ROGERS-LOWERY, C. L.; DIMOCK, R. V., Jr.; Wake Forest University: Encapsulation of the glochidium larva of freshwater mussels on na�ve and resistant host fish

In order to metamorphose into juveniles and subsequently mature into adults, the larvae (glochidia) of freshwater mussels in the order Unionoida must temporarily parasitize the gills, fins, or other external structures of fish. Once attached to the fish, the glochidium is encapsulated by host fish epithelial tissue. We have recently demonstrated that bluegill sunfish (Lepomis macrochirus) develop acquired resistance to glochidial larvae of Utterbackia imbecillis after 2 infections, resulting in reduction of successful metamorphosis during the 3rd and 4th infections and significantly fewer glochidia attaching to fish during the 4th infection. Since the mechanism resulting in acquired resistance is not fully understood, one possible mechanism, differences in the nature of cyst formation on na�ve and resistant fish, was examined. The time course of the formation of cysts on fins of na�ve and resistant fish was monitored by scanning electron microscopy and by time-lapsed video microscopy. The development of cysts was scored according the relative surface area of the glochidia covered by the migration of host epithelial tissue at intervals up to 6 hours post infection. Compared to na�ve fish, cyst formation on resistant fish was significantly delayed. By 6 h post infection, 48.0% of the glochidia on na�ve fish were completely covered by cysts, compared to only 9.6% on resistant fish. Additionally, the cysts on resistant fish were morphologically irregular as compared to those on na�ve fish. The delay and irregularities in cyst formation on resistant fish may result in prolonged exposure of glochidia to innate immune mechanisms in fish mucus and to unsuitable external environment, resulting in increased mortality and reduced success of metamorphosis, and may represent one mechanism of acquired resistance.