Responses of host fish to ectoparasitic larvae of freshwater mussels (Bivalvia: Unionidae).

摘要

The larvae of most freshwater mussels are obligate parasites on fish, on which they attach to fins and gills and metamorphose into juveniles. Since there is a growing body of evidence that fish may become immune to glochidia, the effects of multiple infections of bluegill sunfish (Lepomis macrochirus ) by the glochidia of the freshwater mussel Utterbackia imbecillis on the host-parasite relationship were examined. During the third and fourth infections, the fish exhibited resistance to glochidia in that the success of metamorphosis was decreased as compared to the first two infections and controls and decreased total number of glochidia successfully attaching to fish during the 4th infection.; Because the mechanism resulting in acquired resistance is not fully understood, possible mechanisms, including differences in cyst formation, antibody-mediated immune response, and changes in mucous protease activity, were examined. Once attached to the gills or fins of the fish, the glochidium is encapsulated by host fish epithelial tissue. Compared to naive fish, cyst formation on resistant fish was delayed and was morphologically irregular and may result in prolonged exposure of glochidia to innate immune mechanisms in fish mucus and to unsuitable external environmental factors, resulting in increased mortality and reduced success of metamorphosis. Additionally, fish exhibited an initial antibody response in serum and mucus around day 20 after the 1st infection, which was followed by second antibody response beginning at the end of the 3rd infection. Sera collected from fish at during the primary response contained antibodies against proteins that were present in granular structures located between the mantle tissue and shell of the larval mussel. However, sera collected from fish during the secondary antibody response recognized additional proteins that were present in cells of the larval mantle, most prominently in a ciliated region that will develop into the gills and organs of the juvenile and adult mussel. Finally, when a glochidium attaches to a fish, it is potentially exposed to innate immune factors in the fish skin mucus, including proteases. Treatment with a suite of protease inhibitors revealed that the proteases in the mucus of naive fish are primarily serine proteases, similar to trypsin. However, proteases in the mucus of fish infected 4 times with glochidia were primarily metalloproteases. Overall protease activity was lower in mucus from 4th infection fish as compared to naive fish, and protease levels varied inversely with the antibody response during the course of study. This study suggests that alterations in protease activity may play a role in acquired resistance of fish against glochidia by influencing cyst formation.