Round Hickorynut (Obovaria subrotunda) | U.S. Fish & Wildlife Service (2024)

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Overview

The round hickorynut mussel is a wide-ranging species, historically known from 12 states, though now occurs in nine, as well as the Canadaian Province of Ontario. It is currently found in five major basins: Great Lakes, Ohio (where it is most prevalent), Cumberland, Tennessee, and Lower Mississippi (where it is most rare). It’s a small- to medium-sized mussel up to three inches long, living up to 15 years. It is found in small streams to large rivers, and prefers a mixture of sand, gravel, and cobble stream bottoms.

The round hickorynut and other aquatic species have endured negative influences commonly found in the central and eastern U.S., including: habitat fragmentation from dams and other barriers; habitat loss; degraded water quality from chemical contamination and erosion from poorly managed development, agriculture, mining, and timber operations; direct mortality from dredging and harvest; and the proliferation of invasive species invasive species
An invasive species is any plant or animal that has spread or been introduced into a new area where they are, or could, cause harm to the environment, economy, or human, animal, or plant health. Their unwelcome presence can destroy ecosystems and cost millions of dollars.

Learn more about invasive species

, such as the zebra mussel.

It's currently found in the Great Lakes, Ohio, Cumberland, Tennessee, and Lower Mississippi River major river basins, within the states of Alabama, Indiana, Kentucky, Michigan, Mississippi, Ohio, Pennsylvania, Tennessee, and West Virginia. It is considered extirpated from Georgia, Illinois, and New York.

We, the U.S. Fish and Wildlife Service (Service), were petitioned by the Center for Biological Diversity, Alabama Rivers Alliance, Clinch Coalition, Dogwood Alliance, Gulf Restoration Network, Tennessee Forests Council, West Virginia Highlands Conservancy, Tierra Curry, and Noah Greenwald to list the round hickorynut as an endangered or threatened species under the ESA. This request was part of a 2010 petition to list 404 aquatic, riparian riparian
Definition of riparian habitat or riparian areas.

Learn more about riparian

, and wetland species in the southeastern United States.

Habitat

The round hickorynut exhibits a preference for sand and gravel in riffle, run, and pool habitats in streams and rivers, but also may be found in sandy mud. They can be found in shallow habitats with gentle flows at less than one foot with abundant American water-willow, but in larger rivers are commonly found up to depths of 6.5 feet.

Food

The round hickorynut and other adult freshwater mussels within the genus Obovaria are suspension-feeders, consuming food filtered from the water. Their diet consists of a mixture of algae, bacteria, detritus, and microscopic animals.

Physical characteristics

Mollusks are mostly aquatic and are named from the Latin molluscus, meaning “soft.” Their soft bodies are often enclosed in a hard shell made of calcium carbonate (CaCO3), which functions as an exoskeleton. This shell is secreted by a thin sheet of tissue called the mantle, which encloses the internal organs.

Round hickorynut adults are greenish-olive to dark or chestnut brown, sometimes blackish in older individuals, and may have a yellowish band. The shell is thick, solid, and up to three inches long, but usually is less than 2.4 inches. A distinctive characteristic is that the shell is round, nearly circular. The foot can be pale tan to pale pinkish orange.

Lifecycle

Males release sperm into the water column, which is taken in by the female to fertilize eggs that develop into larvae, remaining in the gill chamber until they are ready for release. Larvae are released from the female in packets, and once in the water these packets are targeted by feeding darters, bursting when bitten by the fish, releasing the larval mussels. These larvae snap shut in contact with fish, attaching to its gills, head, or fins (Vaughn and Taylor 1999, p. 913). Once on the fish, the larvae are engulfed by the host’s tissue that forms a cyst. The cyst protects the glochidia and aids in their maturation. The larvae draw nutrients from the fish and develop into juvenile mussels, days to weeks after initial attachment. The juvenile mussels then drop off the host fish and settle to the bottom.

  • Gatenby, C.M., R.J. Neves, and B.C. Parker. 1996. Influence of sediment and algal food on cultured juvenile freshwater mussels. Journal of the North American Benthological Society 15(4):597–609.
  • Gordon, M.E., and J.B. Layzer. 1989. Mussels (Bivalvia: Unionidea) of the Cumberland River: Review of life histories and ecological relationships. U.S. Fish and Wildlife Service Biological Report 89(15). 99 pp.
  • Parmalee, P.W., and A.E. Bogan. 1998. The freshwater mussels of Tennessee. The University of Tennessee Press, Knoxville, Tennessee. 328 pp.
  • Vaughn, C.C., and C.M. Taylor. 1999. Impoundments and the decline of freshwater mussels: a case study of an extinction gradient. Conservation Biology 13:912920.
  • Watters, G.T., M.A. Hoggarth, and D.H. Stansbery. 2009. The Freshwater Mussels of Ohio. The Ohio State University Press, Columbus, Ohio. 421 pp.
  • Williams, J.D., A.E. Bogan, and J.T. Garner. 2008. Freshwater mussels of Alabama and the Mobile Basin of Georgia, Mississippi, and Tennessee. University of Alabama Press, Tuscaloosa.

Scientific Name

Obovaria subrotunda

Common Name

round hickorynut

FWS Category

Clams

Kingdom

Animalia

Location in Taxonomic Tree

Genus

Obovaria

Species

Obovaria subrotunda

Identification Numbers

TSN:

Geography

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Regulatory Status Change

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Round Hickorynut (Obovaria subrotunda) | U.S. Fish & Wildlife Service (2024)
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