The western pearlshell (''Margaritifera falcata'') is a species of

freshwater bivalve Freshwater bivalves are molluscs of the order (biology), order Bivalvia that inhabit freshwater ecosystems. They are one of the two main groups of freshwater molluscs, along with freshwater snails. The majority of bivalve molluscs are saltwater ...

, a pearl mussel, a

bivalve Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of aquatic animal, aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed b ...

mollusk Mollusca is a phylum of protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant species of molluscs are recognized, making it the second-largest animal phylum after Arthropoda. The ...

in the family

Margaritiferidae Margaritiferidae is a family of medium-sized freshwater mussels, aquatic bivalve molluscs in the order Unionida. Text may have been copied from this source, which is available under Attribution 4.0 International (CC BY 4.0)licence. It is the ...

. This species can be found only in the United States and Canada, where it occurs mostly west of the

Rocky Mountains The Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch in great-circle distance, straight-line distance from the northernmost part of Western Can ...

Distribution

This species of pearl mussel is found in Pacific drainages from

California California () is a U.S. state, state in the Western United States that lies on the West Coast of the United States, Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and shares Mexico–United States border, an ...

north to

British Columbia British Columbia is the westernmost Provinces and territories of Canada, province of Canada. Situated in the Pacific Northwest between the Pacific Ocean and the Rocky Mountains, the province has a diverse geography, with rugged landscapes that ...

and southern

Alaska Alaska ( ) is a non-contiguous U.S. state on the northwest extremity of North America. Part of the Western United States region, it is one of the two non-contiguous U.S. states, alongside Hawaii. Alaska is also considered to be the north ...

.Clarke, A. H. 1981. ''The Freshwater Molluscs of Canada''. National Museum of Natural Sciences, National Museums of Canada, Ottawa. 446 pp.Frest, T.J. and E. J. Johannes. 1995. Interior Columbia Basin mollusk species of special concern. Final report to the Interior Columbia Basin Ecosystem Management Project, Walla Walla, WA. Contract #43-0E00-4-9112. 274 pp. plus appendices. Some scientists consider a number of the coastal and large river populations of this species to be already extirpated, nearly extirpated, or declining rapidly.Brim Box, K., J. Howard, D. Wolf, C. O'Brian, D. Nez, and D. Close. 2006. Freshwater mussels (Bivalvia: Unionoida) of the Umatilla and Middle Fork John Day rivers in eastern Oregon. ''Northwest Science'' 80:95-107 However, this species is still common throughout parts of the northern Rocky Mountains, although some populations in

Montana Montana ( ) is a landlocked U.S. state, state in the Mountain states, Mountain West subregion of the Western United States. It is bordered by Idaho to the west, North Dakota to the east, South Dakota to the southeast, Wyoming to the south, an ...

may be declining. The species is also found east of the

Continental Divide A continental divide is a drainage divide on a continent such that the drainage basin on one side of the divide feeds into one ocean or sea, and the basin on the other side either feeds into a different ocean or sea, or else is endorheic, not ...

in the

headwaters The headwater of a river or stream is the geographical point of its beginning, specifically where surface runoff water begins to accumulate into a flowing channel of water. A river or stream into which one or many tributary rivers or streams flo ...

of the

Missouri River The Missouri River is a river in the Central United States, Central and Mountain states, Mountain West regions of the United States. The nation's longest, it rises in the eastern Centennial Mountains of the Bitterroot Range of the Rocky Moun ...

. Originally these populations were identified as the eastern species ''

Margaritifera margaritifera The freshwater pearl mussel (''Margaritifera margaritifera'') is an endangered species of freshwater mussel, an Aquatic animal, aquatic bivalve mollusc in the family Margaritiferidae. Although the name "freshwater pearl mussel" is often used fo ...

'', but recently scientists have confirmed that the populations are in fact ''M. falcata'', which means that the species somehow was able to cross the divide.Gangloff, M. M. and D.L. Gustafson. 2000. The mussels (Bivalvia: Unionida) of Montana. ''Central Plains Archaeology'' 8(1):121-130 The most likely explanation for this distribution is headwater capture, where pre-glacial watersheds were cut and reconfigured by glacial advance or retreat.

Westslope cutthroat trout The Westslope cutthroat trout (''Oncorhynchus lewisi'')Page, Lawrence M.; Bemis, Katherine E.; Espinosa-Pérez, Héctor S.; Findley, Lloyd T.; Gilbert, Carter R.; Hartel, Karsten E.; Lea, Robert N.; Mandrak, Nicholas E.; Neighbors, Margaret A. ( ...

are thought to have crossed the Continental Divide from the West into the headwaters of the present-day Missouri River during the

Pleistocene The Pleistocene ( ; referred to colloquially as the ''ice age, Ice Age'') is the geological epoch (geology), epoch that lasted from to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fin ...

glaciation, more that 20,000 years ago. Since cutthroat trout are an important host species for the

glochidia 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 ...

larvae of ''M. falcata'', it is likely that the mussels crossed the divide by hitching a ride on the trout.

References

External links

Freshwater Mussels Of The Willamette
documentary on species population and health (

Oregon Field Guide ''Oregon Field Guide'' is a weekly television program produced by Oregon Public Broadcasting focusing on recreation, the outdoors, and environmental issues in the state of Oregon. The show has become part of the Oregon zeitgeist. Steve Amen is th ...

) {{Taxonbar, from=Q3803562 Margaritifera Molluscs of the United States Bivalves described in 1850