The rock pocketbook, Arcidens confragosus (Say 1829), is a large, dark green to black, knob-ridged, fairly thin-shelled mussel that is found in medium sized rivers in a variety of substrates. Its uniquely sculptured, but relatively thin shell distinguishes it from other mussels. The rock pocketbook is found in the Mississippi River and its tributaries. Its range extends as far west and south as Texas, east to Ohio, and to north to Minnesota. However, its abundance varies widely across its geographic range and is uncommon in many states compared to other native mussels. Embryos are brooded from September to June in the female's gills, where they develop into tiny larvae called glochidia. Once the glochidia are expelled from the female's gills, they attach to fish gills or fins by clamping onto them with their valves. The glochidia live as parasites on the host fish until they develop into juvenile mussels, at which point they detach from the fish and fall to the streambed as free-living mussels. Factors that threaten the viability of the rock pocketbook include pollution, habitat disruption and the introduction of non-native aquatic species.
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