Obovaria olivaria
hickorynut

The hickorynut is in the Mississippi drainage, its western range from eastern Minnesota to eastern Arkansas, south to northern Alabama, and north up to western Pennsylvania. In the Great Lakes region it is found in the St. Lawrence drainage from Lake Ontario up to Quebec.

In Michigan, O. olivaria has been recorded in the Detroit River, Grand River, Kalamazoo River, Saginaw River, and Lakes St. Clair and Erie. One specimen was collected from the Menominee River in the upper peninsula, prior to 1936. (Badra, 2004; Burch, 1975; Cummings and Mayer, 1992)

The hickorynut is found in large rivers, and rarely in medium or small sized streams with a good current. Substrates it inhabits include sand, sand/gravel mixes, and gravel. (Cummings and Mayer, 1992; Watters, 1995)

The hickorynut is up to 10.2 cm (4 inches) long , and is oblong or oval in shape. The shell is usually fairly thick and inflated. The anterior end is broadly rounded the posterior end sharply rounded. The dorsal margin is straight and the ventral margin is evenly rounded to both the anterior and posterior ends.

Umbos are low, being raised only slightly above the hinge line. Umbos are also curved inward and positioned slightly toward the anterior end. The beak sculpture is fine, with four or five double-looped ridges.

The periostracum (outer shell layer) is smooth, olive-brown to yellow brown. Younger individuals may have faint rays. Older individuals tend to be darker brown.

On the inner shell, the left valve has two pseudocardinal teeth, which are erect, large, triangular and divergent. There is usually an additional small, thin, low tooth between the beak point and anterior psedocardinal tooth. The two lateral teeth are straight to slightly curved, striated, and moderately long. The right valve has one large, erect, triangular and serrated pseudocardinal tooth. Anterior to this tooth is a smaller (lamellar) tooth. The one lateral tooth straight to slightly curved, striated, and moderately long.

The beak cavity is shallow to moderately deep. The nacre is blue-white and iridescent at the posterior end.

In Michigan, this species can be confused with the round hickorynut, and the round pigtoe. The round hickorynut is more rounded and lacks rays. The round pigtoe has a more bluntly pointed anterior and is usually darker in color. (Cummings and Mayer, 1992; Oesch, 1984; 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. Often they are found buried under the substrate. (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 hickorynut attracts and/or recognizes a 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)

Fish hosts are determined by looking at both lab metamorphosis and natural infestations. Looking at both is necessary, as lab transformations from glochidia to juvenile may occur, but the mussel may not actually infect a particular species in a natural situation. Natural infestations may also be found, but glochidia will attach to almost any fish, including those that are not suitable hosts. Lab transformations involve isolating one particular fish species and introducing glochidia either into the fish tank or directly inoculating the fish gills with glochidia. Tanks are monitored and if juveniles are later found the fish species is considered a suitable host.

In lab trials, Obovaria olivaria glochidia metamorphosed on the lake sturgeon. Natural infestations have not yet been observed. (Brady et al., 2004)

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 olivaria is listed as Special Concern in Michigan and Minnesota. (Hove, 2004)