Rayed Bean
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''Paetulunio fabalis'', the rayed bean, is a
species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...
of
freshwater mussel Freshwater bivalves are molluscs of the 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 species that l ...
, an aquatic
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
Unionidae The Unionidae are a Family (biology), 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 a ...
, the river mussels. It is the only species in the genus ''Paetulunio'', and was formerly classified in ''
Villosa ''Villosa'' is a genus of freshwater mussels, aquatic bivalve molluscs in the family Unionidae. Species Species within the genus ''Villosa'': * '' Villosa amygdala'' * '' Villosa arkansasensis'' - Ouachita creekshell * '' Villosa choctawensi ...
'' until a 2018 study. The species name is derived from Latin, with a general meaning of "small crescent bean". It was first described as ''Unio fabalis'' by
Isaac Lea Isaac Lea (March 4, 1792 – December 8, 1886) was an American publisher, Conchology, conchologist and geologist. He was a partner in the publishing businesses Mathew Carey, Matthew Carey & Sons; Carey, Lea & Carey; Carey, Lea & Blanchard; and Le ...
in 1831, from a specimen from the
Ohio River The Ohio River () is a river in the United States. It is located at the boundary of the Midwestern and Southern United States, flowing in a southwesterly direction from Pittsburgh, Pennsylvania, to its river mouth, mouth on the Mississippi Riv ...
.


Distribution

This species lives in eastern North America. It is native to the drainages of the
Ohio River The Ohio River () is a river in the United States. It is located at the boundary of the Midwestern and Southern United States, flowing in a southwesterly direction from Pittsburgh, Pennsylvania, to its river mouth, mouth on the Mississippi Riv ...
, the
Tennessee River The Tennessee River is a long river located in the Southern United States, southeastern United States in the Tennessee Valley. Flowing through the states of Tennessee, Alabama, Mississippi, and Kentucky, it begins at the confluence of Fren ...
, and
Lake Erie Lake Erie ( ) is the fourth-largest lake by surface area of the five Great Lakes in North America and the eleventh-largest globally. It is the southernmost, shallowest, and smallest by volume of the Great Lakes and also has the shortest avera ...
and
Lake St. Clair Lake St. Clair () is a freshwater lake that lies between the Canadian province of Ontario and the U.S. state of Michigan. It was named in 1679 by French Catholic explorers after Saint Clare of Assisi, on whose feast day they first saw the lake. ...
. Currently, it is thought to occur in
Michigan Michigan ( ) is a peninsular U.S. state, state in the Great Lakes region, Great Lakes region of the Upper Midwest, Upper Midwestern United States. It shares water and land boundaries with Minnesota to the northwest, Wisconsin to the west, ...
,
Indiana Indiana ( ) is a U.S. state, state in the Midwestern United States, Midwestern region of the United States. It borders Lake Michigan to the northwest, Michigan to the north and northeast, Ohio to the east, the Ohio River and Kentucky to the s ...
,
Ohio Ohio ( ) is a U.S. state, state in the Midwestern United States, Midwestern region of the United States. It borders Lake Erie to the north, Pennsylvania to the east, West Virginia to the southeast, Kentucky to the southwest, Indiana to the ...
,
Pennsylvania Pennsylvania, officially the Commonwealth of Pennsylvania, is a U.S. state, state spanning the Mid-Atlantic (United States), Mid-Atlantic, Northeastern United States, Northeastern, Appalachian, and Great Lakes region, Great Lakes regions o ...
,
New York New York most commonly refers to: * New York (state), a state in the northeastern United States * New York City, the most populous city in the United States, located in the state of New York New York may also refer to: Places United Kingdom * ...
,
Kentucky Kentucky (, ), officially the Commonwealth of Kentucky, is a landlocked U.S. state, state in the Southeastern United States, Southeastern region of the United States. It borders Illinois, Indiana, and Ohio to the north, West Virginia to the ...
,
West Virginia West Virginia is a mountainous U.S. state, state in the Southern United States, Southern and Mid-Atlantic (United States), Mid-Atlantic regions of the United States.The United States Census Bureau, Census Bureau and the Association of American ...
, and
Ontario Ontario is the southernmost Provinces and territories of Canada, province of Canada. Located in Central Canada, Ontario is the Population of Canada by province and territory, country's most populous province. As of the 2021 Canadian census, it ...
. In Canada, it is only known from the
Sydenham River The Sydenham River is a river in Chatham-Kent, Lambton County and Middlesex County in southwestern Ontario, Canada, flowing southwest from its source west of London, Ontario and emptying into Lake Saint Clair. The length of the river is and it ...
, with a small population in the north
Thames River The River Thames ( ), known alternatively in parts as the River Isis, is a river that flows through southern England including London. At , it is the longest river entirely in England and the second-longest in the United Kingdom, after the ...
. To the south and west, it occurs as far as the
Maumee River The Maumee River (pronounced ) (; ) is a river running in the Midwestern United States from northeastern Indiana into northwestern Ohio and Lake Erie. It is formed at the confluence of the St. Joseph River (Maumee River), St. Joseph and St. Mar ...
watershed in Indiana and the Duck River in Tennessee. To the east, it occurs as far as the Elk River in West Virginia and the
Allegheny River The Allegheny River ( ; ; ) is a tributary of the Ohio River that is located in western Pennsylvania and New York (state), New York in the United States. It runs from its headwaters just below the middle of Pennsylvania's northern border, nor ...
in Pennsylvania and New York. Shells have been found from the St. Joseph River of
Lake Michigan Lake Michigan ( ) is one of the five Great Lakes of North America. It is the second-largest of the Great Lakes by volume () and depth () after Lake Superior and the third-largest by surface area (), after Lake Superior and Lake Huron. To the ...
, but it no longer exists in this watershed. It has been extirpated from
Lake Erie Lake Erie ( ) is the fourth-largest lake by surface area of the five Great Lakes in North America and the eleventh-largest globally. It is the southernmost, shallowest, and smallest by volume of the Great Lakes and also has the shortest avera ...
, where it once lived in shallow shores and around
islands This is a list of the lists of islands in the world grouped by country, by continent, by body of water, and by other classifications. For rank-order lists, see the #Other lists of islands, other lists of islands below. Lists of islands by count ...
, and the
Detroit River The Detroit River is an List of international river borders, international river in North America. The river, which forms part of the border between the U.S. state of Michigan and the Provinces and territories of Canada, Canadian province of Ont ...
, due to invasive
zebra Zebras (, ) (subgenus ''Hippotigris'') are African equines with distinctive black-and-white striped coats. There are three living species: Grévy's zebra (''Equus grevyi''), the plains zebra (''E. quagga''), and the mountain zebra (''E. ...
and
quagga The quagga ( or ) (''Equus quagga quagga'') is an extinct subspecies of the plains zebra that was endemic to South Africa until it was hunted to extinction in the late 19th century. It was long thought to be a distinct species, but mtDNA ...
mussels.


Description

The rayed bean is a small mussel that is rarely larger than 3.8 cm (1.5 in) in length. The shell is shiny and green, yellowish-green, or brown, and has many wavy, dark green rays. The rays may be obscured in older animals. The shell is thick and solid. The
nacre Nacre ( , ), also known as mother-of-pearl, is an organicinorganic composite material produced by some molluscs as an inner shell layer. It is also the material of which pearls are composed. It is strong, resilient, and iridescent. Nacre is ...
is silvery-white or bluish and iridescent towards the posterior side. On the inside of the shell, it has unusually heavy teeth (structures along the inner hinge line) for a small mussel. Females are generally smaller than males, and more inflated.


Habitat

It lives in small rivers and creeks with gravel or sandy
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, pools, or flats, and sometimes in the shallow areas of lakes. It prefers flowing water in shallow depths and may often be found buried around the roots of vegetation such as water willow and
water milfoil ''Myriophyllum'' (water milfoil) is a genus of about 69 species of freshwater aquatic plants, with a cosmopolitan distribution. The centre of diversity for ''Myriophyllum'' is Australia with 43 recognized species (37 endemic). These submersed aq ...
. The rayed bean is almost always found buried in the substrate, at depths up to 15cm. Both adults and juveniles create byssal threads made of protein to anchor themselves to gravel or other objects in the water. Due to their small size, this adaptation helps them stay in suitable habitat without being swept away by the current.


Ecology

Freshwater mussels in general are filter-feeders that
siphon A siphon (; also spelled syphon) is any of a wide variety of devices that involve the flow of liquids through tubes. In a narrower sense, the word refers particularly to a tube in an inverted "U" shape, which causes a liquid to flow upward, abo ...
oxygen and food such as
phytoplankton Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater Aquatic ecosystem, ecosystems. The name comes from the Greek language, Greek words (), meaning 'plant', and (), mea ...
and
microorganisms A microorganism, or microbe, is an organism of microscopic size, which may exist in its single-celled form or as a colony of cells. The possible existence of unseen microbial life was suspected from antiquity, with an early attestation in ...
from the water. As juveniles they use "foot feeding" to absorb particles through their
foot The foot (: feet) is an anatomical structure found in many vertebrates. It is the terminal portion of a limb which bears weight and allows locomotion. In many animals with feet, the foot is an organ at the terminal part of the leg made up o ...
. During reproduction, males release sperm into the water, which the females take in through their siphon. The female develops the eggs inside her gills. The number of eggs is likely in the thousands. The rayed bean is thought to be a long-term brooder, meaning they hold the developing eggs for several months. Brooding females have been found from May to October. Like most freshwater mussels, 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) have a parasitic phase where they must attach to a host fish. To attract a host fish, the female rayed bean gapes her shell and exposes her white swollen gills that contain the glochidia. She moves papillae on her mantle in rhythmic, zipping motion. Presumably, like similar unionid mussels, when the fish comes near or nibbles her mantle, she will spurt glochidia into the water so that they can attach to the fish's skin and gills. The only known host fish for the larval stage are the
Tippecanoe darter The Tippecanoe darter (''Etheostoma tippecanoe'') is a species of freshwater ray-finned fish, a darter from the subfamily Etheostomatinae, part of the family Percidae, which also contains the perches, ruffes and pikeperches. It is endemic to t ...
and the spotted darter. Other darters have been suggested as possible hosts as well, such as greenside darter, rainbow darter, mottled sculpin, or
largemouth bass The largemouth bass (''Micropterus nigricans'') is a carnivorous, freshwater fish, freshwater, ray-finned fish in the Centrarchidae (sunfish) family, native to the eastern United States, eastern and central United States, southeastern Canada an ...
. After spending 1-2 weeks attached to the fish's gills, the larvae finish metamorphosing and drop off as juvenile mussels. Its lifespan has been estimated at around 20 years.


As a food source

The rayed bean has been found in shell
middens A midden is an old dump for domestic waste. It may consist of animal bones, human excrement, botanical material, mollusc shells, potsherds, lithics (especially debitage), and other artifacts and ecofacts associated with past human occupati ...
at Native American archaeological sites in small percentages. It was not likely preferred food due to its small size. The species is preyed upon by
muskrat The muskrat or common muskrat (''Ondatra zibethicus'') is a medium-sized semiaquatic rodent native to North America and an introduced species in parts of Europe, Asia, and South America. The muskrat is found in wetlands over various climates ...
,
racoon The raccoon ( or , ''Procyon lotor''), sometimes called the North American, northern or common raccoon (also spelled racoon) to distinguish it from other species of raccoon, is a mammal native to North America. It is the largest of the proc ...
,
mink Mink are dark-colored, semiaquatic, carnivorous mammals of the genera ''Neogale'' and '' Mustela'' and part of the family Mustelidae, which also includes weasels, otters, and ferrets. There are two extant species referred to as "mink": the A ...
, river otter,
striped skunk The striped skunk (''Mephitis mephitis'') is a skunk of the genus ''Mephitis (genus), Mephitis'' that occurs across much of North America, including southern Canada, the United States, and northern Mexico. It is currently listed as least concern ...
, feral hogs,
hellbender The hellbender (''Cryptobranchus alleganiensis''), also known as the hellbender salamander, is a species of aquatic giant salamander endemic to the eastern and central United States. It is the largest salamander in North America. A member of the ...
salamander, turtles, aquatic birds, and some fish.


Threats

The main threats to the rayed bean are siltation of rivers, water pollution, and changes in river flow and physical barriers imposed by
dam A dam is a barrier that stops or restricts the flow of surface water or underground streams. Reservoirs created by dams not only suppress floods but also provide water for activities such as irrigation, human consumption, industrial use, aqua ...
s. Invasive dreissenid mussels (
zebra Zebras (, ) (subgenus ''Hippotigris'') are African equines with distinctive black-and-white striped coats. There are three living species: Grévy's zebra (''Equus grevyi''), the plains zebra (''E. quagga''), and the mountain zebra (''E. ...
and
quagga The quagga ( or ) (''Equus quagga quagga'') is an extinct subspecies of the plains zebra that was endemic to South Africa until it was hunted to extinction in the late 19th century. It was long thought to be a distinct species, but mtDNA ...
mussels), and the invasive
round goby The round goby (''Neogobius melanostomus'') is a euryhaline bottom-dwelling species of fish of the family (biology), family Gobiidae. It is native to Central Eurasia, including the Black Sea and the Caspian Sea. Round gobies have established larg ...
also have a major impact.


Habitat loss and alteration

In 2012, it was estimated that the rayed bean had been eliminated from 73% of its historical habitat, leaving only highly fragmented populations. Dams cause dramatic change to
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 ...
and
shoal In oceanography, geomorphology, and Earth science, geoscience, a shoal is a natural submerged ridge, bank (geography), bank, or bar that consists of, or is covered by, sand or other unconsolidated material, and rises from the bed of a body ...
habitats, change flood patterns and water flow, and cause increased sedimentation. Dams block the free passage of host fish, preventing fish from reaching mussel populations, which is detrimental to mussel reproduction. In Ontario, dams cut off the North Thames population from their historic habitat in the south river. Navigation channels, dredging, agricultural run-off, and livestock trampling of river edges cause sedimentation and disrupt mussel habitat. In Ontario, siltation from agricultural and forestry practices are the biggest threat to the species. Excessive silt can impact mussels' feeding and respiration and cause suffocation. Much of the rayed bean's river habitat is surrounded by agricultural land (for example, up to 89% of the
Maumee River The Maumee River (pronounced ) (; ) is a river running in the Midwestern United States from northeastern Indiana into northwestern Ohio and Lake Erie. It is formed at the confluence of the St. Joseph River (Maumee River), St. Joseph and St. Mar ...
drainage in Ohio). Lack of streamside buffers that separate agriculture and development projects from rivers can cause run-off and pollution. Interstitial spaces in the
substrate Substrate may refer to: Physical layers *Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached ** Substrate (aquatic environment), the earthy material that exi ...
are an important part of the rayed bean's habitat because of their burrowing behavior. Large deposits of sedimentation and contaminants in sediment can degrade these spaces.


Pollution

Chemical contaminants come from industrial and municipal discharge, spills, and agricultural and other run-off. Mussels have been shown to have high sensitivity to pollutants such as ammonia, metals, chlorine, and pesticides, and may affect them even in levels determined "safe" by EPA standards. Nutrients such as
nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...
and
phosphorus Phosphorus is a chemical element; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...
come from farm and livestock run-off can deplete dissolved oxygen levels. The Upper Thames River has some of the highest phosphorus levels in the region. Pharmaceutical pollution from wastewater is an emerging but relatively unstudied threat and is common in the rayed bean's habitat.


Invasive species

Dreissenid mussels (
zebra Zebras (, ) (subgenus ''Hippotigris'') are African equines with distinctive black-and-white striped coats. There are three living species: Grévy's zebra (''Equus grevyi''), the plains zebra (''E. quagga''), and the mountain zebra (''E. ...
and
quagga The quagga ( or ) (''Equus quagga quagga'') is an extinct subspecies of the plains zebra that was endemic to South Africa until it was hunted to extinction in the late 19th century. It was long thought to be a distinct species, but mtDNA ...
mussels) are invasive freshwater mussels from eastern Europe that have become firmly established in parts of the rayed bean's range. Dreissenid mussels attach themselves to native mussels, reduce food particles in the water, interrupt reproduction, and foul the water with pseudofeces. Zebra mussels are established in the Great Lakes and Ohio River drainages, have eliminated rayed bean populations from
Lake Erie Lake Erie ( ) is the fourth-largest lake by surface area of the five Great Lakes in North America and the eleventh-largest globally. It is the southernmost, shallowest, and smallest by volume of the Great Lakes and also has the shortest avera ...
and the
Detroit River The Detroit River is an List of international river borders, international river in North America. The river, which forms part of the border between the U.S. state of Michigan and the Provinces and territories of Canada, Canadian province of Ont ...
, and have high potential to spread further. Two introduced fish species, the
round goby The round goby (''Neogobius melanostomus'') is a euryhaline bottom-dwelling species of fish of the family (biology), family Gobiidae. It is native to Central Eurasia, including the Black Sea and the Caspian Sea. Round gobies have established larg ...
and the
black carp The black carp (''Mylopharyngodon piceus'') or Chinese black roach is a species of freshwater ray-finned fish belonging to the family Xenocyprididae, the East Asian minnows or sharpbellies. The black carp is the sole extant species of the genus ' ...
, are aggressive predators of mussels.


Population fragmentation

Many populations are geographically isolated and have become even more so as dams were constructed. Some are below the effective population size needed to sustain the population and maintain genetic diversity.


Localized threats

Several localized threats have been identified for certain populations the rayed bean. These include contaminants from
coal mining Coal mining is the process of resource extraction, extracting coal from the ground or from a mine. Coal is valued for its Energy value of coal, energy content and since the 1880s has been widely used to Electricity generation, generate electr ...
near the Elk River in West Virginia, gravel mining near the Allegheny and Tennessee watersheds, and oil and gas extraction from the Marcellus and Utica Shales. Dreissenid mussels from
Fanshawe Lake Fanshawe Lake is a small man-made lake east of London, Ontario. It is fed by the Thames River from the north, and is separated from the river to the southwest by Fanshawe Dam. The Lake is a popular choice for boaters during the summer months, an ...
in Ontario represent a potentially devastating threat to the Thames River population.


Conservation efforts

It has been listed as Endangered in Canada on the
List of Wildlife Species at Risk As of November 2024, the List of Wildlife Species at Risk has more than 600 entries for Canadian wildlife species considered at varying risks of extinction, including 301 classified as endangered species, 147 threatened species, 200 special conc ...
under the Canadian
Species at Risk Act The ''Species at Risk Act'' (, SARA) is a piece of Canadian federal legislation which became law in Canada on December 12, 2002. It is designed to meet one of Canada's key commitments under the International Convention on Biological Diversity. T ...
since 1999.COSEWIC. 2005
Canadian Species at Risk
Committee on the Status of Endangered Wildlife in Canada. 64 pp., page 13.
In 2012, it was listed as Endangered in the United States under the
Endangered Species Act The Endangered Species Act of 1973 (ESA; 16 U.S.C. § 1531 et seq.) is the primary law in the United States for protecting and conserving imperiled species. Designed to protect critically imperiled species from extinction as a "consequence of e ...
, citing a reduction in range and declining, isolated populations.Federal Register Vol 77, No. 30. Feb. 14, 2012: Determination of Endangered Status for the Rayed Bean and Snuffbox Mussels Throughout Their Ranges It holds protection at the state and provincial levels in several jurisdictions including Ontario, Michigan, Ohio, and Indiana. The species' recent success has been mixed. Some new populations have been found since its Endangered listing in 2012, and the
Allegheny River The Allegheny River ( ; ; ) is a tributary of the Ohio River that is located in western Pennsylvania and New York (state), New York in the United States. It runs from its headwaters just below the middle of Pennsylvania's northern border, nor ...
holds one of the largest current populations and appears to be increasing. A study found robust populations in locations in Ohio and New York in 2024. However, Fish and Wildlife surveys have found it likely extirpated in other locations. Several recent bridges and infrastructure projects have been detrimental to this mussel, and a 2017 cleanup project for a decades-old petroleum spill near the city of
Olean, New York Olean ( ) is a Administrative divisions of New York#City, city in Cattaraugus County, New York, United States. Olean is the largest city in Cattaraugus County and serves as its financial, business, transportation and entertainment center. It is ...
affected thousands of rayed bean that had to be relocated from that area of the Allegheny. There have been several reintroduction efforts in different areas. The status of a 2006-07 translocation of mussels to the Elk River is uncertain, while a relocation to the Duck River in 2009 initially appeared successful. In 2020, over 5,000 rayed bean that had been salvaged from the Olean petroleum cleanup were transferred to the Green River and Licking River in Kentucky, areas where the species had previously been extirpated. Since the life history of the rayed bean has been insufficiently studied, captive breeding is not yet possible.


References

{{Taxonbar, from=Q3013948 Unionidae Taxobox binomials not recognized by IUCN ESA endangered species IUCN Red List endangered species Monotypic bivalve genera