Epioblasma obliquata
catspaw
(Also: Purple Cat's paw pearly mussel)

Historically Epioblasma obliquata was found in the Ohio River drainage and the Detroit River, however its range has been greatly reduced to one population in Indiana.

In Michigan the white catspaw was historically found in the Detroit River and Lake Erie. One specimen was collected from the Raisin River in Monroe County. This species has not been collected in Michigan waters since the 1930s. (Burch, 1975; Carman, 2001)

Specific habitat requirements are unknown since this species is rare. However, populations recently found were in high gradient streams in riffles with gravel. In Michigan this species was historically found in the Detroit River and Lake Erie. (Carman, 2001; van der Schalie, 1938)

The catspaw is up to 5.1 cm (2 inches) long. Females are rectangular or quadrate while males are oval and elongate. The posterior ridge has a wide sulcus or depression between double ridges in males. The ridge is sharp and narrow in females. The shell is usually fairly solid and inflated. The anterior end is rounded, the posterior end bluntly pointed in males. Females have a posterior margin that is extended, truncated, with ribs and a medial notch. The dorsal margin is short and straight and the ventral margin is broadly curved in males and gently curved in females.

Umbos are full and even with the hinge line. Beak sculpture is double looped.

The periostracum (outer shell layer) is smooth, yellow to yellow-tan with fine green rays.

On the inner shell, the left valve has two pseudocardinal teeth, which are wide, elevated and serrated. The two lateral teeth are short and straight to slightly curved, and rough. The right valve has one wide, elevated and serrated pseudocardinal tooth.

The beak cavity is shallow. Although the nacre is white to purple it is iridescent at the posterior end. Michigan specimens have only had a white nacre.

In Michigan, this species could be confused with the northern riffleshell. The northern riffleshell is generally smaller and females lack serrations at the edge of the posterior end of the shell. (Carman, 2001; 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)

Based on growth rings on specimens, the white catspaw probably lives up to 15 years. (Carman, 2001)

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)

While freshwater mussels require a host fish for metamorphosis, the host for the spike is unknown.

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 obliquata is a federally Endangered species in both the United States and Canada. IUCN lists this species as Critically Endangered.

The population in Michigan is actually a subspecies population, Epioblasma obliquata perobliqua. Another subspecies, Epioblasma obliquata obliquata has a purple nacre and does not occur in Michigan.

Epioblasma obliquata is synonomous with Epioblasma obliquata. The genus Epioblasma is synonomous with Dysnomia and Plagiola. (Carman, 2001)