Epioblasma triquetra
snuffbox

The snuffbox mussel is found from western New York and souther Ontario, west to Wisconsin, Iowa and eastern Nebraska, south to Oklahoma, and east to northern Alabama. Within this range it is found in Tennessee, West Virginia, and the Ohio River drainage.

In Michigan this species is found in southeastern Michigan rivers of the Lake Erie drainage, except for the Raisin River. In western Michigan it was historically found in parts of the St. Joseph (Michigan) and Muskegon Rivers. One shell was collected from the Kalamazoo River, although a live individual has not been found. Recent populations have been found in the Grand River. Other museum specimens were collected from the Cass and Chippewa Rivers in the 1930s. (Burch, 1975; Carman and Goforth, 2000; van der Schalie, 1938)

In Michigan, Epioblasma triquetra is found in small- to medium-sized rivers, 7 to 23 m wide. In the southeastern part of Michigan, it is more common in streams between lakes. Its fish host, the logperch, Percina caprodes reproduces in both rivers and lakes, and populations of E. triquetra have been also been found in lakes. However, the invasion of the zebra mussel Dreissena polymorpha, has decimated most lake populations of the snuffbox in southeastern Michigan. (Sherman, 1994; van der Schalie, 1938)

The snuffbox is up to 6.4 cm (2.5 inches) long , and is triangular or elongated in shape. The shell is uniformly solid, and inflated. The anterior end is rounded. Males have a truncated posterior end and females have a more expanded posterior end. The dorsal and ventral margins are straight to slightly curved. Females in general are smaller and can be distinguished from males by the ribs and teeth-like sculpture at the posterior end.

Umbos are broad and slightly raised above the hinge line. The beak sculpture is has three to four double-looped bars, although these may be worn and faint.

The periostracum (outer shell layer) is smooth, yellow to yellow-green. The shells also have dark green rays with blotches or chevron markings.

On the inner shell, the left valve has two pseudocardinal teeth, which are high, thin and triangular. The two lateral teeth are short, slightly curved, and slightly striated. The right valve has one high, triangular, thin, triangular pseudocardinal tooth. The one right lateral tooth is short, slightly curved and slightly striated.

The beak cavity is moderately to fairly deep. Although the nacre is white and iridescent at the posterior end.

In Michigan, this species can be confused with the slippershell, and deertoe. The snuffbox is generally smaller, and females are tooth-like at the posterior end. Slippershells are smaller (therefore growth lines are closer together) and lack the black blotches and chevrons. The deertoe is generally larger than the snuffbox and is more rounded in shape. (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. Demographic data in Michigan for this species suggest they may live from 14-20 years.

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.

Female Epioblasma triquetra will trap the host fish between its valves, then pump glochidia onto the fish. (Barnhart and Roston, 2005; Oesch, 1984; Sherman, 1994)

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. (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 triquetra metamorphosed on the logperch, the black-sided darter, the Ozark sculpin, the banded sculpin, and the black spotted topminnow. The logperch is likely its main host species, given the distribution and numbers of glochidia that successfully transform on the logperch. (Barnhart, Riusech, and Baird, 1998; Hill, 1986; Sherman, 1994)

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 triquetra is on state endangered lists in Illinois, Indiana, Michigan, Missouri, Ohio, Virginia and Wisconsin. In Minnesota it is listed as threatened. In Canada it is federally Endangered under the Species At Risk Act. (Environment Canada, 2003; Hove, 2004)

The genus Epioblasma was formerly known (synonomous) as Dysnomia. This species has also been called Plagiola triquetra.