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freshwater mussels

By Jeff Shearer

"At Riverside commercial shelling was found in progress. Here one fisherman, with an assistant, had taken 20 tons of shells up to the date of our visit (July 27, 1913). ...About 3 miles below Riverside there was another fisherman who had taken out about 15 tons of shells." Coker and Southall (1915) on the James River mussel resources

Like the Mississippi, Illinois, and Ohio rivers, the James River was host to a booming button industry in the early 1900s. But today, a walk along the banks of the James River would scarcely remind us of the prolific mussel fishery that existed in these waters. A midden (pile of shells) left by a muskrat or a sandbar scattered with shells following a flood often provides the only window into the world of freshwater mussels beneath the water's surface. But if one were to wade knee-deep into the James River and look at these hidden treasures first hand, you would find a mussel community indicative of many rivers and streams throughout the U.S., both in terms of biological function and ecological threats.

Freshwater mussels (family Unionidae) are bivalved (two-shelled) mollusks that inhabit a variety of streams, rivers and lakes. Like other mollusks, mussels are best characterized by their hard shell covering, but the actual organism lives within the shell. A mussel's body is made up of muscular and membranous tissue that contains structures for feeding, movement, respiration, reproduction and response to stimuli (both chemical and physical).

Mussels are filter feeders, siphoning plankton, organic matter and nutrients from the water. This feeding process also acts as a natural filter for the stream, cleansing the water of nutrients and toxins. A muscular "foot" that extends and contracts to push the mussel along the substrate controls movement. This process is extremely slow and few mussels move more than one hundred yards throughout their entire life, which may extend past 50 years for some species.

Due to a sedentary lifestyle, mussels rely on their unique reproductive strategy to colonize new areas. When water temperatures and other environmental variables reach ideal conditions, male mussels release sperm into the water column. The sperm are siphoned during the respiration process by female mussels and used to fertilize eggs internally. The fertilized eggs develop into larvae known as glochidia. The glochidia are released into the water by the female and attach to a fish's gills as parasites. Different mussel species rely specifically on certain fish species as hosts. Some mussels have even developed "lures" that resemble a small fish to attract a potential fish host. The glochidia use a fish's gills as a site for development although this parasitic stage rarely has any detrimental impact to a fish. The mobile nature of fish allows glochidia to be transported to other water bodies and stream reaches. After several weeks, the glochidia detach from their fish host and settle into suitable substrate where they develop towards an adult mussel.

Perhaps the most interesting feature of mussels is their shell. Mussel shells consist of either a left and right valve hinged together by teeth structures and ligament tissue. Muscle tissue inside controls the opening and closing of the shells. Shell features include a beak (known as an umbo), hinge, pseudocardinal teeth, lateral teeth, and various ridges, nodules, and keels. All of these features are used to distinguish between species. The white, pearly color (known as nacre) of the shell's inside is what made them of value for buttons. But only those mussels with dense, heavy shells were of value, such as the Threeridge (Amblema plicata) found in the James River.

The United States is home to more mussel species than any other country in the world. The Clinch and Green rivers in Tennessee are world renowned for their mussel diversity. While South Dakota rivers do not have the diversity of their eastern counterparts a variety of mussel species can be found within the state. Thirty-four mussel species have been documented in South Dakota, mostly occurring in the James and Big Sioux rivers, Missouri River below Gavin's Point Dam, and Minnesota River tributaries in northeastern South Dakota. Additionally, the zebra mussel (Dreissena polymorpha) and Asian clam (Corbicula fluminea), both exotic species, have been recently documented in the Missouri River below Fort Randall and Gavin's Point dams.

Despite the diversity of mussel fauna found within the U.S., no other group of animals (aquatic or terrestrial) is as imperiled or has faced as many extirpations. The U.S. Fish and Wildlife Service lists seventy species as federally endangered or threatened, many more are protected at the state level. South Dakota mussel fauna are no exception to this national trend. The Winged mapleleaf (Quadrula fragosa) and Scaleshell (Leptodea leptodon) are the two federally listed species in South Dakota. These species are extremely rare with only one shell of the Scaleshell being reported in recent years. The Winged Mapleleaf is presumed extirpated as only a few old, relic shells have been found reported in recent surveys. Twenty-five of the South Dakota's thirty-four mussel species are recognized as vulnerable, imperiled or critically imperiled.

Reasons for mussel declines are diverse and wide-ranging. Overharvest of shells during the button industry boom of the early 1900s severely depleted some populations. Habitat changes associated with siltation, channel alterations, and flow reduction impact entire mussel assemblages. Impoundments that block host fish movement also restrict mussel distribution. Mussel's bioaccumulate nutrients, metals, and pesticides as these substances are filtered from the water. The introduction of zebra mussels also presents a threat to native mussel populations through direct competition with food sources. Pearls in freshwater mussels are rare and of low quality compared to those found in oysters. However, individuals in search of these hidden treasures may harvest hundreds of mussels just to find one pearl.

Today a variety of conservation efforts are underway to reverse the decline in mussel populations. The propagation of mussels at fish hatcheries has been an especially valuable technique for preserving imperiled species. Other efforts may involve physically moving mussel beds downstream of construction sites to more secure upstream locations. Harvest regulations have become quite restrictive in most states. In fact, most commercial harvest of mussels today takes place at hatchery facilities through artificial propagation. However, many conservation efforts have met with mixed results. Sometimes even the most intensive efforts can not undo a century's worth of impacts. The James River will probably never possess the mussel diversity it had 200 years ago. Too often the shells we observe along a river are all that remain as evidence of a once unique component to the stream's aquatic community.

Jeff Shearer is an aquatic ecologist for South Dakota Game, Fish & Parks

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