Obovaria subrotunda

Geographic Range

The round hickorynut is found in the Ohio, Tennessee and Cumberland drainages. Its northern range includes the Lake Erie drainage in Michigan, east to western New York and western Pennsylvania. In the south it is found in the Tombigbee drainage, including northern Alabama. Its western range is from eastern Arkansas to southeastern Illinois.

In Michigan the records for this species are mainly in the Belle, Clinton, Huron and Raisin drainages as well as Lakes St. Clair and Erie. One specimen is recorded from the Grand River (probably pre-1920s), however the data are scant and this species has not been found in subsequent intense surveys on the Grand and its drainages.

• Biogeographic Regions
• nearctic nearctic
  • native native

Habitat

The round hickorynut is found in medium headwater to large streams in areas with swift to slow water. Substrates it inhabits include sand and gravel.

• Habitat Regions
• freshwater freshwater

• Aquatic Biomes
• rivers and streams

Physical Description

The round hickorynut is up to 7.6 cm (3 inches) long , and is round in shape. The shell is usually fairly thick and inflated. The anterior end is rounded. The posterior end is rounded in males and somewhat truncated in females. The dorsal margin is straight to slightly curved and the ventral margin is usually curved.

Umbos are low, being raised only slightly above the hinge line. The beak sculpture has coarse, concentric double-looped ridges pointed posteriorly. The sculpture is usually only seen in younger individuals.

The periostracum (outer shell layer) is smooth, yellow to yellow-brown. Older specimens tend to be more brown or blackish. The round hickorynut lacks rays, but is generally lighter at the posterior end, almost with a two-toned coloration.

On the inner shell, the left valve has two pseudocardinal teeth , which are small, serrated and divergent. The two lateral teeth are slightly curved, thin, and short. The right valve has one small, serrated and divergent pseudocardinal tooth. The lateral tooth is similar to the right valve.

The beak cavity is shallow to moderately deep. Although the nacre is white, occasionally it has a salmon tint and is iridescent at the posterior end.

In Michigan, this species can be confused with the hickorynut , and the round pigtoe . The hickorynut is not as round and lacks the two-toned colors. The round pigtoe is more compressed, more angular at the posterior end and is generally uniformly brown.

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

• Sexual Dimorphism
• sexes shaped differently

Development

Fertilized eggs are brooded in the marsupia (water tubes) up to 11 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 a 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.

• Development - Life Cycle
• metamorphosis metamorphosis

Reproduction

Age to sexual maturity for this species is unknown. Unionids are gonochoristic (sexes are separate) and viviparous. The glochidia, which are the larval stage of the mussels, are released live from the female after they are fully developed.

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 internally fertilized in the suprabranchial chambers, then pass into water tubes of the gills, where they develop into glochidia.

Obovaria subrotunda is a long-term brooder. In the Huron River in Michigan, it was gravid from early August to mid-June. It probably spawns from mid-June to August in Michigan.

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

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
• pre-fertilization
  • provisioning
• pre-hatching/birth
  • provisioning
    • female

Lifespan/Longevity

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.

Behavior

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.

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

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. Mantle flaps in the lampsilines are modified to attract potential fish hosts. How the round hickorynut recognizes and/or attracts its fish host is unknown.

Glochidia respond to touch, light and some chemical cues. In general, when touched or a fluid is introduced, they will respond by clamping shut.

• Communication Channels
• chemical chemical

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

Food Habits

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.

• Primary Diet
• planktivore planktivore
• detritivore detritivore

• Plant Foods
• algae
• phytoplankton phytoplankton

• Other Foods
• detritus detritus
• microbes

• Foraging Behavior
• filter-feeding filter-feeding

Predation

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.

Ecosystem Roles

While freshwater mussels require a host fish for metamorphosis, the host species for Obovaria subrotunda has not yet been determined.

• Ecosystem Impact
• parasite parasite

Economic Importance for Humans: Positive

Mussels are ecological indicators. Their presence in a water body usually indicates good water quality.

Economic Importance for Humans: Negative

There are no significant negative impacts of mussels on humans.

Conservation Status

Obovaria subrotunda is considered Endangered in Michigan. Under the Species At Risk Act, this species is federally Endangered in Canada. The IUCN Red list considers O. subrotunda Lower Risk, near threatened.

Additional Links

Encyclopedia of Life

Contributors

Renee Sherman Mulcrone (author).

References

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.

Carman, S. 2001. Special animal abstract for Obovaria subrotunda (round hickorynut) . Lansing, Michigan: Michigan Natural Features Inventory. Accessed October 10, 2005 at http://web4.msue.msu.edu/mnfi/abstracts/aquatics/Obovaria_subrotunda.pdf .

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 .

Environment Canada, 2003. "Species At Risk" (On-line). Accessed September 21, 2005 at http://www.speciesatrisk.gc.ca/default_e.cfm .

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.