North America’s Diverse and Highly Beneficial Native Freshwater Mussels Provide an Array of Valuable Ecosystem Services to our Aquatic Ecosystems

By November 13, 2022 News

Michigan’s Once Diverse and Extraordinarily Abundant Mussel
Populations Now in Severe Decline or Extinct

by Scott Brown

Snuffbox, pimpleback, white catspaw, elk toe, slipper shell, Wabash pig toe, fat mucket, deertoe, three ridge, maple leaf, and three horned warty back represent just a small sample of the hundreds of unique common names that humans have assigned over the course of the past two hundred fifty years to members of two indigenous families of freshwater mussels that inhabit North American waters. In spite of their only vaguely descriptive and sometimes amusing common names, aquatic ecologists who have observed the “flamboyant” reproductive strategies and studied the unique life cycles of freshwater mussels have suggested that the venerable creatures of the substrate represent one of our planet’s most fascinating and grossly under-appreciated animals.

The vast majority of the freshwater mussels that are indigenous to North American rivers, streams, and lakes are members of the diverse Order Unionoida, and includes two hundred eighty-six species within fifty-eight genera of the Family Unionidae, and five species representing two genera within the Family Margaritiferidae. Even though approximately one thousand freshwater mussel species within the Order Unionoida inhabit freshwater ecosystems distributed across the planet, North American rivers, streams, and lakes situated east of the Rocky Mountains continue to support at least one third of the species within the diverse Order, and therefore host the greatest diversity of freshwater mussels on earth. It is also important to note that freshwater ecosystems distributed throughout Canada and the United States, including many located within the Laurentian Great Lakes region, and especially Lake Michigan, also currently host massive infestations of two highly invasive non-native species of freshwater mussels within the Dreissena genera of the Family Dreissenidae: Dreissena rostriformis bugensis – quagga mussels; and Dreissena polymorpha – zebra mussels. Moreover, North American fresh and brackish water ecosystems also host two increasingly widespread exotic invasive mollusks within the Corbicula genera of the Family Cyrenidae: Corbicula Corbicula fluminea – Asian clams; and Corbicula largillierti – freshwater clams.

Ranging in size from three to twenty-five centimeters (1.18 – 9.84 inches), native adult freshwater mussels within the Families Unionidae and Margaritiferidae possess calcareous exoskeletons that consist of two distinctive hinged shells that are referred to as valves that provide both structure and protection to an otherwise highly vulnerable gelatinous body. A highly variable set of characteristics that include size, shape, thickness, texture, color, and pattern forming special features such as ridges, rays, chevrons, bumps, and warts that often adorn the distinctive shells of freshwater mussels serve to enable the ability of aquatic ecologists to reliably identify each species in the field. Freshwater mollusks possess a delicate soft tissue body that consists of a mouth, a relatively large stomach, a kidney, an intestine, an in-current siphon, an ex-current siphon, large filamentous gills that enable the extraction of oxygen, a foot that allows the unique creature to slowly move short distances, and remain anchored to substrates even in the presence of strong currents, and ligaments that permit the organism to open and close its surrounding shells. Comprised of neurons and glial cells that are supported by a network of connective tissue, Unionid mussels also possess a rudimentary sensory system that allows the sightless creatures to sense motion induced by nearby fish or potential predators. Representing the longest living invertebrates on earth, freshwater mussels living in optimal habitat are capable of achieving lifespans that often exceed fifty years.

The extraordinary reproductive cycle of Unionid freshwater mussels begins as sperm originating from the mantle cavity of a male mussel is ejected through their ex-current aperture, and is taken via the surrounding water column (hopefully) into a nearby female’s mantle cavity through their in-current aperture. Eggs that become fertilized move from the gonads of the female to their gills where they ripen and gradually metamorphosize into first larval stage mussels referred to as glochidia. Ranging in size from 0.05 to 0.5 millimeters, to those observing the tiny organisms through a microscope, depending upon species, mature glochidia are triangular, oblong, or circular in shape, and appear as miniature bi-valve mussels. Characterized by the presence of sensory hairs on their mantle, and either a larval thread or hook-like structure that protrudes from their partially open shell, the next stage of the reproductive cycle of freshwater mussels begins when up to one million mature glochidia are expelled by the female, and a tiny fraction of those become attached to the gills, skin, or fins of a fish that serves as parasitic host. Freshwater mussel populations located in the Laurentian Great Lakes region often rely upon smallmouth bass or walleye to serve as hosts for glochidia. Upon attachment to a compatible host fish the mature glochidia forms a protective cyst that acts as a parasite while extracting critical growth nutrients that support continued growth and development of the now fast maturing glochidia. Following a period that depending upon species ranges from ten to thirty days, the now juvenile mussel falls off their parasitic host, and enters the third and final stage of development that occurs within the substrates of the host freshwater ecosystem. Lasting from one to eight years, freshwater mussels in the juvenile stage of life complete their internal development, achieve exponential shell growth, and reach sexual maturity. It is important to note that the vast majority (99.999%) of glochidia ultimately fail to become attached to a host fish, and, due to the fact they are not yet capable of living independently in the substrates, die within hours of being expelled from their parent female mussel.

Representing an extraordinary strategy that has evolved in order to enhance an otherwise extremely low glochidia survival rate, species within the Lampsilis genera of the Family Unionidae have evolved the unique ability to deploy their mantle in a manner that bears extraordinary resemblance to the appearance of a small fish. Replete with natural looking markings and false eyes, the realistic decoy moves in the current and serves to attract the attention of fish in the area. Potential host fish that approach and attempt to prey upon the decoy are abruptly doused with a dense cloud comprised of hundreds of thousands of glochidia in the hope that a few of the tiny organisms will succeed in establishing a life sustaining parasitic relationship while attached to the gills, skin, or fins of a fish whose immune system has evolved to accommodate the presence of mature glochidia. Aquatic ecologists familiar with the reproductive processes of freshwater mussels recognize that the unique relationship that exists with fish that are capable of providing support to larval stage reproductive propagules represents an extraordinarily innovative strategy that has evolved in order to provide the sessile creatures with the capacity to successfully colonize upstream habitats. The very fact that at least small populations of approximately one thousand freshwater mussel species within the diverse Order Unionoida continue to be observed in rivers, streams, and inland lakes distributed across the planet serves as a de facto indication of the existence of aquatic habitat that is capable of supporting the ecologically sensitive creatures.

The incredible size, abundance, and diversity of populations of native freshwater mussel that were often present well over a century ago in many North American rivers strongly suggests that optimal habit for members of the Family Unionidae and Family Margaritiferidae is primarily found in large northern temperate rivers whose sensitive aquatic ecosystems are protected from the negative influences of their surrounding watersheds by forests, marshes, wetlands, and densely vegetated riparian corridors. The capacity of large northern temperate rivers to support abundant freshwater mussel communities is ultimately contingent upon the existence of relatively high quality aquatic ecosystems whose continuously flowing pollution and sediment free waters are capable of supporting much higher dissolved oxygen content in contrast to the still waters of inland lakes and ponds, for example, and healthy, moderately productive littoral zones that are capable of providing filter feeding mollusk communities with an abundance of the phytoplankton, diatoms, and other tiny organisms that they depend upon for sustenance. Moreover, due to their glacial origins, the waters of many large northern temperate rivers are also capable of providing the abundant calcium carbonate concentrations that freshwater animals such as mollusks and snails rely upon for the development of their protective shells. Northern temperate rivers that have maintained their capacity to support abundant freshwater mussel populations are also characterized by host fish friendly benthic habitat such as woody debris, boulders, and stones, as well as by natural shorelines, and near shore shallow areas featuring abundant emergent aquatic plants that provide optimal habitat for juvenile host fish. It is important to note, however, that certain members of the Family Unionidae and Family Margaritiferidae exist only in northern temperate inland lakes, streams, and small rivers characterized by slow moving or still waters. The freshwater mussel known as lake floater (Pyganodon lacustris), for example, is primaily observed inhabiting substrates that are situated in wind and wave protected coves and bays of inland lakes. Given the fact that native mollusks are capable of living for periods of up to fifty years, the existence of abundant and diverse freshwater mussel communities accompanied by large quantities of the abandoned shells of previous generations serves as a reliable bio-indicator of the long-term existence of habit conditions that were ultimately capable of supporting the ecologically sensitive bell weather species.

Native freshwater mussels are powerful ecosystem engineers that are capable of rendering highly beneficial ecological services that contribute to achieving and sustaining healthy, diverse aquatic ecosystems. First and foremost, freshwater mussels are highly efficient filter feeders that are perhaps best known for their extraordinary collective capacity to transform turbid, light deprived waters into relatively clear waters by removing algae, bacteria, suspended particulate, and organic matter, allowing life sustaining sunlight to penetrate deeper into the water column, and enabling highly beneficial submerged aquatic plants to colonize larger, deeper areas of the ecosystem. The propensity of freshwater mussels to filter out and utilize suspended algae, inorganic particulate, and organic matter also enables the capacity of the unique substrate-borne creatures to effectively sequester phosphorus, nitrogen, and carbon that would otherwise be available to fuel exponential growth of light attenuating phytoplankton. Freshwater mussels also contribute to sustaining a host of other ecosystem-friendly creatures by converting filtered materials into important sources of food that would otherwise be unavailable for consumption by the myriad of fish, crayfish, amphibians, reptiles, birds, and mammals that often forage within aquatic ecosystems. Diverse in size and shape, the durable abandoned shells of freshwater mussels that have completed their life cycles also provide protective physical spaces that serve as optimal benthic habitat for aquatic insects, and nesting sites for small fish.

Once abundant freshwater mussel communities are also known to have made significant contributions to sustaining North America’s indigenous tribes in centuries past, and in particular the mound-building tribes of the Midwest who placed a high value on the shells of the substrate-borne creatures that were efficiently utilized for making tools, jewelry, and pottery, and upon their meat that was considered an important source of protein rich food. In sharp contrast to the responsible and ultimately sustainable manner in which North American indigenous tribes utilized freshwater mussels to support their culture for many centuries, large scale commercial exploitation of the extraordinarily abundant populations of freshwater mussel that once existed in many of North America’s large rivers, and particularly the Illinois, Columbia, and Ohio Rivers, did not begin until the late 19th century. Recognized by textile entrepreneurs of the time for the considerable economic value of their pearly, durable shells that were turned into the tens of millions of buttons of various sizes that were in demand by a flourishing garment industry, freshwater mussels were removed en masse from the ecologically sensitive substrates of rivers distributed across North America between 1890 and 1950 in order to support the two hundred button factories that were in operation during the period. Large scale, grossly unsustainable exploitation of freshwater mussel populations in North America lasted for well over half a century, and ended only in response to the advent of plastic that than become a cheaper, and much more readily available source of durable material for manufacturing buttons in the early 1950’s.

Coupled with the historically significant fact that extraordinarily diverse and abundant native freshwater mussel communities that inhabited many large North American rivers were being exploited for their valuable shells at an unsustainable rate, the rampant pace of industrialization that was also occurring on much of the continent within the late 19th and early 20th centuries was driving equally unsustainable rates of deforestation that ultimately caused the permanent loss, and/or severe degradation of freshwater habitat that was capable of supporting the ecologically sensitive species such as freshwater mussels has also been caused by the fact that many large rivers have been extensively dredged to allow their once relatively shallow channels to accommodate large commercial cargo vessels that are now deployed to transport coal, iron ore, and other heavy industrial commodities. Not surprisingly, large North American rivers such as the Ohio and Illinois are now widely recognized as the most extensively polluted rivers in the world.

The extent of the loss of freshwater habitat that was once capable of supporting abundant and diverse freshwater mussel communities in North America is best measured by the fact that up until the early decades of the 20th century the substrates of many large rivers such as the Ohio, Columbia, Illinois, and the Wabash “were paved with mussels.”  Loss of freshwater habitat that is capable of supporting abundant freshwater mussel populations has unfortunately continued at a steady pace for well over one hundred years. For this reason, readers should not be surprised to learn that nearly three-quarters of North America’s once extraordinarily abundant native freshwater mussel species are now classified as endangered, threatened, species of special concern, or, as in the case of as many as thirty-eight ecologically sensitive species, have now passed into extinction.