The flat pigtoe or Marshall's mussel (Pleurobema marshalli) is a species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 of freshwater mussel in the family Unionidae

Unionidae

Unionidae is a family of freshwater mussels, the largest in the order Unionoida, the bivalve mollusks sometimes known as river mussels, naiads, or simply as unionids.The range of distribution for this family is world-wide...

, the river mussels. It is native to Alabama

Alabama

Alabama is a state located in the southeastern region of the United States. It is bordered by Tennessee to the north, Georgia to the east, Florida and the Gulf of Mexico to the south, and Mississippi to the west. Alabama ranks 30th in total land area and ranks second in the size of its inland...

 and Mississippi

Mississippi

Mississippi is a U.S. state located in the Southern United States. Jackson is the state capital and largest city. The name of the state derives from the Mississippi River, which flows along its western boundary, whose name comes from the Ojibwe word misi-ziibi...

, but it has not been seen since 1980. Though it is still listed as critically endangered on the IUCN Red List

IUCN Red List

The IUCN Red List of Threatened Species , founded in 1963, is the world's most comprehensive inventory of the global conservation status of biological species. The International Union for Conservation of Nature is the world's main authority on the conservation status of species...

 and as an endangered species

Endangered species

An endangered species is a population of organisms which is at risk of becoming extinct because it is either few in numbers, or threatened by changing environmental or predation parameters...

 on the US Endangered Species List, it is likely extinct

Extinction

In biology and ecology, extinction is the end of an organism or of a group of organisms , normally a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point...

.

This mussel was last seen in a stretch of the Tombigbee River

Tombigbee River

The Tombigbee River is a tributary of the Mobile River, approximately 200 mi long, in the U.S. states of Mississippi and Alabama. It is one of two major rivers, along with the Alabama River, that unite to form the short Mobile River before it empties into Mobile Bay on the Gulf of Mexico...

 before the habitat was destroyed by the installation of the Tennessee-Tombigbee Waterway

Tennessee-Tombigbee Waterway

The Tennessee–Tombigbee Waterway is a 234-mile man-made, artificial waterway that extends from the Tennessee River to the junction of the Black Warrior-Tombigbee River system near Demopolis, Alabama, United States. The Tenneessee-Tombigbee Waterway links commercial navigation from the nation’s...

. No living or freshly dead specimens have been seen since.