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Freshwater Mussels: Engineering Ecosystems One Shell at a Time
(Released August 2011)

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  by Natalie Abram  

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Ecology and Benefits from Mussels

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USFWS biologist studies zebra mussels
US Fish & Wildlife Service biologist Nick Rowse studies non-native zebra mussel populations in the Lower St. Croix River.

For a Malacologist to study freshwater populations, the freshwater ecology also needs to be considered. This interaction between the abiotic forces such as water temperature, pH, hydrology, geology, geomorphology, pollution, sunlight regimes, and overall habitat deterioration on biota has been identified (Ricciardi and Rasmussen 1222). Innately, organisms and mussels react to a changing environment. But if the conditions change too rapidly, this endangers the population and could cause extinction (Voshell 226).

While some freshwater ecosystems shift due to seasons, precipitation, and natural phenomena, most systems are directly modified by humans. The first major modification is impoundments. Dam construction reduces natural flow from the headwaters to downstream and disconnects mussel fauna (Bates 235). Any locks and dams disrupt the habitat and fragments the currents to the connecting tributaries and rivers. Behind the dam, excessive sediments linger, causing siltation and degraded water quality (Lydeard and Mayden 804). If the geomorphology of the mussel beds is replaced from gravel, sand or any other substrate, this could reduce the ability of the mussels to burrow underground. The impoundments could also prevent mussels from finding their fish hosts for the larvae, thus further reducing the populations.

Another unnatural alteration comes in the form of pollution. The two major types are point source pollution and non-point source pollution. If a particular manufacturing factory residing on a waterway can be traced to dumping heavy metals or waste, the pollution has a source. Whereas, if pollution enters from unknown and numerous entities, it is considered nonsource pollution. Excessive nitrogen, phosphorus, and fertilizer loads from nearby watersheds drain directly into water areas and cause Eutrophication (Weber 169). When algal blooms occur, the exponentially growing phytoplankton suck out all of the nutrients and oxygen from the water column. This directly affects the filter-feeding of mussels. Some pollution events are so dramatic, all residents in a mussel bed perish (Strayer et al 443). Heavy metals and inorganic molecules make their way into the mussel by several ways. Unwanted chemicals can enter with typical filter-feeding, through bioaccumulation from food particles in the column, and from exposure in the sediments (437). This can affect all life stages, including the glochidia on a host to well-established adults.

Since mussels experience pollution acutely through feeding, they are considered ‘bioindicators’. Their potential decline in an area could indicate that pollution is spreading and affecting the decline of the water quality. Rapid assessment of the area should occur by alerting water officials. Other biota could be affected, “however, changes in water characteristics do not affect mussels solely directly, but also indirectly by influencing the density and the composition of the ichthyofauna” (Weber 169). Mussel communities can filter thousands of gallons of water per day and actually improve water quality in some areas. They can reduce the turbidity of the water and naturally biofilter out pollutants from the water. This actually aids municipal treatment facilities.
Mussel Festival
The 3rd Annual Mussel Festival, Bath County, VA, YouTube

Some declines occur naturally or because of anthropogenic influences, while others are from direct competition. Nevertheless, some invasive, exotic, nonnative mussels are competing with the ever-fragile native populations (Strayer 75). These alien species include the zebra mussel and the Asian clam (Dreissena and Corbicula). Both have fast reproductive rates, filtration rates, and early maturity. They also have a tendency of fouling adult mussel beds and basically smothering them (Strayer 78). Invasive species use the native mussels as both substrate and as a food source, slowly reducing the communities.

Zebra Mussels
Fat mucket mussel (Lampsilis siliquoidea) covered with invasive zebra mussels, US Fish & Wildlife Service
Observing the naturally occurring populations has led to food web studies in freshwater systems. Mussels are suspension-feeders, herbivores and consumers and have been found to feast primarily on green algae, diatoms, rotifers, zooplankton and detritus (Nichols and Garling 875). All food sources contain organic matter, which is a necessity for mussel growth, shell maintenance, and metabolism. Freshwater mussels can also be considered as food sources for waterfowl, non-native mussel invaders, and muskrats (Diggins and Stewart 517). Finally, mussels can enrich the freshwater environment in several ways. As mussels burrow into the sediment, they mix-up the lower layers, which reintroduces deposited nutrients and dissolves oxygen back into the water column (Vaughn and Hakenkamp 1431). Nutrient cycling is one of the fundamental processes in the freshwater ecosystem.  Mussels can be labeled ‘ecosystem engineers’ because of their shell production. Shells requires a large amount of calcium carbonate (CaCO3 ), which does not break down easily after the mussel dies, so shells become a refuge for other organisms or as substrate for fouling mussels (Guitérrez et al 80).

1911 Clammers
Clammers standing atop a mound of mussels killed to make mother-of-pearl buttons, circa 1911, US Fish & Wildlife Service
Humans have always benefited from mussels. For thousands of years, freshwater mussels have been used as food sources, for ornaments, pearl jewelry, buttons and for other uses (Strayer et al 429). These findings were uncovered by archeologists, anthropologists and biologists. The Native Americans are well-documented to have utilized freshwater mussels as a food source. Mounds of abandoned shells have been unearthed near freshwater waterways in America (Parmalee and Klippel 424). The use of mussels has shifted from a food source to a manufacturing resource.  In the 1800’s and early 1900’s (before plastics were invented) all buttons for clothing originated from mussel shells (Coker 13). Millions of mussels were killed not for the consumption of flesh, but for the content of their shells. Archived images and videos still exist that display the punch-like procedures to obtain the perfectly circular beginnings of buttons.  Just like many other great resources, mussels have been overharvested. Currently, freshwater mussels are used for pearl cultivation (Williams et al 8). The jewelry industry now uses the most domestic freshwater mussels. “Most shells harvested are sold to the Japanese pearl industry, where they are cut and ground into beads that form the nucleus of cultured pearls” (8). Freshwater pearls are sought after because of their gorgeous coloring which comes from the nacre of the internal shell. Naturally, the nacre would surround the nuclei injected into the living mussel and over time add layers upon layers of calcium carbonate making the pearl. Pearl-making by mussels happens naturally without the assistance of aquaculturists. If those natural pearls are harvested, the cost for consumers increases. “Pearly mussels are among the most imperiled of all organisms in North America and elsewhere around the world” (Strayer et al 430). At this rate, do future mussel populations have a chance?

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