Epioblasma torulosa
tubercled blossom

Historically, the northern riffleshell was found in the Ohio river drainage in Illinois, Indiana, Kentucky, Ohio, Pennsylvania, and west Virginia, and into southeastern Michigan and southwestern Ontario. Today its range is restricted to southwestern Ontario, the Green River in Kentucky, Big Darby Creek in Ohio and French Creek, LeBoeuf Creek and Allegheny River in Pennsylvania.

In Michigan this species was historically found in Lake Erie drainages. Specimens were collected from the Clinton River and lower Huron River, however no individuals have been found since the 1930s. Recent populations were found in the Black River up to 1998, the lower Raisin River up to 1977, and the Detroit River in the 1990s. However, with the invasion of the zebra mussel into the Detroit River, the population is likely extirpated from Michigan. (Burch, 1975; Carman and Goforth, 2000; Hoeh and Trdan, 1985)

In Michigan, this species was historically found in the Black River and in old lake plain areas. In general the riffleshell is found in well-oxygenated waters in large streams and rivers mainly in riffles and runs with fine to coarse gravel. (Carman and Goforth, 2000; Watters, 1995)

The northern riffleshell is up to 7.6 cm (3 inches) long . Males are oblong in shape with a broad sulcus along the posterior ridge. Females are oblong but have a broad, fragile, posterior end. The shell is thick anteriorly and compressed. The anterior end is rounded, the posterior end broadly rounded. Females are slightly truncated posteriorly and and males have an indent. The dorsal margin is curved and the ventral margin is gently curved in the males and may be slightly arched in the females .

Umbos are low and somewhat turned forward. The beak sculpture is fine, with double-looped ridges.

Males may have small tubercles from the beaks to the posterior-ventral margin on the lateral surface. A broad sulcus or depression runs on the posterior ridge of the males, but the sulcus is less prominent in females. The periostracum is greenish, yellowish or tan with numerous fine green rays.

On the inner shell, the left valve has two pseudocardinal teeth, which are triangular, divergent and rough. The two lateral teeth are short, straight and rough. The right valve has one triangular, divergent pseudocardinal tooth, with two smaller teeth on both sides. The one right lateral tooth has a much smaller tooth below.

The beak cavity is moderate to shallow. The nacre is white, and iridescent posteriorly.

In Michigan, the males of this species can be confused with the three-horned wartyback. However, the three-horned wartyback has distinct large knobs alternating from side to side. (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)

Growth rings of specimens suggest this species lives up to 15 years. (Carman and Goforth, 2000)

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 spike 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)

Fish hosts are determined by looking at both lab transformations 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, Epioblasma torulosa metamorphosed on the banded darter, the bluebreast darter, brown trout and the mottled sculpin. Other hosts are likely for Lake Erie populations. (Watters, 1996)

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.

Epioblasma torulosa is a federally Endangered species in both the United States and Canada. It is Critically Endangered on the IUCN Red List.

Epioblasma torulosa is likely extirpated from Michigan due to dredging in the Black River and the introduction of the zebra mussel in the Detroit River. However, extant populations are found across Lake Erie in Ontario, Canada drainages. (Environment Canada, 2004a)

The genus Epioblasma is synonomous with Dysnomia and Plagiola.

The northern riffleshell is also considered as a subspecies, Epioblasma torulosa rangiana and is sometimes listed as Epioblasma rangiana. Another subspecies, the tubercled blossom, Epioblasma torulosa torulosa is not in Michigan, but is also considered federally endangered. (Cummings and Mayer, 1992; Watters, 1995)