Pleurobema sintoxia is synonomous with Pleurobema coccineum.
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 round pigote attracts its fish host is unknown. However, white glochidial packets that are released resemble invertebrates that fish would eat.
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: visual ; chemical
Perception Channels: visual ; tactile ; vibrations ; chemical
The round pigtoe is listed as state endangered in Iowa, threatened in Minnesota, and special concern in Michigan.
US Federal List: no special status
CITES: 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.
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.
In lab trials, Pleurobema sintoxia glochidia metamorphosed on the bluntnose minnow, northern redbelly dace, southern redbelly dace and spotfin shiner.
Ecosystem Impact: parasite
Species Used as Host:
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 round pigtoe was historically found in the upper Mississippi drainages and the St. Lawrence drainage. Generally it is found from western New York and Ontario west through Wisconsin and Minnesota to the southeastern portion of South Dakota, south to Oklahoma, Arkansas, Mississippi and Alabama. Its eastern range includes western Virginia, West Virginia, and western Pennsylvania.
In Michigan P. sintoxia is found in mid-Michigan drainages, mainly from the Muskegon and Saginaw River systems on through the southern river systems. Specimens have also been collected in the Sturgeon River in the upper peninsula.
Biogeographic Regions: nearctic (Native )
The round pigtoe is generally widespread from small to large rivers. Substrates it inhabits include mud, sand and gravel with moderate flows.
In Lake St. Clair it is found in nearshore areas less than 1 m with firm, sandy 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 round pigtoe is up to 10.2 cm (4 inches) long , and is round and compressed in headwaters, triangular and inflated in rivers. The shell is usually fairly thick. The anterior end is broadly rounded, the posterior end rounded to bluntly pointed. The dorsal margin is straight to slightly curved and the ventral margin is gently curved.
Umbos are low, being raised only slightly above the hinge line, and can be centered on the shell or more anterior. The beak sculpture with two elevated ridges on the umbo or posterior ridge.
The periostracum (outer shell layer) is green-brown, red-brown or light brown with green rays. Older specimens tend to be more brown and lack rays.
On the inner shell, the left valve has two pseudocardinal teeth, which are rough, large, and grooved. The anterior tooth is about one third the size of the posterior tooth. The two lateral teeth are straight, short, and somewhat striated. The right valve has one large, erect, notched and grooved pseudocardinal tooth. The one lateral tooth has high, broad and striated.
The beak cavity is shallow to moderately deep. Although the nacre is white, occasionally it is has a pink or salmon tint and is iridescent at the posterior end.
In Michigan, this species is most difficult to distinguish from the headwater forms of the Wabash pigtoe, and is often only distinguished by looking at the teeth on the inner part of the valves of the shells. The Wabash pigtoe has a deeper beak cavity. When present in females, glochidial packets from the Wabash pigtoe also tend to be orangish while the round pigtoe has white glochidial packets. Other similar species include the hickorynut and the round hickorynut, which are generally more inflated. The hickorynut is more slightly more oval in shape. The round hickorynut is more rounded and generally lighter at the posterior end.
Range length: 10.2 (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.
Pleurobema sintoxia is a short-term brooder. In southeastern Michigan, it was gravid from mid May to late July, and probably spawns in early May.
Breeding interval: The round pigtoe 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 (External ); 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)
Pleurobema sintoxia, the round pigtoe, is a species of freshwater mussel, an aquatic bivalve mollusk in the family Unionidae, the river mussels.
This species is native to eastern North America. It is found in the drainages of the Ohio River, the St. Lawrence River, and the Great Lakes.
The Canadian Species at Risk Act listed it in the List of Wildlife Species at Risk as being endangered in Canada.[1]
Pleurobema sintoxia, the round pigtoe, is a species of freshwater mussel, an aquatic bivalve mollusk in the family Unionidae, the river mussels.
