Arcidens confragosus

Geographic Range

The historical range of Arcidens confragosus, the rock pocketbook mussel, includes the Mississippi River and its tributaries, as well as gulf coastal rivers from the Colorado River in Texas east to the Mobile River System in Alabama. It is currently distributed in the Mississippi River Basin from Minnesota south to Louisiana and from Ohio west to eastern Kansas. In the south, the rock pocketbook mussel's distribution is from eastern Texas east to western Alabama along the Gulf of Mexico. (Martinez, et al., 2002; "Aridens confragosus", 2013; Roe, 2002)


The rock pocketbook mussel occurs in medium to large rivers in a wide range of substrates including silt, sand, mud, gravel, cobble, and bolder. The mussels depend on a stable substrate for burrowing and require good water quality and quantity for feeding, breathing, and reproducing. These mussels typically inhabit unpolluted waters that are rich in oxygen, calcium, and suspended food particles. The rock pocketbook mussel occurs in waters ranging from approximately 10 centimeters to 1 meter in depth with slow to swift currents. (Buchanan, 1980; Jennings, 2012; "Aridens confragosus", 2013; Murray and Leonard, 1962; Roe, 2002)

  • Aquatic Biomes
  • rivers and streams
  • Range depth
    .10 to 1 m
    0.33 to 3.28 ft

Physical Description

A molluscan body has six fundamental parts. These include the shell, head, soft body, mantle, the gill filaments, and the foot. The shell provides a protective outer skeleton. Inside the shell is the head, which is poorly defined. The soft body is a thickened central mass containing the essential organs. It is attached to the top of the paired valves. Exterior to the gills is a covering of tissue termed the mantle. The mantle lines the inner surface of the shell and body. The gill filaments also line the shell. The forward, bottom part of the body forms the foot.

The rock pocketbook mussel is a relatively large and inflated mussel. The shell is elliptical and can range from thin to moderately thick. The periostracum can be dark green, brown, or black. The mussel's anterior end is rounded and its posterior end is squared or bluntly pointed. The ventral margin is straight or slightly rounded. The umbos is full and elevated above the hinge line and located near the middle of the shell. The beak sculpture consists of two rows of large knobs or heavy, double-looped ridges. These become irregular folds or ridges as the individual matures. The psuedocardial teeth are present, the lateral teeth are poorly developed, and the inner shell is white. The rock pocketbook mussel can measure anywhere from approximately 7 centimeters to 18 centimeters in length. The rock pocketbook resembles the three-ridge mussel (Amblema plicata) and the washboard (Megalonaias nervosa). It is distinguishable from these other species by its distinct coarse beak sculpture, thin shell, and reduced lateral teeth. (Jennings, 2012; Martinez, et al., 2002; "Aridens confragosus", 2013; Roe, 2002)

  • Sexual Dimorphism
  • sexes alike
  • Range length
    7 to 18 cm
    2.76 to 7.09 in


The mussel life cycle begins when a male mussel releases sperm into the aquatic habitat. Females draw the sperm through their incurrent siphon. Fertilized eggs are brooded in the marsupia where they develop into larvae called glochidia. The glochidia are then released into the water. The glochidia attach to the gill filaments or body surface of a host fish where they live as temporary parasites. The glochidia initially attach by clasping fins, scales, or gill filaments of the fish with their valves. Within hours of attachment, the glochidia are encapsulated by the host's epithelial tissue. Host species for Arcidens confragosus include American eel (Anguilla rostrata), gizzard shad (Dorosoma cepedianum), rock bass (Ambloplites rupestris), white crappie (Pomoxis annularis) and freshwater drum (Aplodinotus grunniens). Within a few weeks, the larval glochidia metamorphose into juvenile mussels and drop from the host fish. Juveniles are found in the substrate where they develop into adults. ("Reproductive Cycle in Mussels", 2003; Arey, 1921; Jennings, 2012; Jirka and Neves, 1992; Lefevre and Winterton, 1910; "Aridens confragosus", 2013)


Mussels are polygyndandrous. Gametogenesis is generally thought to be brought on by changes in temperature. Males release sperms into the surrounding water. The sperm enter females in the area through their incurrent siphon. (Jennings, 2012)

The primary stimulus for reproduction in mussels is a change in temperature. When the ambient temperature rises or falls within a certain limit, gamete production and fertilization begin. Other environmental cues that may affect reproduction include but are not limited to density-dependent factors, diurnal rhythms, and parasites.

Sperm are released into the water by males. These sperm are carried by currents and taken in by the inhalant siphon of females in the area. When the females take sperm in, eggs are released into chambers in the gills. The sperm are carried to the unfertilized eggs that are retained in the gill marsupia. The eggs are subsequently fertilized and retained in the gills. The rock pocketbook mussel is a bradydtictic species, meaning that it retains its larvae over-winter. Gravid females release the glochidia through the excurrent siphon. Females can release anywhere from hundreds to hundreds of thousands of glochidia. The glochidia use fish as hosts. ("Status and Life History of the Three Assessed Mussels", 2007; Jennings, 2012; "Aridens confragosus", 2013)

  • Breeding interval
    The rock pocketbook breeds once yearly.
  • Breeding season
    Females brood their young from September through June.
  • Range number of offspring
    Hundreds to Hundreds of thousands

Females provide some parental care, by retaining the fertilized eggs and then later the glochidia in a marsupia. It is not until a host fish is nearby that the females release the glochidia into the water, so that they will have a host to hatch on to, and can safely proceed with development. After the glochidia are released, they are independent of the mother and receive no more care. ("Status and Life History of the Three Assessed Mussels", 2007; Jennings, 2012; "Aridens confragosus", 2013)

  • Parental Investment
  • pre-fertilization
    • provisioning
    • protecting
      • female
  • pre-hatching/birth
    • provisioning
      • female


Early survivorship of mussels is low, but adult survivorship is high. Fresh water mussels are extremely variable in terms of life-span. Life-spans can range from less than 5 years to more than 100 years depending on the species. The age of a mussel can be determined by examining the annual rings on the shell. The lifespan of Arcidens confragosus is unknown, as no demographic data on this species has been recorded. ("Freshwater Mussels of the Upper Mississippi River System", 2006; Jennings, 2012)


Arcidens confragosus is primarily sedentary, but can use a highly muscular and flexible foot for movement such as burrowing or anchoring into the substrate. Many juvenile bivalves crawl a considerable distance before locating a suitable habitat to settle in. Crawling is accomplished by extending the foot, anchoring its tips with mucus or with a muscular attachment, and then contracting the foot muscles to pull the body forward. The capacity for movement is reduced in adults. (Jennings, 2012; "Aridens confragosus", 2013)

Communication and Perception

The head of bivalves is enclosed entirely inside the valves. An assortment of nerve chords coordinates foot and valve movement. Receptors in the foot assist with orientation, positioning, and burrowing. Tissues that are in direct contact with the external environment contain developed sensory organs. In addition, there are a variety of sensory organs on the mantle edge and siphons of the mussel.

Photoreceptor cells detect changes in light intensity that are indicators of diurnal rhythms and animal movements. Other sense organs can detect vibrations and direct touch. Strong stimuli cause the valves to close abruptly. This is a common predator defense in all fresh-water bivalve species.

Glochidia can respond to touch, light, and chemical cues. (Arey, 1921; Jennings, 2012; Watters, 1995)

Food Habits

Arcidens confragosus eats by filtering bacteria, protozoans, algae, and other organic matter out of water. Mussels draw water into their body via an incurrent siphon. In general, mussels use their branched gills as feeding organs to remove small suspended particles from the water. Water is brought to the gills by ciliary action. Water then moves through the tubes located in the gills where gas exchange takes place. Food is then carried in ciliary tracts from the gills to the palps where it is sorted and moved to the mouth and finally into the stomach. Mussels subsequently expel the filtered water through an excurrent siphon. Temperature, food concentrations, food particle size, and body size greatly influence feeding rates.

Parasitic glochidia absorb blood and nutrients from a host via mantle cells that phagocytize host tissue. (Arey, 1921; Jennings, 2012; "Aridens confragosus", 2013; Watters, 1995)


In general, Unionids are preyed upon by carnivorous mammals such as muskrats, raccoons, mink, and otter. Smaller individuals are likely also eaten by many species of carnivorous fish. In response to a threat, Arcidens confragosus can shut its valves, protecting the soft inner body parts. (Cummings and Mayer, 1992; Watters, 1995)

Ecosystem Roles

Mussels have important functions in aquatic environments. They are a middle link in the food-chain and help to maintain water quality. Mussels serve as a food source for many aquatic and terrestrial animals. They can also act as host to parasites such as trematodes. Mussels can act as important conduits of energy fixed by photosynthesis in phytoplankton to higher trophic levels in the ecosystem. Mussels are filter feeders and, consequently, act as water clarifiers and organic nutrient sinks. Mussels may also be significant aquatic decomposers because they filter suspended organic detritus and bacteria.

In addition, mussels are long lived and sensitive to changes in water quality. Consequently, mussels serve as an indicator of ecosystem health. The disappearance of mussels from a river or lake can serve as a signal that other aquatic species may be at risk. (Jennings, 2012; Martinez, et al., 2002)

Species Used as Host
Commensal/Parasitic Species

Economic Importance for Humans: Positive

Mussels are ecological indicators. Their presence in a body of water typically indicates good water quality. Mussels are ideal biomonitors because they are easily collected, sedentary, and long-lived. Mussels also bioconcentrate contaminants. Mussel shells can be analyzed to determine if and when toxic chemicals were accumulated.

Native Americans and others recognize the value of fresh-water mussels as a food source and for the construction of tools, utensils, and pottery. In addition, mussels are used for jewelry, as currency, and for trading. (Jennings, 2012)

  • Positive Impacts
  • food
  • body parts are source of valuable material

Economic Importance for Humans: Negative

There are no known adverse effects of Arcidens confragosus on humans.

Conservation Status

Arcidens confragosus has no special conservation status, and is listed as a species of "least concern" by the IUCN. Possible threats to the rock pocketbook mussel include degradation of mussel habitat via sediment loading, erosion, pollutants from improper agricultural, forestry, and coal mining practices, gravel mining, channel modifications, over-harvest, illegal collecting, population isolation, and introduction of non-native species such as the zebra mussel (Dreissena polymorpha). The rock pocketbook mussel is the only member of the genus Arcidens. As such, the rock pocketbook mussel should be given special consideration for protection. (Jennings, 2012; Roe, 2002)


Meredith Schlenner (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


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.


mainly lives in water that is not salty.


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


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.


having the capacity to move from one place to another.

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.)


an animal that mainly eats plankton


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


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Environmental Protection Agency. Status and Life History of the Three Assessed Mussels. 2007. Accessed March 26, 2013 at

Arey, L. 1921. An experimental study on glochidia and the factors underlying encystment. Journal of Experimental Zoology, 33: 463-499.

Buchanan, A. 1980. Mussels (Naiades) of the Meramec River Basin: Issue 17 of Aquatic Series. Columbia, Missouri: Missouri Department of Conservation.

Cummings, K., C. Mayer. 1992. Field guide to freshwater mussels of the Midwest. Champaign, Illinois: Illinois Natural History Survey Manual 5.

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Martinez, D., D. Spooner, T. Adornato, S. Dudding, C. Vaughn. 2002. Occurrence of the Rock Pocketbook Mussel, Arcidens confragosus (Bivalvia: Unionidae), in the Poteau and Deep Fork Rivers of Oklahoma. Proceedings of the Oklahoma Academy of Science, 84: 79-80. Accessed March 26, 2013 at

Murray, H., A. Leonard. 1962. Handbook of unionid mussels in Kansas: Issue 28 of University of Kansas Museum of Natural History. University of Kansas: Museum of Natural History 1962.

Roe, K. 2002. "Conservation Assessment for the Rock Pocketbook (Arcidens confragosus) Say, 1892" (On-line pdf). Accessed March 23, 2013 at

Watters, G. 1995. A guide to the freshwater mussels of Ohio. Columbus: Ohio Department of Natural Resources.

van der Schalie, H. 1938. The naiad fauna of the Huron River, in southeastern Michigan. Miscellaneous Publications of the Museum of Zoology, University of Michigan, 40: 1-83.