The pimpleback is found throughout the Mississippi drainage through mid-Michigan and into Lake Erie. Its range extends from central New York west to southeastern North Dakota, south to eastern Texas and east through northern Mississippi.
- Habitat Regions
- Aquatic Biomes
- rivers and streams
The pimpleback is up to 10.2 cm (4 inches) long , and rounded. The shell is usually fairly thick, heavy and compressed to moderately inflated. The anterior end is rounded, the posterior end straight and somewhat truncated. The dorsal margin is straight and short and the ventral margin is rounded.
Umbos are raised above the hinge line. The beak sculpture has three to four rounded ridges.
The anterior half of the periostracum (outer shell layer) is smooth and two-thirds of the posterior half are covered with pustules or bumps. The shell is yellow-brown to light brown. Younger specimens have a broken green ray present extending from the umbo to the ventral margin.
On the inner shell, the left valve has two pseudocardinal teeth, which are erect, large, and striated. The two lateral teeth are short to slightly curved and striated. The right valve has one large, grooved, erect triangular pseudocardinal tooth. The area between the pseudocardinal and lateral tooth, the interdentum, is broad.
The beak cavity is deep. The nacre is white and iridescent at the posterior end.
In Michigan, this species can be confused with the purple wartyback and maple leaf. The maple leaf is more quadrate and lacks the green ray. The purple wartyback has a dorsal wing and purple nacre. (Cummings and Mayer, 1992; Oesch, 1984; Watters, 1995)
- Sexual Dimorphism
- sexes alike
- Range length
- 10.2 (high) cm
- 4.02 (high) in
The pimpleback's fertilized eggs are brooded in the marsupia (water tubes) up to 2 months, where they develop into larvae, called glochidia. The glochidia are then released into the water where they must attach to the gill filaments and/or general body surface of the host fish. After attachment, epithelial tissue from the host fish grows over and encapsulates the glochidium, usually within a few hours. The glochidia then metamorphoses into a juvenile mussel within a few days or weeks. After metamorphosis, the juvenile is sloughed off as a free-living organism. Juveniles are found in the substrate where they develop into adults. (Arey, 1921; Lefevre and Curtis, 1910)
- Development - Life Cycle
Age to sexual maturity for this species is unknown. In general, gametogenesis in unionids is initiated by increasing water temperatures. The general life cycle of a unionid, includes open fertilization. Males release sperm into the water, which is taken in by the females through their respiratory current. The eggs are fertilized in the suprabranchial chambers, then pass into water tubes of the gills, where they develop into larvae, called glochidia.
- Key Reproductive Features
- seasonal breeding
- gonochoric/gonochoristic/dioecious (sexes separate)
- Breeding interval
- The pimpleback breeds once, probably in spring.
- Breeding season
- In Michigan, the breeding season is probably in early to mid-May.
- Range gestation period
- 2 (high) months
Females brood fertilized eggs in their marsupial pouch. The fertilized eggs develop into glochidia. There is no parental investment after the female releases the glochidia.
- Parental Investment
The age of mussels can be determined by looking at annual rings on the shell. However, no demographic data on this species has been recorded.
Mussels in general are rather sedentary, although they may move in response to changing water levels and conditions. Although not thoroughly documented, the mussels may vertically migrate to release glochidia and spawn. (Oesch, 1984; Watters, 1995)
Communication and Perception
The middle lobe of the mantle edge has most of a bivalve's sensory organs. Paired statocysts, which are fluid filled chambers with a solid granule or pellet (a statolity) are in the mussel's foot. The statocysts help the mussel with georeception, or orientation.
Mussels are heterothermic, and therefore are sensitive and responsive to temperature.
Unionids in general may have some form of chemical reception to recognize fish hosts. How the pimpleback attracts and/or recognizes its fish hosts is unknown.
Glochidia respond to both touch, light and some chemical cues. In general, when touched or a fluid is introduced, they will respond by clamping shut. (Arey, 1921; Brusca and Brusca, 2003; Watters, 1995)
- Communication Channels
In general, unionids are filter feeders. The mussels use cilia to pump water into the incurrent siphon where food is caught in a mucus lining in the demibranchs. Particles are sorted by the labial palps and then directed to the mouth. Mussels have been cultured on algae, but they may also ingest bacteria, protozoans and other organic particles.
The parasitic glochidial stage absorbs blood and nutrients from hosts after attachment. Mantle cells within the glochidia feed off of the host’s tissue through phagocytocis. (Arey, 1921; Meglitsch and Schram, 1991; Watters, 1995)
- Plant Foods
- Other Foods
- Foraging Behavior
Unionids in general are preyed upon by muskrats, raccoons, minks, otters, and some birds. Juveniles are probably also fed upon by freshwater drum, sheepshead, lake sturgeon, spotted suckers, redhorses, and pumpkinseeds.
Unionid mortality and reproduction is affected by unionicolid mites and monogenic trematodes feeding on gill and mantle tissue. Parasitic chironomid larvae may destroy up to half the mussel gill. (Cummings and Mayer, 1992; Watters, 1995)
- Known Predators
- muskrat, Ondatra zibethicus
- mink, Neovison vison
- raccoon Procyon lotor
- otter, Lontra canadensis
- turtles, Testudines
- hellbenders, Cryptobranchus
- freshwater drum, Aplodinotus grunniens
- sheepshead, Archosargus probatocephalus
- lake sturgeon, Acipenser fulvescens
- shortnosed sturgeon, Acipenser brevirostrum
- spotted suckers, Minytrema melanops
- common red-horse, Moxostoma
- catfish, Siluriformes
- pumpkinseed, Lepomis gibbosus
Fish hosts are determined by looking at both lab metamorphosis and natural infestations. Looking at both is necessary, as lab transformations from glochidia to juvenile may occur, but the mussel may not actually infect a particular species in a natural situation. Natural infestations may also be found, but glochidia will attach to almost any fish, including those that are not suitable hosts. Lab transformations involve isolating one particular fish species and introducing glochidia either into the fish tank or directly inoculating the fish gills with glochidia. Tanks are monitored and if juveniles are later found the fish species is considered a suitable host.
- Ecosystem Impact
Economic Importance for Humans: Positive
Mussels are ecological indicators. Their presence in a water body usually indicates good water quality.
Because of its thick and sturdy shell,is harvested for use by the pearl industry. The shell is sliced and then ground into beads (called "slugs") which are placed in pearl producing oysters. The oysters secrete a nacre over the slugs. The finished pearl has a nucleus of freshwater mussel shell and an outer layer from the saltwater oyster.
Economic Importance for Humans: Negative
There are no significant negative impacts of mussels on humans.
Renee Sherman Mulcrone (author).
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.
- 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.
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
- 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
movements of a hard surface that are produced by animals as signals to others
uses sight to communicate
reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.
Arey, L. 1921. An experimental study on glochidia and the factors underlying encystment. J. Exp. Zool., 33: 463-499.
Brusca, R., G. Brusca. 2003. Invertebrates. Sunderland, Massachusetts: Sinauer Associates, Inc..
Burch, J. 1975. Freshwater unionacean clams (Mollusca: Pelecypoda) of North America. Hamburg, Michigan: Malacological Publications.
Coker, R., F. Shira, H. Clark, A. Howard. 1921. Natural history and propagation of fresh-water mussels. Bull. Bur. Fish., 37: 77-181.
Cummings, K., C. Mayer. 1992. Field guide to freshwater mussels of the Midwest. Champaign, Illinois: Illinois Natural History Survey Manual 5. Accessed August 25, 2005 at http://www.inhs.uiuc.edu/cbd/collections/mollusk/fieldguide.html.
Graf, D. 2002. Historical biogeography and late glacial origin of the freshwater pearly mussel (Bivalvia: Unionidae) faunas of Lake Erie, North America. Occasional Papers of Mollusks, 6: 175-211.
Haag, W., M. Warren. 1997. Host fishes and reproductive biology of six freshwater mussel species from the Mobile Basin, USA. Journal of the North American Benthological Society, 16: 576-585.
Hoeh, W., R. Trdan. 1985. Freshwater mussels (Pelecypoda: Unionidae) of the major tributaries of the St. Clair River, Michigan. Malacological Review, 18: 115-116.
Hove, M. 2004. "Links to each state's listed freshwater mussels, invertebrates, or fauna" (On-line). Accessed September 21, 2005 at http://www.fw.umn.edu/Personnel/staff/Hove/State.TE.mussels.
Howard, A. 1914. Experiments in propagation of fresh-water mussels of the Quadrula group. Rep. of the U.S. Commissioner of Fisheries for 1913, Appendix 4: 1-52.
Lefevre, G., W. Curtis. 1912. Experiments in the artificial propagation of fresh-water mussels. Proc. Internat. Fishery Congress, Washington. Bull. Bur. Fisheries, 28: 617-626.
Lefevre, G., W. Curtis. 1910. Reproduction and parasitism in the Unionidae. J. Expt. Biol., 9: 79-115.
Meglitsch, P., F. Schram. 1991. Invertebrate Zoology, Third Edition. New York, NY: Oxford University Press, Inc.
Oesch, R. 1984. Missouri naiades, a guide to the mussels of Missouri. Jefferson City, Missouri: Missouri Department of Conservation.
Watters, G. 1995. A guide to the freshwater mussels of Ohio. Columbus, Ohio: 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.