Pyganodon grandis
giant floater

The giant floater is a widespread species, found throughout te Mississippi and Missouri river drainages, the St. Lawrence drainage,, Gulf of Mexico through Louisiana and Texas. In Canada it is in the Interior basin from western Ontario to Alberta.

In Michigan, P. grandis is found throughout streams, lakes and rivers in the state. In general, it is more common in lakes, but is widespread throughout river systems. (Burch, 1975)

The giant floater is found in all habitats, but mainly in lakes or slower moving waters, including backwaters of streams, rivers, and impoundments. It can colonize newly impounded streams. Generally, it is found in substrates of mud or sand, but can also be found in gravel. (Cummings and Mayer, 1992; Oesch, 1984; van der Schalie, 1938; Watters, 1995)

The giant floater is up to 25.4 cm (10 inches) long , and is elliptical or elongated in shape. The shape often varies. The shell is usually fairly thin and inflated. The anterior end is broadly rounded, the posterior end bluntly pointed. The dorsal hinge line is slightly curved and the ventral margin is straight or slightly curved.

Umbos are full, raised slightly above the hinge line and are situated slightly towards the anterior part of the shell. The beak sculpture has three to five double-looped ridges.

The periostracum (outer shell layer) is smooth, yellow to yellow-green with rays in younger individuals. Older specimens tend to be more brown.

On the inner shell, the left valve lacks pseudocardinal and lateral teeth. There is a slight thickening sometimes where the lateral tooth would be.

The beak cavity is broad and shallow. The nacre varies from silvery white, yellow, pink or copper.

In Michigan, this species can be confused with Pyganodon lacustris, paper pondshell, creeper, and cylindrical papershell. Pyganodon lacustris is generally more elongate. The paper pondshell has flattened umbos. The creeper may have a slightly waved and thickened hinge. The cyclindrical papershell generally has a beak sculpture of concentric, single loops. (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. (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. How the giant floater attracts its fish hosts 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. (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.

Glochidial metamorphosis and natural infestations have been observed for bluegill, green sunfish, black crappie, rock bass, largemouth bass, bluntnose minnow, central stoneroller, common shiner, striped shiner, blackchin shiner, blacknose shiner, brook silverside, Iowa darter, Johnny darter, rainbow darter, and yellow perch.

In lab trials, glochidial metamorphosis was observed on banded killifish, golden topminnow, longnose gar, longear sunfish, pumpkinseed, creek chub, golden shiner, redfin shiner, blacknose dace, brook stickleback (Arey, 1932; Howells, 1997; Lefevre and Curtis, 1910; Penn, 1939; Trdan and Hoeh, 1982; Tucker, 1928)

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.

Pyganodon grandis is fairly common throughout its range. However, it is considered Threatened in Vermont. (Hove, 2004)

The genus Anodonta is synonomous with Pyganodon. Because of the variation in shell morphology, Pyganodon grandis was once considered to be several species. (Hoeh, 1990)