Pleurobema coccineum

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.

• Biogeographic Regions
• nearctic nearctic
  • native native

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.

• Habitat Regions
• temperate temperate
• freshwater freshwater

• Aquatic Biomes
• benthic benthic
• rivers and streams

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.

• Other Physical Features
• ectothermic ectothermic
• heterothermic heterothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• sexes alike

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.

• Development - Life Cycle
• metamorphosis metamorphosis

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 .

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.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• fertilization fertilization
  • internal internal
• oviparous oviparous

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.

• 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.

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.

• Key Behaviors
• parasite parasite
• motile motile
• sedentary sedentary

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.

• Perception Channels
• visual visual
• tactile tactile
• vibrations vibrations

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.

• Primary Diet
• planktivore planktivore
• detritivore detritivore

• Plant Foods
• algae
• phytoplankton phytoplankton

• Other Foods
• detritus detritus
• microbes

• Foraging Behavior
• filter-feeding filter-feeding

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.

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.

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.

• Ecosystem Impact
• parasite parasite

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.

• 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.

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 .

Additional Links

Encyclopedia of Life

BioKIDS Critter Catalog

Contributors

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

References

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/ .

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 .