Simpsonaias ambigua

The salamander mussel is found in the Ohio River drainages, north through Lake Erie in southern Michigan, west to Iowa sandsouth to northern Alabama. Its eastern range extends from Pennsylvania south to the western half of Tennessee.

In Michigan S. ambigua is found mainly in the Lake Erie drainages. Historical records are mainly from the Tifflin River, Macon Creek (Raisin River), Lake Erie and the Detroit River. Recently, spent shells were found in the Black and Pine Rivers in Sanilac County. (Burch, 1975; Carman, 2002; Oesch, 1984)

This species is usually only found where its host, Necturus maculosus is found, in areas of silt or sand within medium to large rivers or lakes. They are often under large flat stones, where their hosts occur. (Carman, 2002; Cummings and Mayer, 1992; Watters, 1995)

The salamander mussel is up to 5.1 cm (2 inches) long , and is elliptical and elongate. The shell is usually fairly thin, compressed in the male and inflated in the female. The anterior end and posterior ends are rounded. The dorsal and ventral margins are straight.

Umbos are raised only slightly above the hinge line. The beak sculpture is fine, with three or four double-loops.

The periostracum (outer shell layer) is smooth, grayish, or yellow-brown. Older specimens tend to be more brown.

On the inner shell, the left and right valves each have one small, low and rounded pseudocardinal tooth. Lateral teeth are absent on both valves. The beak cavity is shallow. The nacre is white and iridescent posteriorly.

In Michigan, this species can be confused with the cylindrical papershell. The salamander mussel is slightly smaller, elongate and compressed. The beak sculpture can be used to help distinguish between both species. The salamander mussel also has small pseudocardinal teeth, which are absent in the cylindrical papershell. (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)

Age to sexual maturity for this species is unknown. Unionids are gonochoristic (sexes are separate) and viviparous. The glochidia, which are the larval stage of the mussels, are released live from the female after they are fully developed.

In general, gametogenesis in unionids is initiated by increasing water temperatures. The general life cycle of a unionid, includes open fertilization. Males release sperm into the water, which is taken in by the females through their respiratory current. The eggs are internally fertilized in the suprabranchial chambers, then pass into water tubes of the gills, where they develop into glochidia.

Simpsonaias ambigua is probably a long-term brooder, spawning in the summer and retaining glochidia until the following spring. (Lefevre and Curtis, 1912; Oesch, 1984; Watters, 1995)

Females brood fertilized eggs in their marsupial pouch. The fertilized eggs develop into glochidia. There is no parental investment after the female releases the glochidia.

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 salamander mussel attracts or recognizes 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 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.

As the common name implies, the host fish for this mussel is a salamander--the mudpuppy, Necturus maculosus. (Barnhart, et al., 1998; Howard, 1915)

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

There are no significant negative impacts of mussels on humans.

Simpsonaias ambigua is often rare within its range. It is listed as endangered in Illinois and Michigan, Threatened in Minnesota and Wisconsin, and Special Concern in Indiana. Under the Canadian Species At Risk Act, it is considered Endangered. The IUCN Red List considers this species Lower Risk, Conservation Dependent. (Environment Canada, 2003; Hove, 2004)

Simpsonais was formerly known as (is synonomous) with Simpsoniconcha.