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 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 ; infrared/heat ; tactile ; vibrations ; chemical
Elliptio dilatata is listed as Threatened in Illinois, and Special Concern in Minnesota and North Carolina.
US Federal List: no special status
CITES: no special status
IUCN Red List of Threatened Species: no special status
Fertilized eggs are brooded in the marsupia (water tubes) up to three 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.
While freshwater mussels require a host fish for metamorphosis, the host for the spike is unknown.
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 spike is found in the Mississippi River drainage, St. Lawrence system and the Gulf of Mexico drainage from Florida to the Guadalupe River, Texas.
In Michigan E. dilatata is found in streams and rivers throughout the upper and lower peninsulas of Michigan.
Biogeographic Regions: nearctic (Native )
Small to large streams and occasionally lakes. Usually the substrate it inhabits is sand-gravel or mud-gravel. The spike is absent from shifting sands.
In the Huron River, the spike was mainly found on firm substrates of sand and gravel.
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 spike is up to 12.7 cm (5 inches) long , and is elongate and elliptical. The shell is usually fairly thick and compressed. The anterior end is rounded, the posterior end rounded to slightly pointed. The dorsal and ventral margins are straight to slightly curved.
Umbos are low, and not raised above the hinge line. The beak sculpture has three or four parallel ridges.
The periostracum (outer shell layer) is smooth, green-brown in younger specimens, dark brown to black in older specimens.
On the inner shell, the left valve has two divergent, erect, triangular and striated pseudocardinal teeth. The two lateral teeth are short to slightly curved, and rough. The right valve has one large, erect, triangular and serrated pseudocardinal tooth, with a small tooth to the anterior end. The single lateral tooth on the right valve is slightly curved and rough.
The beak cavity is shallow. Although the nacre is almost always purple, and only occasionally white.
In Michigan, this species is most easily confused with the eastern elliptio in northern Michigan. The eastern ellipse is rhomboidal and more compressed. In lower Michigan, similar species include the black sandshell and the kidneyshell. The black sand shell has a thicker shell and the posterior end is more pointed or saber-shaped. Besides being more of a kidney shape, the posterior end of the kidneyshell is more bluntly pointed, and the shell color is more yellow.
Range length: 12.7 (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.
In the Huron River in Michigan, Elliptio dilatata was gravid from mid-May to early August. It probably breeds in early May.
Breeding interval: The spike breeds once in the warmer months of the year.
Breeding season: In Michigan, the breeding season is probably early to mid-May.
Range gestation period: 2.5 (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)
