''Obovaria arkansasensis'', the Ouachita creekshell, is a

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

freshwater mussel Freshwater bivalves are one kind of freshwater mollusc, along with freshwater snails. They are bivalves that live in fresh water as opposed to salt water, which is the main habitat type for bivalves. The majority of species of bivalve molluscs ...

, an aquatic bivalve mollusc in the family

Unionidae The Unionidae are a family of freshwater mussels, the largest in the order Unionida, the bivalve molluscs sometimes known as river mussels, or simply as unionids. The range of distribution for this family is world-wide. It is at its most divers ...

, the river mussels. It is endemic to certain rivers and streams in the

Ouachita Mountains The Ouachita Mountains (), simply referred to as the Ouachitas, are a mountain range in western Arkansas and southeastern Oklahoma. They are formed by a thick succession of highly deformed Paleozoic strata constituting the Ouachita Fold and Thru ...

, Arkansas, and has a complex life history including its larvae being

parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson ha ...

on a fish host.

Description

The Ouachita creekshell grows to about in length. The shell is thin but has robust hinge teeth, especially at the anterior end. There are sometimes a few fine ribs on the posterior end of the shell. The periostracum is olive or yellowish-brown. This species is dimorphic with the males being ovate and the females being less compressed laterally and with the posterior end truncated and sometimes with a notch on the posterior margin. ''Obovaria arkansasensis'' shows great similarity in general appearance to '' Obovaria jacksoniana''. However, its shell

morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines * Morphology (archaeology), study of the shapes or forms of artifacts * Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies ...

and life cycle are different, as are the species of host fish with which it is associated. The latter is sometimes considered a synonym of ''Obovaria arkansasensis''.

Distribution and habitat

The Ouachita creekshell is found at about 30 sites in small and medium-sized streams in the

in south west Arkansas. It is restricted to the

headwaters The headwaters of a river or stream is the farthest place in that river or stream from its estuary or downstream confluence with another river, as measured along the course of the river. It is also known as a river's source. Definition The ...

of the

Ouachita River The Ouachita River ( ) is a river that runs south and east through the U.S. states of Arkansas and Louisiana, joining the Tensas River to form the Black River near Jonesville, Louisiana. It is the 25th-longest river in the United State ...

and the Saline River drainage systems including the Caddo River and Little Missouri River. It is found buried in the sand or gravel at the bottom of

riffle A riffle is a shallow landform in a flowing channel. Colloquially, it is a shallow place in a river where water flows quickly past rocks. However, in geology a riffle has specific characteristics. Topographic, sedimentary and hydraulic indica ...

s and glides and sometimes slower moving stretches of water, but not in lakes.

Biology

Ouachita creekshells are filter feeders and orient themselves with their siphons upstream. They tend to remain buried in the sediment sometimes with their posterior ends uncovered. The females come to the surface when the weather warms up in spring in order to release their larvae. Like other fresh water mussels in the family Unionidae, the Ouachita creekshell has a complex life history involving a larval stage known as a

glochidium The glochidium (plural glochidia) is a microscopic larval stage of some freshwater mussels, aquatic bivalve mollusks in the families Unionidae and Margaritiferidae, the river mussels and European freshwater pearl mussels. These larvae are t ...

which attaches itself to the

gill A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are ...

, fin or skin of a suitable host fish. The actual host fishes used by Ouachita creekshells are unknown but suitability trials in the laboratory showed that the shadow bass ('' Ambloplites ariommus'') might be the primary host and that other possible hosts include the Creole darter ('' Etheostoma collettei''), the greenside darter ('' Etheostoma blennioides'') and the green sunfish ('' Lepomis cyanellus''). The female Ouachita creekshell produces a lure for potential host fish consisting of movements of papillae on the foot creating a wave action of the mantle. Fish that investigate this get showered in glochidia which have been brooded within the mussel's mantle cavity over the winter period. The glochidia encyst on the surface of the fish and feed on fish tissue for several weeks. They undergo metamorphosis while encysted before dropping off the host and settling on the stream bed as juvenile creekshells. The fish are unharmed by this process.

Status

The Ouachita creekshell is endemic to Arkansas and is classified as a S2 species. This means that it is of special concern as it is very rare, either having fewer than 20 localities at which it is found or having a small number of individuals at only a very few locations. In either case, it is in danger of extinction. When considering how to conserve the mussel, the management of suitable host fish is important. The chief threat to both is habitat destruction and degradation. The impoundment of waters by damming streams restricts fish movement and causes increased sedimentation below the dam and has a deleterious effect on mussel populations. Competition from the introduced species of mussel, '' Corbicula fluminea'', may also be a cause for concern.

References

{{Taxonbar, from=Q106020101, from2=Q3012758 arkansasensis Freshwater animals of North America Molluscs of the United States Endemic fauna of Arkansas Endangered fauna of the United States Molluscs described in 1862 Taxa named by Isaac Lea Ouachita River