Western fanshell, Cyprogenia aberti. 

The fanshells have specially modified marsupial gills that produce elongate, wormlike conglutinates.  The conglutinates are composed primarily of sterile eggs.   Fertile eggs are located on the surface and rupture to release the glochidia when the conglutinate is bitten by a host fish.  The release of each conglutinate is a prolonged process, and fish may often attack the conglutinate while it is still trailing from the female mussel's excurrent siphon.  The conglutinates are durable and can withstand considerable "handling".   Logperch and fantail darters are suitable hosts for the glochidia.

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Cyprogenia aberti from the Spring River, Cherokee Co, KS. C. aberti is the only congener of the Federally endangered C. stegaria.  Both are rare. Diagram of Cyprogenia showing the coiled marsupial gills (Lefevre and Curtis 1912) The wormlike conglutinates of C. aberti are up to 5 cm long.
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Close-up of the "head" end Ranks of fertile eggs bearing glochidia are arrayed along the edges. The conglutinate is composed primarily of sterile eggs Transmitted light shows regions of sterile and fertile eggs more clearly
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The glochidia of Cyprogenia do not gape widely Reniform shape of the closed glochidia is distinctive.  Darters contemplating a conglutinate as it is released by a female fanshell Fanshell glochidia encysted on the gills of logperch (Percina caprodes).

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