Life History and Biology
DESCRIPTION: The rayed bean is a small mussel usually less than 1.8 inches in length. Shell is thick and solid and the outline is elongate or ovate in males and elliptical in females, and moderately inflated in both sexes, but more so in females. Surface texture is smooth with a green, yellowish-green, or brown color, with numerous wavy dark-green rays of various widths. The color of the nacre (mother-of-pearl) is silvery white or bluish and iridescent. Key characters useful for distinguishing the rayed bean from other mussels is its small size, thick valves, unusually heavy teeth for a small mussel, and color pattern.
RANGE: The rayed bean was historically known from 106 streams, lakes, and some man-made canals in Illinois, Indiana, Kentucky, Michigan, New York, Ohio, Pennsylvania, Tennessee, Virginia, West Virginia, and Ontario in parts of the upper Lake Michigan drainage. Rayed bean are gone from 76% of the historic streams and this species no longer occurs in hundreds of miles of former habitat in the Maumee, Ohio, Wabash, and Tennessee Rivers and their tributaries. Rayed bean are currently known from ten streams in the lower Great Lakes system and 15 streams and one lake in the Ohio River system, distributed in Indiana, Michigan, New York, Ohio, Pennsylvania, West Virginia (reintroduced), and Ontario. Relatively few of the remaining 25 streams are thought to harbor populations large enough to assure long-term viability.
In Pennsylvania the rayed bean are largely restricted to the middle Allegheny River, French Creek and lower reaches a few of their tributaries.
HABITAT: The rayed bean habitat is varied and includes small to medium sized streams to larger rivers and lakes. The species often occurs in or near shoal or riffle areas, deep slow runs and in the shallow, wave-washed areas of glacial lakes. Substrates typically include gravel and sand. Rayed bean are sometimes associated with the roots of vegetation in and adjacent to riffles and shoals, but the species also lives in relatively deep-water (10 to 20 feet) and sparsely vegetated habitat.
REASONS FOR CURRENT STATUS: The decline of the rayed bean is primarily the result of habitat loss and degradation. Chief among the causes of decline are impoundments, channelization, chemical contaminants, mining, exotic species, and sedimentation. The majority of the remaining populations of the rayed bean are generally small and geographically isolated. The patchy distributional pattern of populations in short river reaches makes them much more susceptible to extirpation from single catastrophic events, such as toxic chemical spills. Furthermore, this level of isolation makes natural repopulation of any extirpated population impossible without human intervention.