Elliptio complanata); eastern floater (Pyganodon cataracta); triangle floater (Alasmidonta undulata); alewife floater (Anodonta implicata); eastern lampmussel (Lampsilis radiata); eastern pondmussel (Ligumia nasuta); eastern pearlshell (Margaritifera margaritifera); and squawfoot (Strophitus undulatus).

The researchers also discovered the state’s populations of freshwater mussels are facing significant threats. They “are rare or localized and should be considered high conservation priorities in Rhode Island,” the co-authors wrote in their study.

The introduction of “The Status of Freshwater Mussels in Rhode Island” noted North America contains a high proportion of the world’s freshwater mussels, but “several mussel extinctions have already occurred and many other species are imperiled.”

Corey Pelletier, a fisheries biologist for the Rhode Island Department of Environmental Management stationed at the Great Swamp Management Area, said those eight species remain in Rhode Island, but he noted “there are certain species that have very localized distributions and are at higher risk of extirpation.”

In fact, six of the state’s freshwater mussel species are species of greatest conservation need, according to Pelletier. Those species are the triangle floater, alewife floater, eastern lampmussel, eastern pondmussel, eastern pearlshell, and the squawfoot.

He said freshwater mussels are “one of the most imperiled group of animals in the United States” and noted many of these species are indicators of water quality and ecosystem health.

“Pollution in the form of roadway runoff, agricultural runoff, and other chemicals and metals that are discharged into waterways are the primary threats to freshwater mussels,” Pelletier wrote in a recent email to ecoRI News.

He also noted that freshwater mussels rely on specific host fish species for reproduction. Mussel species that require specific hosts, such as wild brook trout and river herring, “may be at risk due to declines or limitations of these species”

“Few freshwater mussel surveys have been completed in recent years,” he said “The Division of Fish and Wildlife hopes to be able to utilize eDNA in the future to better understand and document species distributions.”

There are nearly 300 native freshwater mussel species in North America, compared to 12 in Europe, according the U.S. Fish & Wildlife Service. Of those, about 70% are in decline, 40% are listed as threatened, 21% are listed as endangered, and 7% are extinct.

Their numbers took a beating in the early 1900s when big-river dam-building exploded. Thousands of river miles were flooded, the flow of water slowed, and river bottoms were buried in layers of muck. Mussels, which need to siphon moving water to breathe, suffocated.

Their populations haven’t really recovered, which is too bad for anglers, hunters, birdwatchers, wildlife enthusiasts, and conservationists. Healthy mussel populations typically indicate a healthy aquatic system, which usually means good fishing and good water quality for waterfowl and other wildlife species.

But the remaining pockets of healthy freshwater mussel populations are under an increasing amount of stress.

The Xerces Society for Invertebrate Conservation has noted “destruction of habitat has been one of the biggest impacts to freshwater mussels, including the construction of dams that have altered how rivers flow and function.

The Portland, Ore.-based nonprofit has also noted water diversions for human uses that decrease streamflow, combined with warming waters and changing precipitation patterns, have and will continue to threaten mussel populations.

“Pollution and toxic chemicals also impact mussels, which can be especially sensitive to certain contaminants,” according to the Xerces Society. “The same characteristics that have made them so valuable to protecting our water quality (sedentary nature, ability to reach high density, and filtering lifestyle) also make them vulnerable to a wide range of stressors to which they may ultimately succumb.”

Freshwater mussels are members of a large group of animals called mollusks, which includes saltwater mussels, clams, snails, squid, slugs, and octopuses. They are part of the benthos, which is the community of fauna and flora that occupies the bottom of a waterbody.

Mussels play many important roles in freshwater ecosystems, especially in cool, flowing streams. They feed by filtering water through their siphons. In fact, they are one of nature’s greatest natural filtration systems. Not only do they stabilize freshwater ecosystems, but they also continually protect and improve water quality. A single mussel can filter 5-10 gallons of water a day.

They also filter algae out of water, but may filter at a slower rate if the water is dominated by toxic blue green algae (cyanobacteria), according to a 2013 paper published by the University of Rhode Island.

Cyanobacteria is a growing problem in Rhode Island and throughout southern New England because of warming air and water temperatures and the proliferation of fertilizer containing nutrients such as nitrogen and phosphorus.

“Mussels consume zooplankton, detritus (dead and decaying matter), and algae. Therefore, they help maintain the proper balance of organisms typical of waterbodies that are oligotrophic or mesotrophic (have a low or medium level of nutrients),” the authors of the four-page paper wrote. “The mussels themselves serve as a food source for fish and other wildlife [such as river otters].”

Note: Christopher Raithel retired in 2018 after nearly 40 years as a rare species biologist at the Rhode Island Department of Environmental Management. Raymond Hartenstine served as a volunteer field biologist.

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