Western Fanshell

Like all mussels, Western Fanshell are omnivores that primarily filter feed on a wide variety of microscopic particulate matter suspended in the water column, including phytoplankton, zooplankton, bacteria, detritus, and dissolved organic matter (Haag 2012, p.26). Juveniles likely pedal feed in the sediment, whereas adults filter feed from the water column.

Western fanshell have a thick, compressed to moderately inflated, round to triangular shell that is up to 3 inches in length. The posterior ridge is prominent and raised with a shallow sulcus from umbo to middle of the ventral margin.

Western fanshell periostracum is a dull tan with a distinctive ray pattern from bands of tiny pigment flecks. The shell has a wrinkled or rough appearance.

Western fanshell typically prefer large creeks and rivers with good water quality, moderate to swift current and gravel-sand substrate.

The life cycle of western fanshell, like most freshwater mussels, is complex, relying on host fish for successful reproduction. Male mussels release their sperm into the water column where it is siphoned in by females. Once fertilization has taken place in the gills of the female mussel, mature microscopic glochidia, or larva, are released by the females where they must attach themselves to the gills, and or, fins of fish hosts to continue developing. Western fanshell are bradyticitc, meaning long-term, brooders that typically spawn from August to October and usually release their larvae in early spring.

Each mussel species has specific fish species, also called the host fish, that are needed by the glochidia to keep growing to ultimately transform into juveniles. After a few weeks of living as parasites, they drop off and land on the river bottom where they grow into adults. Western fanshell host fish include the logperch (Percina caprodes), slenderhead darter (Percina phoxocephala), fantail darter (Etheostoma flabellare), orangebelly darter (Etheostoma radiosum) and rainbow darter (Etheostoma caeruleum).