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[Photo: Phil Myers, Museum of Zoology, University of Michigan]
Virginia Opossum Didelphis virginiana
Description:Virginia opossums are heavy-set mammals roughly the size of a large cat. They have gray fur, whiskers, pointed snouts, long scaly tails and opposable thumbs. Virginia opossums are the only marsupials in the US, carrying their young in a fur-lined pouch. These opossums grow to a length of 13.8-15.8 inches (35-40 cm) with a tail length of 8.5-18.5 inches (22-47 cm). The males are larger than females with weights ranging from 1.8-14.1 pounds (0.8-6.4 kg) and 0.7-8.1 pounds, (0.3-3.7 kg), respectively.
Life History:The mating season of the Virginia opossum lasts from January to July and sexual maturity occurs by the first mating season. Virginia opossums have a 13 day gestation period, with average litter size ranging from 7-9 young. Virginia opossums are omnivorous, primarily feeding on insects and carrion. They are nocturnal and live 1-2 years.
Habitat:Virginia opossums prefer wet areas, near streams and swamps, but can also adapt to drier conditions. They are primarily found in forests.
Distribution:Didelphis virginiana
occur in Central America at the southernmost portion of their range, north to Ontario, Canada, and throughout the United States east of the Rocky Mountains. Virginia opossums can occur, although rarely, along the west coast of the US. This species is common in the Mississippi Alluvial and Southeast Coastal Plain ecoregion.
Status:Virginia opossums appear to be extending their northern range. These opossums occur at a rate of approximately 1 for every 10 acres in the wild, and are hunted as a human food source in some areas of the US. These animals are well adapted to humans and provide an important role in the ecosystem because they will consume food and garbage that other animals do not.
Mississippi Alluvial & Southeast USA Coastal Plains Ecoregion
Map of Level 3 Ecoregions within the Mississippi Alluvial and Southeast US Coastal Plains Ecoregion [Figure: Houston Advanced Research Center]
The Mississippi Alluvial and Southeast USA Coastal Plains Ecoregion stretches along the eastern border of Arkansas, south to Louisiana, and follows the Louisiana coastline east to Mississippi. This ecoregion is characterized by flat plains and wetlands and is defined by its relationship with the Mississippi River. Before human modification, the Mississippi Alluvial and Southeast Coastal Plains were largely comprised of forests, oxbow lakes, and bayous. Many of the lakes, bayous and wetlands were intermittent, dependent upon the flood waters of the Mississippi River.
The soil is comprised of alluvium and coastal marine deposits, fine to medium-textured forest soils and organic soils. This region receives 43-71 inches (110-180 cm) of yearly rainfall. It is characterized by warm winters and hot, humid summers with annual average temperatures ranging from 55-80 F (13-27 C). The vegetation in this ecoregion is dominated by bottomland forests comprised of ash, oak, tupelo and bald cypress forests. Another prominent forest type is the drier, southern mixed forest type comprised of beech, sweet gum, magnolia, oak, pine, saw palmetto and loblolly shortleaf pine. Crops such as soybeans, rice, and cotton comprise much of the cultivated landscape.
Forestry and agriculture are the two main industries in the area, with growth of citrus, rice, soybeans, and cotton. Tourism, fishing and logging are also important activities. A significant human impact on this ecoregion is the altered hydrology of the Mississippi River. Channelization and levees have significantly changed the way the Mississippi flows and altered the interaction of the river with its physical and biological environment.
Invasive Species Spotlight: Zebra Mussel
[Photo: USGS]
Zebra mussel Dreissena polymorpha
Description:The zebra mussel is a small shellfish with a striped zebra-like pattern on its shell. It is similar in appearance to the quagga mussel (another invasive mussel) with brown or black stripes over a cream or yellow-colored shell. However, as the name "polymorpha" implies, the zebra mussel can take varied forms. The zebra mussel is 0.25-1.5 inches (0.6-3.8 cm) long with a D-shaped shell. It attaches to surfaces using an organ called a byssus, which consists of thread-like material.
Life History:Zebra mussels are filter feeders, feeding on mostly algae, but also detritus and zooplankton. They live from 3-9 years and reach sexual maturity at about one year of age (shell length 0.24-0.35 inches or 6-9 mm). Zebra mussels undergo external fertilization and spawn for several weeks as water temperatures reach 12 C or greater (May-Sept) and can spawn year-round in waters that maintain temperatures >12 C.
Habitat:Zebra mussels are a successful invader because they can tolerate a wide range of habitat conditions. They live in both freshwater and estuarine conditions and at temperatures ranging from 0-35 degrees Celsius (C). They can tolerate low dissolved oxygen conditions and can successfully attach to a wide variety of surfaces. Zebra mussels prefer a pH near 7.5, flow velocities of <2 meters/second and require calcium levels of >20 parts per million (ppm).
Distribution:Dreissena polymorpha
are native to Eurasia (the Black, Caspian, and Azov seas). The species was first found in the Great Lakes in 1988. It was introduced to the Great Lakes in ballast water from a cargo ship originating from the Black Sea. Once introduced into the Great Lakes, the species quickly spread down the entire length of the Mississippi River. Zebra mussels have now been established in most major river systems and many inland lakes east of the Missouri River, with isolated populations as far west as California.
Status:Zebra mussels have rapidly spread due to their ability to effectively disperse as both juveniles (veligers) and adults. There are no native mussels which maintain a bysus in the adult form. This adult dispersal attribute has allowed it to quickly invade North America's waters, inflicting millions of dollars in damage from biofouling, and disrupting native ecosystems by significantly reducing phytoplankton abundance. Zebra mussels are considered one of the world's 100 worst alien invaders by the Invasive Species Specialist Group.
[Photo: Ken Bouc, Nebraska Game and Parks Commission]
Pallid Sturgeon Scaphirhynchus albus
Description:The pallid sturgeon is a large, scaleless fish with a shark-like body and appearance. It is one of the largest fish in the Mississippi system, weighing from 26-86 pounds (12-39 kg). The sturgeon has a long, bony, shovel-shaped snout, four prominent barbels, and a toothless sucker mouth. Pallid sturgeons also have many bony plates running the length of their body.
Life History:Pallid sturgeons can live more than forty years, reaching sexual maturity at 7-12 years of age. The pallid sturgeon only spawns every 3-5 years. Pallid sturgeons feed on fish and aquatic invertebrates (immature insects).
Habitat:The pallid sturgeon prefers the sandy bottoms and gravel bars in the swift flowing water of large turbid rivers such as the Missouri and Mississippi. The habitats created by the intermittent flooding of these two rivers are also important to the various life stages of the pallid sturgeon.
Distribution:Scaphirhynchus albus
can be found throughout the Missouri and Mississippi Rivers, and near the mouths of larger tributaries. Its range extends over 3,350 miles.
Status:This species is a federally listed (1990) endangered species. Although the pallid sturgeon is widely distributed, it is rare within its range. The sturgeon is declining due to alteration of its habitat in the Missouri and Mississippi Rivers and length of time it takes an individual to reach sexual maturity. Pallid sturgeons have experienced loss of habitat due to dam and levee construction, and channelization. By altering the natural flow and flooding cycles of the Mississippi and Missouri Rivers, humans have reduced flood plain habitat and blocked migration routes that are important to the sturgeon's survival. The sturgeon has also been affected by water quality decline and historical over-harvesting.
