Paetulunio fabalis, the rayed bean, is a species of freshwater mussel, an aquatic bivalve mollusk in the family Unionidae, the river mussels. It is the only species in the genus Paetulunio, and was formerly classified in Villosa until a 2018 study.[2][3]
| Paetulunio fabalis | |
|---|---|
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Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Mollusca |
| Class: | Bivalvia |
| Order: | Unionida |
| Family: | Unionidae |
| Tribe: | Lampsilini |
| Genus: | Paetulunio Watters, 2018 |
| Species: | P. fabalis
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| Binomial name | |
| Paetulunio fabalis (Lea, 1831)
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| Synonyms | |
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The species name is derived from Latin, with a general meaning of "small crescent bean".
Distribution
editThis species lives in eastern North America. It is native to the drainages of the Ohio River, the Tennessee River, and Lake Erie and Lake St. Clair.
Currently, it is thought to occur in Michigan, Indiana, Ohio, Pennsylvania, New York, Kentucky, West Virginia, and Ontario. In Canada, it is only known from the Sydenham River, with a small population in the north Thames River.[4]
Description
editThe 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 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.[5]
Habitat
editIt lives in small rivers and creeks with gravel or sandy riffles, 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.[4] The rayed bean is almost always found buried in the substrate, at depths up to 15cm.[6]
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.[4]
Ecology
editFreshwater mussels in general are filter-feeders that siphon (mollusc) oxygen and food such as phytoplankton and microorganisms from the water. As juveniles they use "foot feeding" to absorb particles through their foot.
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, and brooding females have been found from May to October. Like most freshwater mussels, the larvae have a parasitic phase where they must attach to a host fish. To attract a host fish, the female rayed bean moves papillae on her mantle in rhythmic, zipping motion.
The only known host fish for the larval stage are the Tippecanoe darter 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. After spending 1-2 weeks attached to the fish's gills, the larvae finish metamorphosing and drop off as juvenile mussels.[7]
Its lifespan has been estimated at around 20 years.[7]
Conservation
editThe species is listed as Endangered in the United States under the Endangered Species Act. It is listed as Endangered in Canada on the List of Wildlife Species at Risk under the Canadian Species at Risk Act.[8]
The species' recent success has been mixed. A study found robust populations in locations in Ohio and New York in 2024,[9] 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.[10]
References
edit- ^ "Villosa fabalis". NatureServe. Retrieved 2011-12-02.
- ^ "Molluscabase - Paetulunio Watters, 2018". www.molluscabase.org. Retrieved 2023-09-21.
- ^ "MUSSELpdb | valid gen. Paetulunio species". mussel-project.uwsp.edu. Retrieved 2023-09-21.
- ^ a b c Committee on the Status of Endangered Wildlife in Canada. "Rayed bean (Villosa fabalis): COSEWIC assessment and status report 2010". Retrieved 31 August 2024.
- ^ USFWS. "Species profile for rayed bean". Retrieved 30 August 2024.
- ^ Grabarkiewicz, Jeff (2012). "Habitat Use and Community Structure of Unionid Mussels in Three Lake Erie Tributaries". Thesis for Masters of Science. doi:10.13140/RG.2.1.2256.7762.
- ^ a b USFWS. "Species Status Assessment Report for the Rayed Bean". Retrieved 30 August 2024.
- ^ COSEWIC. 2005. Canadian Species at Risk. Committee on the Status of Endangered Wildlife in Canada. 64 pp., page 13.
- ^ Ford, David F.; Grabarkiewicz, Jeff; Benshoff, Adam; Foltz, David A.; Kriege, Mitchell; Spaeth, John (2024-05-15). "Population Demographic Data from Four Populations of the Federally Endangered Rayed Bean, Paetulunio (Villosa) fabalis (Mollusca: Unionidae)". Freshwater Mollusk Biology and Conservation. 27 (1). doi:10.31931/fmbc-d-22-00008. ISSN 2472-2944.
- ^ USFWS. "Rayed Bean 5-Year Review 2023" (PDF). Retrieved 31 August 2024.
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