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 fragile papershell recognizes and/or 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.
Communication Channels: chemical
Perception Channels: visual ; tactile ; vibrations ; chemical
Leptodea fragilis is listed as Endangered in Vermont and Threatened in Virginia.
US Federal List: no special status
CITES: 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 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.
Natural infestations of Leptodea fragilis have been found on the freshwater drum (also called the sheepshead). However, no glochidial metamorphosis has been observed.
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 fragile papershell is found throughout the Mississippi drainage, in the Gulf of Mexico drainage from Alabama to Texas. It is also found in the St. Lawrence river drainage from Lake St. Clair to Quebec City.
In Michigan, L. fragilis is found in the lower peninsula from the Muskegon River drainages to the southern part of the state. In eastern Michigan this species is found through the Saginaw and Lake Erie drainage systems.
Biogeographic Regions: nearctic (Native )
The fragile papershell is found in various sized water bodies, from headwater streams to rivers and lakes. Substrates it inhabits include mud and gravel, but is sometimes found in sand.
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 fragile papershell is up to 15 cm (6 inches) long , and is oval or oblong in shape. The shell is usually fairly thin and compressed. Younger individuals have a dorsal wing. The anterior end is uniformly rounded and the posterior end broadly rounded with an acute angle, depending on the wear on the wing. The dorsal margin varies depending on the wing. The ventral margin is uniformly rounded.
Umbos are low, slightly elevated above the hinge line, and situated more near the anterior end. The beak sculpture has three or four weak ridges, the first concentric and the second double looped.
The periostracum (outer shell layer) is smooth, and younger individuals are light yellow-tan, sometimes with green bands. Older specimens tend to be darker yellow-brown. Usually umbos are darker than the rest of the shell.
On the inner shell, the left valve has two pseudocardinal teeth, which are thin and the second tooth is sometimes absent. The two lateral teeth are thin, short and slightly curved. The right valve has one thin and low pseudocardinal tooth. The lateral tooth is thin and slightly curved.
The beak cavity is shallow or absent. The nacre is bluish-white or yellowish, pink or purple under the umbo or lateral tooth, and iridescent throughout.
In Michigan, this species can be confused with the white heelsplitter or pink heelsplitter. The white heelsplitter is darker in color, more rounded in shaped, usually rayed, and has a purple nacre. The pink heelsplitter is also darker and has a more broadly rounded posterior end.
Range length: 15 (high) cm.
Other Physical Features: ectothermic ; heterothermic ; bilateral symmetry
Sexual Dimorphism: sexes alike
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.
Leptodea fragilis is a long-term brooder. Although there are no recorded data, it probably spawns in the summer months in Michigan.
Breeding interval: The fragile papershell breeds once in the warmer months of the year.
Breeding season: In Michigan, the breeding season is probably in the summer months.
Range gestation period: 10 (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)
