Villosa fabalis

Rayed Bean

FWS Focus

Overview

Characteristics
Overview

The rayed bean is one of the smallest freshwater mussels found in North America. Its common name is derived from its resemblance to a large bean covered with rays. The species was listed as endangered in 2012 due to the destruction and modification of its habitat, pollution, sedimentation, and non-native invasive species invasive species
An invasive species is any plant or animal that has spread or been introduced into a new area where they are, or could, cause harm to the environment, economy, or human, animal, or plant health. Their unwelcome presence can destroy ecosystems and cost millions of dollars.

Learn more about invasive species
, including the zebra mussel.

Scientific Name

Villosa fabalis
Common Name
rayed bean
FWS Category
Clams
Kingdom

Location in Taxonomic Tree

Genus
Species

Identification Numbers

TSN:

Characteristics

Characteristic category

Physical Characteristics

Characteristics
Physical Characteristics

Rayed bean spend most of their life in a small area of the stream bed. They are typically completely or partially buried in the substrate.  They are relatively sedentary though they do have the ability to move around with the use of their muscular foot.  Mussels insert their "foot" into the sand or gravel and pull themselves forward, inching their way along the bottom. Adult and juveniles may produce fibers that allow them to attach themselves to substrate particles.

Size & Shape

The rayed bean is a small freshwater mussel, usually less than 1.5 inches long. The species is sexually dimorphic, meaning males and females differ in appearance. The male’s shell shape is generally elongated, whereas the female’s is smaller and elliptical. The inside of the shell is white.

Color & Pattern

The rayed bean shell is smooth-textured and green, yellowish-green, or brown with numerous dark-green wavy lines. The inside of the shell is white.

Characteristic category

Life Cycle

Characteristics
Reproduction

The life cycle of the rayed bean, like most freshwater mussels, is unusual and complex. Males release sperm into the water column that is then siphoned by females to fertilize their eggs. Fertilized eggs develop into microscopic larvae, called glochidia, within special gill chambers. After brooding for up to 7 months, females expel mature glochidia, which then must attach to the gills or fins of specific host fish species to complete development into juvenile mussels. If successfully attached to a host fish, glochidia mature within a few weeks. Juvenile mussels then drop off and continue to grow, if they fall onto appropriate substrate. Using host fish allows the rayed bean to move upstream and populate habitats it could not otherwise reach.

Life Cycle

The rayed bean has a complicated life history that is tightly linked to freshwater fishes. Fertilized eggs develop into larvae, called glochidia, in the female mussels. Glochidia, when released from the female, must come in contact with a passing fish and attach to the gills, fins, or body of that fish. During this parasitic stage, the mussel glochidia are relatively harmless to their fish host. The mussel-host fish relationship helps disperse a basically immobile creature (the mussel), within and between aquatic systems. Rayed bean are "host specific" in that their glochidia can only survive on a specific species of fish. If a glochidium attaches to a fish that is not a suitable host species, it will not survive. After several weeks, the glochidia free themselves from the host, drift to the substrate and begin their lives as juvenile mussels.

Life Span

Information on the longevity of the rayed bean is lacking, but it is generally thought that they may live up to 20 years. The age at which they reach sexual maturity is unknown but it is estimated to occur at around age 3 or 4.

Characteristic category

Habitat

Characteristics
Habitat

The rayed bean generally lives in smaller, headwater creeks, but it is sometimes found in large rivers and wave-washed areas of glacial lakes. It prefers gravel or sand substrates, and is often found in and around roots of aquatic vegetation. Adults spend their entire lives partially or completely buried in substrate, filtering water through their gills to remove algae, bacteria, detritus, microscopic animals, and dissolved organic material for food.

River or Stream
Characteristic category

Food

Characteristics
Food

Rayed bean spend most of their life in a small area of the stream bed. They are typically completely or partially buried in the substrate.  They are relatively sedentary though they do have the ability to move around with the use of their muscular foot. Mussels insert their "foot" into the sand or gravel and pull themselves forward, inching their way along the bottom. Adult and juveniles may produce byssal threads that allow them to attach themselves to substrate particles.

Geography

Characteristics
Range

The rayed bean historically occurred in at least 115 streams, lakes, and some human-made canals in 10 States: Illinois, Indiana, Kentucky, Michigan, New York, Ohio, Pennsylvania, Tennessee, Virginia, and West Virginia; and Ontario, Canada.  The mussel occurred in parts of the upper (Lake Michigan drainage) and lower Great Lakes systems, and throughout most of the Ohio and Tennessee River systems.  During historical times, the rayed bean was fairly widespread and locally common in many Ohio River system streams based on collections made over a several-decade period. 

Extant populations of the rayed bean are known from 34 streams and 1 lake in seven States and 1 Canadian province: Indiana, Michigan, New York, Ohio, Pennsylvania, Tennessee, and West Virginia  and Ontario, Canada.

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