Alewife Floater
Scientific Name: Anodonta implicata
Classification: State Threatened
Photo Credit: NCWRC
The hinge plate and teeth are lacking in this species as they are in all members of the genus Anodonta. Umbos extend above the hinge line. The alewife floater is easily distinguished from the eastern floater, Pyganodon cataracta, by the former having a pronounced thickening of the shell below the pallial line in the anterior-ventral region. The periostracum is generally smooth and may range in color from yellow to brown-black. Young individuals may have green rays over the surface of the shell.
Habitat Preferences:
Johnson (1970) states that this species appears to prefer clean sand/gravel substrates in relatively fast flowing water. However, at the Chowan River, NC site the water has an almost imperceptible flow (B. Adams, personal communication).
Life History
The alewife (Alosa pseudoharengus) appears to be the alewife floater's dominant fish host (Johnson 1946). According to Ortmann (1911), all Anodonta species appear to be bradytictic (long term brooders).
Distribution
The alewife floater ranges from Nova Scotia and New Brunswick, Canada, to the Potomac River in Virginia and Maryland (Johnson 1946, 1970, Burch 1975). A disjunct population is also known from the Chowan River in North Carolina (Shelley 1983). William Adams (pers. comm.) found live individuals in the Chowan River during the Summer of 1989. This species has also been documented from the Roanoke River below the last dam at Weldon down to Washington County.
Distribution by County: Chowan River Basin: Hertford Co. (Meherrin River, Chowan River); Gates Co. (Chowan River). Roanoke River Basin: Northampton Co. (Roanoke River); Washington Co. (Roanoke River). NOTE: All headwater areas that flow into these occupied habitats should receive special management.
Life History
The alewife (Alosa pseudoharengus) appears to be the alewife floater's dominant fish host (Johnson 1946). According to Ortmann (1911), all Anodonta species appear to be bradytictic (long term brooders).
During the spawning season, mature males release prodigious quantities of sperm into the water. For fertilization to occur, these sperm must pass into the incurrent apertures of sexually mature females of the same species. The sperm travel through the aperture into the suprabranchial chamber where the eggs are held. The fertilized eggs are then transferred into the gill chambers. The gill chambers form a modified brood pouch called the marsupium. While in the marsupium, the fertilized eggs metamorphose from an embryo into the larval form known as the glochidium. Glochidia may be released after a few weeks, or mature glochidia may be held in the brood chamber for months, depending upon the species.
The glochidia of most freshwater mussels are obligate parasites of fish. Once the glochidia have matured within the gills of the female mussel, they must attach to the gills or fins of a suitable fish species to go through yet another metamorphosis into free-living mussels. Fish may become infested with glochidia in several ways. The glochidia of some mussel species are lightweight and float in the water column after they are released. These are "eaten" by fish but instead of passing into the fish’s digestive tract, the glochidia attach to its gills. Other glochidia are heavy, and once they are released, they sink to the bottom of the stream. These glochidia are more likely to attach to the fins of fish as they swim over them.
Females of several mussel species use mimicry to lure fish hosts to themselves before they release their glochidia. The mantle tissue of these species are modified in such a way that they look like prey fish or insects to the fish host. When the female waves these tissues, the fish host may attack the "minnow" and instead of a meal receives a mouthful of glochidia. The glochidia then attach to the gills of the fish. Other mussel species use a kind of fishing lure instead of their own tissues. These mussels produce a gelatinous matrix around the mature glochidia before they are released. This matrix and the glochidia together are called a conglutinate. Some mussel species completely release the conglutinate, and it drifts into the water column or onto the substrate, looking much like a worm or insect, where it is "eaten" by the fish host. Other mussel species release the conglutinate, but it remains tethered to the female mussel by a "fishing line" of the matrix material. The conglutinate waves about in the water column and is "eaten" by the fish host.
Once the glochidia have successfully attached to the fish host, they remain attached for a period of time that varies by species. While attached, the glochidia metamorphose into juvenile mussels, developing a true heart, liver, digestive tract, and muscular foot. When the metamorphosis is complete, the juvenile mussels excyst from the fish host and begin an independent life.