Plethobasus cyphyus

Geographic Range

Plethobasus cyphyus, the sheepnose mussel, is found in large rivers ranging across the Midwest and Southeastern parts of the United States. Currently, it is found in Alabama, Illinois, Indiana, Iowa, Kentucky, Minnesota, Mississippi, Missouri, Ohio, Pennsylvania, Tennessee, Virginia, West Virginia and Wisconsin. Historically, this muscle was widespread throughout the Mississippi River system. (Wolf, et al., 2012)


The sheepnose mussel inhabits a wide variety of habitats in large river systems and streams. Plethobasus cyphyus is typically found in shallow areas with moderate to swift currents flowing over sand and gravel. Additionally, they have been found in areas containing mud, cobble, boulders and in deep ruts. The degradation of these habitats have largely lead to the decline of P. cyphyus. ("Endangered and Threatened Wildlife and Plants; Determination of Endangered Status for the Sheepnose and Spectaclecase Mussels Throughout Their Rang", 2012)

  • Aquatic Biomes
  • rivers and streams

Physical Description

Features that distinguish the sheepnose mussel from other mussels include its general shape, color, and presence of central tubercles. They are a medium-sized mussel with a somewhat inflated and elongated shape. Male and female sheepnose mussels differ in the general shape of their shell. Female mussels have a round, shortened posterior end, where as males are oval. The exterior shell of P. cyphyus can be yellow to dark brown and contains dark concentric ridges that reflect growth periods. The exterior shell is rayless and possesses a smooth texture with a shiny appearance. The sheepnose has a broad and shallow depression on the outside of its shell that lies between the tubercles and posterior ridge. This depression is known as the sulcus. The inner shell of P. cyphyus has a shallow beak cavity and is white in color. ("Endangered and Threatened Wildlife and Plants; Determination of Endangered Status for the Sheepnose and Spectaclecase Mussels Throughout Their Rang", 2012; "Sheepnose (a freshwater mussel)", 2013; Rose, 1820; Sietman, 2003)

  • Sexual Dimorphism
  • sexes shaped differently
  • Range length
    13 (high) cm
    5.12 (high) in


Plethobasus cyphyus has a complex life cycle that begins when a female uses her incurrent siphon to draw in male sperm for the fertilization of her eggs. Within the female gills, the fertilized eggs develop into glochidia, the microscopic larval stage. Once the glochidia mature, the female releases them into the water column where they must form a parasite-host relationship with specific freshwater fish. The released glochidia use their valves to clamp onto the gills, fins, or scales of freshwater fish where they encyst into the tissue and live as a parasite to the fish host. A variety of fish species have been identified that glochidia use as their parasite source for growth and development. Some of these include sauger (Sander canadensis), creek chub (Semotilus atromaculatus), and spottail shiner (Semotilus atromaculatus). Over the course of several weeks, the parasite-host interaction fully transforms the glochidia parasite stage into a juvenile mussel by allowing for the development of a foot, gills, and many internal organs. After the complete transformation of the juvenile mussel is complete, the young mussel drops from the fish host to begin its life on a stream bottom where it may grow into an adult. ("Endangered and Threatened Wildlife and Plants; Determination of Endangered Status for the Sheepnose and Spectaclecase Mussels Throughout Their Rang", 2012; Rose, 1820; Wolf, et al., 2012)


The reproduction system of Plethobasus cyphyus requires the use of water flow. Males release sperm into the water where it is transported by current. Downstream, females siphon the sperm from the water as they are siphoning for food and respiration. The release of sperm usually takes place in the early summer, in response to changes in water temperature. ("Endangered and Threatened Wildlife and Plants; Determination of Endangered Status for the Sheepnose and Spectaclecase Mussels Throughout Their Rang", 2012; Wolf, et al., 2012)

Once a female takes in a free floating sperm through the incurrent siphon, the process of fertilization occurs. Once glochidia are formed, they are expelled in a jellylike mass called a conglutinates from the female. The glochidia attach to the fish's gills where they undergo a parasitic stage and grow. Upon the complete transformation into a juvenile mussels, the detachment from the fish host introduces the mussel to a new habitat where they live independently and grow into adult mussels. While the sexual maturity age for the 'sheepnose Plethobasus cyphyus' is unknown, but given its longevity, it is estimated that it is reached after a few years. ("Endangered and Threatened Wildlife and Plants; Determination of Endangered Status for the Sheepnose and Spectaclecase Mussels Throughout Their Rang", 2012; Wolf, et al., 2012)

  • Breeding interval
    Plethobasus cyphyus breeds once a year.
  • Breeding season
    Sheepnose mussels breed in the early months of the summer season.
  • Range number of offspring
    Hundreds to Millions

Parental investment of the sheepnose mussel is carried out only by the mother during the beginning stage of the offspring's development. Upon fertilization, the fertilized eggs are brooded within gills of the female where they obtain oxygen until they develop into the larval stage called glochidia. The parental involvement of the female ends once her offspring reach the glochidia stage, in which they are released into the water column for further development.

Freshwater male mussels are not involved in parental investment of the young. The only developmental contribution the male makes towards the offspring is the release of sperm into the water column to fulfill fertilization. ("Endangered and Threatened Wildlife and Plants; Determination of Endangered Status for the Sheepnose and Spectaclecase Mussels Throughout Their Rang", 2012)


Many mussel species are known as long-lived animals, which involves living for decades, centuries or longer. The lifespan of a Plethobasus cyphyus mussel is determined by whether or not the glochidia successfully find and attach to freshwater fish necessary to complete development. Unsuccessful glochidia do not survive to become adults while successful individuals grow to the adult stage where they may live for as long as 30 years. ("Endangered and Threatened Wildlife and Plants; Determination of Endangered Status for the Sheepnose and Spectaclecase Mussels Throughout Their Rang", 2012)

  • Range lifespan
    Status: wild
    30 (high) years


One interesting feature of sheepnose is that during the larval stage of mussel development, the glochidia behave as parasites to freshwater fish. The mother mussel imitates a food source and when the fish comes near to investigate, the glochidia become attached to the fish. After development, the sheepnose move around using their foot and hatchet shaped mussel. Through contractions, they are able to travel along the river/stream bottom by burrowing their foot into the substrate. Because of their primarily sedentary behavior, they remain in one place for an extended period of time. (Rose, 1820)

Home Range

Freshwater mussels tend to live within the bottom sediments of lakes and streams. Golchidia travel with their fish host until they develop, where they then drop off to enter their new habitat among other freshwater mussels, known as a multi-species community or mussel beds. (Rose, 1820)

Communication and Perception

Freshwater mussels have many sensory organs that are located on the middle lobe of the mantle edge. They are generally responsive to chemical and tactile stimulation. If they are introduced to a fluid or are physically touched, glochidia will respond to the recognized fish host by clamping shut. Adult freshwater mussels have fluid filled chambers that include a solid granule or pellet, known as statocysts. Statocysts are located in the mussels foot and they are typically paired. The mussels use their paired statocysts as a way to orient themselves to new habitats. ("Sheepnose (a freshwater mussel)", 2013)

  • Other Communication Modes
  • mimicry

Food Habits

Sheepnose mussels use their incurrent and excurrent siphon as primary food filter sources. They draw water into their body through their incurrent siphon, where the food and oxygen is removed by their gills. Food that they filter from the water usually involves algae, bacteria, protozoans, and other organic matter. Once the water is filtered and the food is taken in, the mussel discards the filtered water through their excurrent siphon. (Rose, 1820)


Muskrats (Ondatra zibehicus) are the largest predator of freshwater mussels. They drag the mussel out of their water habitat, onto the shore where they try to break the shell with their teeth or they wait for the mussel shell to open by leaving the mussel out of the water, causing it to die. Other predators of freshwater mussels are freshwater fishes, crayfish, and humans. Crayfish (Cambarus bartonii) typically prey on juvenile mussels, preventing the development into adult mussels. Some fish species evolve an immune response to resist golchidia causing them to drop off the fish host and end development.

Freshwater mussels avoid predation by burrowing into the sediment of rivers and streams. For respiration purposes, mussels leave their posterior region out of the sediment, leaving them more vulnerable to predation as well as environmental factors. ("Endangered and Threatened Wildlife and Plants; Determination of Endangered Status for the Sheepnose and Spectaclecase Mussels Throughout Their Rang", 2012)

Ecosystem Roles

The sheepnose mussel has an important role within the ecosystem in that it supports species diversity. Their sedentary lifestyle makes them an easy prey target for predators, making them an important food source within the food chain. The diet of Plethobasus cyphyus maintains various freshwater microorganism populations including plankton, algae and detritus. As a result of their filter feeding diet, they prevent future problematic explosions of these populations. Glochidia are also parasitic on several species of fish, including sauger (Sander canadensis), creek chub (Semotilus atromaculatus), and spottail shiner (Semotilus atromaculatus). ("Endangered and Threatened Wildlife and Plants; Determination of Endangered Status for the Sheepnose and Spectaclecase Mussels Throughout Their Rang", 2012)

Species Used as Host

Economic Importance for Humans: Positive

The freshwater mussel Plethobasus cyphyus is beneficial in that it has ecocentric value as a native species and provides a means of decoration as jewelry. Humans have used freshwater mussels for thousands of years for food, jewelry, tools, utensils, and pottery temper. In the 1890's the pearl button industry boomed and shell buttons on clothes became a fashion trend. Muscatine, Iowa was the center of industry for hundreds of river towns along the Mississippi and thousands of tons of mussels were harvested. The demand was so high that by 1900, the Illinois and Wabash rivers were depleted of mussels. By the 1940s, cheaper plastic buttons replaced shell buttons.

Mussel populations are indicators of water quality because they filter particles from the water. Mussel health is indicative of pollution levels, and mussel deaths reflect toxic water quality levels. (Winhold, 2004)

  • Positive Impacts
  • food
  • body parts are source of valuable material
  • research and education

Economic Importance for Humans: Negative

There are no known adverse effects of Plethobasus cyphyus on humans.

Conservation Status

Plethobasus cyphyus is listed as an endangered species by the IUCN. Habitat degradation is the greatest contributing factor towards the listing. Examples of habitat loss includes the damming of rivers, siltation and the impacts of invasive zebra mussels. Currently, the mussels are fully protected under the Endangered Species Act. Attempts have been made to prevent and stop the spread of the invasive zebra mussels that suffocate native freshwater mussels, like the sheepnose. Funded research is being carried out by many federal and state agencies in areas that have sheepnose mussel populations. These funded research projects are being conducted in order to gain further insight on the sheepnose. ("Sheepnose (a freshwater mussel)", 2013)


Katie Dose (author), Minnesota State University, Mankato, Robert Sorensen (editor), Minnesota State University, Mankato, Angela Miner (editor), Animal Diversity Web Staff.



living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


uses smells or other chemicals to communicate


an animal that mainly eats decomposed plants and/or animals


particles of organic material from dead and decomposing organisms. Detritus is the result of the activity of decomposers (organisms that decompose organic material).


animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

female parental care

parental care is carried out by females


union of egg and spermatozoan


a method of feeding where small food particles are filtered from the surrounding water by various mechanisms. Used mainly by aquatic invertebrates, especially plankton, but also by baleen whales.


A substance that provides both nutrients and energy to a living thing.


Referring to a burrowing life-style or behavior, specialized for digging or burrowing.


mainly lives in water that is not salty.


An animal that eats mainly plants or parts of plants.


having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

internal fertilization

fertilization takes place within the female's body


offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).


A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.


imitates a communication signal or appearance of another kind of organism

native range

the area in which the animal is naturally found, the region in which it is endemic.


an organism that obtains nutrients from other organisms in a harmful way that doesn't cause immediate death


photosynthetic or plant constituent of plankton; mainly unicellular algae. (Compare to zooplankton.)


the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

seasonal breeding

breeding is confined to a particular season


remains in the same area


reproduction that includes combining the genetic contribution of two individuals, a male and a female


uses touch to communicate


that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).


movements of a hard surface that are produced by animals as signals to others


uses sight to communicate


Department of the Interior, Fish & Wildlife Services. Endangered and Threatened Wildlife and Plants; Determination of Endangered Status for the Sheepnose and Spectaclecase Mussels Throughout Their Rang. Docket No. FWS–R3–ES–2010–0050; 4500030113]. United States Fish & Wildlife Service: Daniel M. Ashe. 2012. Accessed March 19, 2013 at

2008. "Federally Listed Endangered, Threatened and Candidate Species for Minnesota" (On-line). Minnesota Department of Agriculture. Accessed April 01, 2013 at

2013. "Sheepnose (a freshwater mussel)" (On-line). United States Fish & Wildlife Service. Accessed March 19, 2013 at

Rose, D. 1820. "Plethobasus cyphyus" (On-line). Accessed March 24, 2013 at

Sietman, B. 2003. "Field Guide to the Freshwater Mussels of Minnesota" (On-line). Minnesota Department of Natural Resources. Accessed April 01, 2013 at

Winhold, L. 2004. "Unionidae" (On-line). Animal Diversity Web. Accessed April 01, 2013 at

Wolf, K., M. Hove, B. Sietman, S. Boyer, D. Hornbach. 2012. Additional Minnows and Topminnow Identified as Suitable Hosts for the Sheepnose, Phethobasus cyphyus (Rafinesque, 1820). Newsletter of the Freshwater Mollusk Conservation Society, 14/3: 7. Accessed April 01, 2013 at