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 fawnsfoot attracts or if it recognizes its fish host is unknown.
Glochidia respond to both touch, light and some chemical cues. In general, when touched or a fluid is introduced, they will respond by clamping shut.
Communication Channels: chemical
Perception Channels: visual ; tactile ; vibrations ; chemical
Truncilla donaciformis is not currently considered for conservation status lists.
US Federal List: no special status
CITES: no special status
State of Michigan List: no special status
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.
Development - Life Cycle: metamorphosis
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.
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.
Lab trials have not been conducted to determine the fawnsfoot's fish host. Natural infestations were observed for freshwater drum and sauger. However, this does not conclusively show that these fish are suitable fish hosts.
Ecosystem Impact: parasite
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.
Plant Foods: algae; phytoplankton
Other Foods: detritus ; microbes
Foraging Behavior: filter-feeding
Primary Diet: planktivore ; detritivore
The fawnsfoot is found in the Mississippi drainage and the St. Lawrence system. In general it occurs from western Pennsylvania through Minnesota, south to eastern Texas, and west to Alabama.
In Michigan, T. donaciformis is found in Lake Erie, the lower Raisin River, and on the Grand River, Black Lake and Bear Lake in the western end of the state.
Biogeographic Regions: nearctic (Native )
The fawnsfoot is generally found in rivers and lakes in slower moving waters, with sand or sandy-mud substrates.
Habitat Regions: freshwater
Aquatic Biomes: rivers and streams
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.
The fawnsfoot is up to 5.1 cm (2 inches) long, is elongate and solid. The anterior end is rounded and the posterior end pointed. The dorsal margin is nearly straight and the ventral margin is smoothly rounded.
Umbos are prominent, centrally located, and raised slightly above the hinge line. The beak sculpture has five or six fine, double-looped bars.
The periostracum (outer shell layer) is smooth, yellow to greenish-brown with many green v-shaped zig-zag (chevron) markings.
On the inner shell, the left valve has two large, erect and divergent pseudocardinal teeth. The two lateral teeth are straight to slightly curved, striated and moderately long. The right valve has one erect, roughened and compressed pseudocardinal tooth. Anterior to this tooth is a smaller (lamellar) tooth. The one lateral tooth is narrow and has fine striations.
The beak cavity is moderately shallow to moderately deep. The nacre is white and iridescent posteriorly.
In Michigan, this species can be confused with the snuffbox and deertoe. The snuffbox is more square in shape, more inflated and as corrugations (small ridges) on the dorsal slope. The deertoe is more rounded in shape.
Range length: 5.1 (high) cm.
Other Physical Features: ectothermic ; heterothermic ; bilateral symmetry
Sexual Dimorphism: sexes shaped differently
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.
Known Predators:
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.
Truncilla truncata is likely a long-term brooder, releasing glochidia in the spring. After this release, the mussels spawn, and glochidia are brooded until the following spring.
Breeding interval: The fawnsfoot breeds once in the warmer months of the year.
Breeding season: In Michigan, the breeding season is probably spring to early summer.
Range gestation period: 11 (high) months.
Key Reproductive Features: seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; fertilization (Internal ); viviparous
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.
Parental Investment: pre-fertilization (Provisioning); pre-hatching/birth (Provisioning: Female)
