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, including the zebra mussel.
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.
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.
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.
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.
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.
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.
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.
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.
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