Freshwater Mussels — Benefits and Threats

The Elephant Ear, Elliptio crassidens. Photo by Matt Ashton

Freshwater mussels are a distinctive component of the biological communities found in aquatic habitats throughout North America. Freshwater ecosystems of the Southern Appalachians in particular are home to a diverse array of mussel species.

Lifecycle of Freshwater Mussels

As part of their reproductive cycle, the Wavy-rayed Lampmussel attracts fish, which nip at the mussel display – in black – for a meal. Instead, the larval mussels, contained in packets between the black flaps, are released and may attach to the fish. Credit: Gary Peeples, USFWS

Mussels inhabit the sediment of streams and lakes throughout the region and possess a unique lifecycle. After fertilization, larvae (also called glochidia) are released by female mussels to attach to the gills and fins of fishes. These larvae mature into juveniles on the host fish and subsequently leave the host to further develop into adults on the stream or lake bottom. By attaching to fish hosts, mussels are able to disperse over substantial distances for a relatively immobile animal.

Mussels employ a variety of strategies to infect host fishes. Perhaps most unusually, some mussel species possess a modified structure that resembles a prey insect or fish to attract a predatory fish. When the fish attacks this structure, known as a lure, the female releases glochidia into the fish’s mouth which then attach to the gills to complete the lifecycle.

Benefits to Stream Health

Freshwater mussels live on the stream bottom. Filter feeders, they open their shell slightly to bring water in from the river, filter it to get their food, then return clean water to the stream. Credit: Gary Peeples, USFWS

Mussels provide a number of important benefits to stream health. Most notably, mussels feed on organic matter suspended in the water column. This “filter feeding” removes material from the water, including potentially harmful algae, bacteria, and chemical compounds.

The natural waste produced by mussels in turn serves as food for aquatic insects. Mussels are an important component of the diets of mammal species such as muskrats and raccoons as well.

Additionally, mussels can play a major role in shaping the physical features of streams. As they bury in soft substrates, large mussel populations can stabilize stream bottoms and banks, thereby reducing the probability of severe erosion. Mussel shells also serve as habitats for algae and invertebrate species.

Threats to Freshwater Mussels

Mussel inventory on the Little Tennessee River. Credit: Gary Peeples, USFWS

Despite their importance to aquatic ecosystems, freshwater mussels are a highly imperiled group. In North Carolina for example, approximately half of the native species are in need of conservation according to the North Carolina Wildlife Resources Commission.

Threats to freshwater mussels include a variety of factors that negatively impact aquatic biodiversity, such as degraded water quality and sedimentation. Mussels are also sensitive to the loss of host fish species required to complete their lifecycles. Stream blockages — ranging in size from road culverts to dams — can fragment fish populations and inhibit access to these hosts, in addition to the altering effects these structures have on stream habitats and their fauna.

The preservation and restoration of aquatic habitats supporting mussel populations will not only protect this group in need of conservation, but also improve entire ecosystems via the water filtration and other ecosystem services provided by these species.

~ Article contributed by volunteer Patrick Ciccotto, Ph.D., NC State University