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Pleurobema coccineumround pigtoe

By Nick Rydell

Geographic Range

Pleurobema coccineum, the round pigtoe, inhabits the eastern parts of North America. This area ranges from Ontario, south to Alabama, and from South Dakota at its west end to New York at its easternmost end. All the states and provinces in its distribution include Alabama, Arkansas, Illinois, Indiana, Iowa, Kansas, Kentucky, Michigan, Minnesota, Missouri, Nebraska, New York, Ohio, Oklahoma, Pennsylvania, South Dakota, Tennessee, West Virginia, Wisconsin, and Ontario. It is also present in the Mississippi and Ohio River drainages, and Lake Michigan, Lake Huron, Lake St. Clair, and Lake Erie drainages. ("COSEWIC assessment and status report on the round pigtoe Pleurobema sintoxia in Canada", 2004; Michigan State University Board of Trustees., 2007; Minnesota Department of Natural Resources, Division of Ecological Resources., 2008)

Habitat

The round pigtoe is primarily found in larger rivers, but can be found in smaller rivers. It prefers habitats that include mostly coarse sand and gravel, with mud substrates, and a fast current. ("COSEWIC assessment and status report on the round pigtoe Pleurobema sintoxia in Canada", 2004; Michigan State University Board of Trustees., 2007; Minnesota Department of Natural Resources, Division of Ecological Resources., 2008)

  • Aquatic Biomes
  • benthic
  • rivers and streams
  • Range depth
    0.9 to 6.1 m
    2.95 to 20.01 ft

Physical Description

The round pigtoe's shell appearance is highly dependent on the habitat in which it is found. The shells of those found in smaller rivers tend to be rounder, more compressed and have low beaks. The opposite is true for those in larger rivers, having more triangular and inflated shells. Valves range from moderately thick to heavy, with the beak sculpture consisting of 2 or 3 ridges. The outside of the shell is smooth, with a dark brown color that may at times contain some green rays in adults and more tan in juveniles. The nacre on the inside of the shell is typically white, but at times can vary from pink to salmon colored. The shells of males and females are morphologically similar. ("COSEWIC assessment and status report on the round pigtoe Pleurobema sintoxia in Canada", 2004; Michigan State University Board of Trustees., 2007; Minnesota Department of Natural Resources, Division of Ecological Resources., 2008)

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

Development

Once females receive sperm from males, the embryos develop. Females then retain the glochidia in their gills until they release them near potential host fish. If the glochidia do not attach to a host, they generally die within 1 to 2 days. The glochidia attach to the gills of the fish and begin to metamorphose into juveniles if the fish is a suitable host species. Most will metamorphose into juvenile mussels in 6 to 8 weeks. If the glochidia fall off their fish host before they can mature into juveniles, they do not survive. When larval development is complete, the juvenile mussels drop off host fish, onto sand or gravel that is suitable for growth,and will grow into mature adult mussels. (Marzec, 2004)

Reproduction

Male mussels release sperm into the water current, which are then drawn in by females through their incurrent siphon. Spawning is triggered by warmer water temperatures and usually occurs in late spring to early summer for Pleurobema coccineum. ("COSEWIC assessment and status report on the round pigtoe Pleurobema sintoxia in Canada", 2004; Marzec, 2004; Michigan State University Board of Trustees., 2007; Minnesota Department of Natural Resources, Division of Ecological Resources., 2008)

The fertilized eggs are brooded in the female's gills from mid-May through July, where they develop into larvae called glochidia. Species in this genus are generally considered short term brooders, as they spawn in spring and release their glochidia during the summer. Typically, the single female will release all of its conglutinates on a single day, around the middle of June. The glochidia will then attach to the gills of a host fish to complete development. ("COSEWIC assessment and status report on the round pigtoe Pleurobema sintoxia in Canada", 2004; Culp, et al., 2009; Marzec, 2004; Michigan State University Board of Trustees., 2007; Minnesota Department of Natural Resources, Division of Ecological Resources., 2008)

  • Breeding interval
    Pleurobema coccineum reproduces once yearly.
  • Breeding season
    Reproduction takes place in spring and summer.

The round pigtoe is tachytictic, with females brooding their young short-term, from mid-May through July, before they are released as glochidia. After the release of glochidia there is no longer any parental investment. (Minnesota Department of Natural Resources, Division of Ecological Resources., 2008)

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

Lifespan/Longevity

Mussels in general are long-lived animals. Members of many species may live for several decades. Maximum life-span for some unionids is over 50 years, with the round pigtoe likely living to over 20 years of age. (Michigan State University Board of Trustees., 2007; Minnesota Department of Natural Resources, Division of Ecological Resources., 2008)

  • Average lifespan
    Status: wild
    20 years

Behavior

Round pigtoe mussels can move around with the use of their foot, which is a muscle that can be extended out of their shell. The foot is then partially buried and contracted to pull itself along the bottom of the river bed. However, they are primarily sedentary, spending most of their lives buried in the sediment of their aquatic habitats. They are often found in multi-species communities called mussel beds. (Minnesota Department of Natural Resources, Division of Ecological Resources., 2008)

Communication and Perception

Little is known about the communication and perception of the round pigtoe. Unionids in general seem to be able to detect vibrations and light. Glochidia in particular seem to be sensitive to touch, which they use to attach to hosts. (Winhold, 2004)

Food Habits

The round pigtoe eats by filtering water through its incurrent siphon, which then passes through its gills. Oxygen, bacteria, protozoa, algae and other organic matter are filtered out and consumed. The waste and water is then excreted through its excurrent siphon. (Michigan State University Board of Trustees., 2007; Minnesota Department of Natural Resources, Division of Ecological Resources., 2008)

Predation

The muskrat is the primary predator of Pleurobema coccineum. Other common predators of adult mussels include mink, river otter, raccoon, striped skunk, hellbenders, turtles, fish, some species of birds, and humans. (Tyrrell and Hornbach, 1998; Winhold, 2004)

Along with their shells, mussels will bury themselves into the lake or stream bed to avoid predators. However, adult unionids do not have true siphons and must leave part of their posterior end exposed from the sediment, to allow for respiration. This exposure can make the mussels vulnerable to predation. (Winhold, 2004)

Ecosystem Roles

The glochidia of the round pigtoe are parasites of several types of fish during their reproduction process including bluegill, spotfin shiner, bluntnose minnow and northern redbelly dace. Mature round pigtoe are filter feeders of bacteria, protozoans, algae, detritus and other organic matter. By doing so, they aid in the processing of detritus and microorganisms. Mussels are also a food sources for a great variety of organisms, and their shells provide a substrate for invertebrate species to attach themselves. (Marzec, 2004; Minnesota Department of Natural Resources, Division of Ecological Resources., 2008; Tyrrell and Hornbach, 1998; Winhold, 2004)

Species Used as Host

Economic Importance for Humans: Positive

As filter feeders, the round pigtoe improves water quality and can be used as a bioindicator for the health of its aquatic habitats. ("COSEWIC assessment and status report on the round pigtoe Pleurobema sintoxia in Canada", 2004; Marzec, 2004; Minnesota Department of Natural Resources, Division of Ecological Resources., 2008; Tyrrell and Hornbach, 1998)

  • Positive Impacts
  • research and education

Economic Importance for Humans: Negative

There are no known adverse effects of Pleurobema coccineum.

Conservation Status

The round pigtoe is listed as common (G4) in North America by the IUCN. However, it is listed as endangered in Iowa and Pennsylvania, threatened in Minnesota, special concern in Michigan and as a species of special interest in Ohio. Pleurobema coccineum has disappeared from over half of its range in the Great Lakes, likely due to the invasive zebra mussel, as well as habitat destruction and pollution. In these states, conservation effects are underway to prevent the further decline of this species. ("COSEWIC assessment and status report on the round pigtoe Pleurobema sintoxia in Canada", 2004)

Other Comments

With the shells of the round pigtoe being so variable depending on habitat, the species was named several times. The round pigtoe is synonymous with Pleurobema coccineum and Pleurobema sintoxia. (Cummings, K. & Cordeiro, J, 2012)

Contributors

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

Glossary

Nearctic

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

benthic

Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.

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.

detritivore

an animal that mainly eats decomposed plants and/or animals

detritus

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

ectothermic

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

fertilization

union of egg and spermatozoan

filter-feeding

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.

freshwater

mainly lives in water that is not salty.

heterothermic

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

iteroparous

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).

metamorphosis

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.

motile

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.

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

parasite

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

phytoplankton

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

planktivore

an animal that mainly eats plankton

seasonal breeding

breeding is confined to a particular season

sedentary

remains in the same area

sexual

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

tactile

uses touch to communicate

temperate

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).

vibrations

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

visual

uses sight to communicate

References

COSEWIC. COSEWIC assessment and status report on the round pigtoe Pleurobema sintoxia in Canada. CW69-14/398-2004E-PDF. Ottawa: Committee on the Status of Endangered Wildlife in Canada. 2004. Accessed March 26, 2013 at http://people.cst.cmich.edu/zanat1d/COSEWIC_Round_Pigtoe.pdf.

Culp, J., A. Shepard, M. McGregor. 2009. Fish Hosts and Conglutinates of the Pyramid Pigtoe (Pleurobema rubrum). Southeastern Naturalist, Vol. 8, No. 1: 19-22. Accessed April 28, 2013 at http://www.jstor.org/stable/25599292.

Cummings, K. & Cordeiro, J, 2012. "Pleurobema sintoxia" (On-line). The ICUN Redlist of Threatened Species. Accessed March 16, 2013 at http://www.iucnredlist.org/details/173061/0.

Marzec, M. 2004. "Host fish and life cycle of the round pigtoe mussel." (On-line). Accessed March 26, 2013 at http://realscience.breckschool.org/upper/research/2003-04photos/Research2004/MeggieMarzec.pdf.

Michigan State University Board of Trustees., 2007. "Round pigtoe" (On-line pdf). Accessed March 16, 2013 at http://www.dnr.state.mn.us/rsg/profile.html?action=elementDetail&selectedElement=IMBIV35070.

Minnesota Department of Natural Resources, Division of Ecological Resources., 2008. "Rare Species Guide: An online encyclopedia of Minnesota's rare native plants and animals" (On-line). Accessed March 20, 2013 at www.dnr.state.mn.us/rsg.

Tyrrell, M., D. Hornbach. 1998. Selective Predation by Muskrats on Freshwater Mussels in 2 Minnesota Rivers. Journal of the North American Benthological Society, Vol. 17, No. 3: 301-310. Accessed April 28, 2013 at http://www.jstor.org/stable/1468333.

Winhold, L. 2004. "Unionidae" (On-line). Animal Diversity Web. Accessed March 27, 2013 at http://animaldiversity.ummz.umich.edu/accounts/Unionidae/.

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