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About Freshwater Mussels

A nearly face-sized mussel is held in a person's two hands. The shell is a beautiful reddish-brown.
(Photo: USFWS)

Freshwater mussels are the animals you never knew you couldn’t live without. Like pollinators, freshwater mussels provide vital services that underpin entire ecosystems, but unlike butterflies and bumblebees, these animals are not showy. In fact, they aren’t often even noticeable, hidden below the water’s surface and tucked in among the rocks, mud, or sand. But below the surface, these creatures are far more fascinating than a first glance might suggest.

 

What Is a Mussel?

A gray-shelled freshwater mussel is shown in this crisp underwater photo.
(Photo: Roger Tabor / USFWS)

Commonly-eaten mollusks like oysters, marine clams, and marine mussels may come to mind when you think of the word "mussel." Although they inhabit very different habitats and have a unique life cycle, freshwater mussels are also a food source⁠—serving as energy-rich prey for birds, otters, muskrats, raccoons, and other wildlife⁠—and are recognized by Tribes as a “first food” of important cultural significance. (However, please note that harvest of western mussels is prohibited in many places to protect declining populations!)

Freshwater mussels are bivalves, meaning they have two halves (valves) to the shell that protect their soft body parts. They have no head, eyes, ears, or appendages⁠⁠—with the exception of a single foot, which they use to burrow down into mud or sand. They use internal gills to filter food and oxygen from water, and to reproduce.

There are more than 900 species of freshwater mussels (belonging to the order Unionida) distributed among six families. North America is home to about 300 animals in the families Unionidae and Margaritiferidae.

 

Life Cycle

Freshwater mussels have a life cycle that is uniquely adapted to colonizing new habitats and sustaining populations in riverine and still-water habitats. Unlike other bivalve mollusks, which may rely on planktonic larvae to drift and settle in new habitat, freshwater mussels use mobile host fish to travel and to complete metamorphosis. It begins when male mussels release sperm into the water, where it is taken in by females nearby or downstream. This is a sensitive stage because poor water quality can keep sperm from reaching females. If successful, fertilized eggs will develop into embryos inside the female’s special internal brooding pouch (called a marsupium). When they are mature, they are released as larvae (called glochidia) into the open water. This stage is critical, as larvae must find host fish to attach to in a short period of hours or days or they will die. Host fish are particular species of fish on which mussels can attach (to head, fins, or gills) and form a cyst. This causes little harm to the host because the “infection” stage is short and more of a low density irritant.

 

A graphic depicting the freshwater mussel life cycle, including being carried by fish during an early stage of development.

 

How does a mussel find a host fish? They don’t have eyes to see, but mussels have special adaptations to bring the host fish to them. Western North American species of mussel can release glochidia in a clump (called a conglutinate) that looks like decaying flesh- an attractive food source to fish. Eastern North American species have an even more interesting method: Some mussels have developed “lures”, which are extensions of their mantle and used to attract the attention of host fish. You have to see it to believe it⁠—check out this article by Wired magazine.

After they have attached, mussels hang on as fish move through or among streams, rivers, or lakes, usually over the course of a few weeks. When they are ready, mussels emerge from the cyst as juveniles and drop into (hopefully!) good habitat. At that point, they burrow into the stream or lake bottom, where they will grow into adults. If conditions are good, this is where mussels will spend the next 10, 20, or for some species, as much as 60-100 years of their life! As you can see, mussels and fish have a symbiotic relationship⁠—mussels maintain water quality for fish and concentrate nutrients that feed macroinvertebrates, that in turn feed fish. In return, mussels rely upon fish to produce the next generation.

 

    Unseen Heroes

    Even though only a fraction of one percent of Earth’s water is freshwater located in rivers and lakes, it is arguably our greatest natural resource. Freshwater mussels support other species that enrich our rivers and lakes and improve water quality by filtering gallons of water each day.

    A man leans over two tanks of water: one that has had mussels in it for a while, and therefore has clear water, and one that is cloudy and brownish.

    In this photo, both aquariums were filled with water from the nearby stream. A small collection of western pearlshell (Margaritifera falcata) was added to the aquarium at left. After three hours, the difference in water quality with and without the mussels is clear. (Photo: Zachary Moore / USFWS)

    A shell with a lot of texture, being held aloft in someone's hand, has a crayfish's face and claws poking out!

    Mussel beds provide habitat for many aquatic life. This photo shows a crayfish using an abandoned mussel shell as shelter. (Photo: Xerces Society / Emilie Blevins)

     

    As filter-feeding powerhouses, freshwater mussels are constantly removing impurities, sequestering heavy metals, and even removing pharmaceuticals and bacteria such as E. coli.

     

    Freshwater mussels filter out nutrients as water flows by, making food sources more visible and available for other organisms to eat. Mussel beds also create habitat for other aquatic invertebrates, which in turn are eaten by fish. Empty mussel shells are a refuge for crayfish, snails, and fish. Decaying shells are a slow-release source of calcium, phosphorous, and nitrogen.

      

    Mussels themselves are a food source, as energy-rich prey for birds, otters, muskrats, raccoons, and other wildlife, and are recognized by Tribes as a “first food” of important cultural significance. However, harvest of western mussels is prohibited in many places to protect declining populations.

     

    Freshwater mussels are long-lived and sensitive to environmental changes, making them excellent bioindicators. Incredibly, some species can live for up to 100 years and are relatively stationary. As we can look at tree rings to measure the age of a tree and learn about the climate and conditions of a forest over many years, mussel biologists can determine the age of freshwater mussels by counting the rings etched into shells and make observations about long-term stream health.