Obovaria subrotunda
round hickorynut

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. (Burch, 1975; Carman, 2001; Cummings and Mayer, 1992)

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. (Cummings and Mayer, 1992; Watters, 1995)

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. (Cummings and Mayer, 1992; Watters, 1995)

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. (Arey, 1921; Lefevre and Curtis, 1910)

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)

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. (Arey, 1921; Brusca and Brusca, 2003; Watters, 1995)

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)

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)

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

There are no significant negative impacts of mussels on humans.

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

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. (Environment Canada, 2003)