Persistence of host response against glochidia larvae in Micropterus salmoides.

Host fish acquire resistance to the parasitic larvae (glochidia) of freshwater mussels (Unionidae). Glochidia metamorphose into juvenile mussels while encysted on host fish. We investigated the duration of acquired resistance of largemouth bass, Micropterus salmoides (Lacepède, 1802) to glochidia of the broken rays mussel, Lampsilis reeveiana (Call, 1887). Fish received three successive priming infections with glochidia to induce an immune response. Primed fish were held at 22-23 degrees C and were challenged (re-infected) at intervals after priming. Metamorphosis success was quantified as the percent of attached glochidia that metamorphosed to the juvenile stage and were recovered alive. Metamorphosis success at 3, 7, and 12 months after priming was significantly lower on primed fish (26%, 40%, and 68% respectively) than on control fish (85%, 93%, and 92% respectively). A second group of largemouth bass was similarly primed and blood was extracted. Immunoblotting was used to detect host serum antibodies to L. reeveiana glochidia proteins. Serum antibodies were evident in primed fish, but not in naive control fish. Acquired resistance of host fish potentially affects natural reproduction and artificial propagation of unionids, many of which are of conservation concern.

Cross-resistance of largemouth bass to glochidia of unionid mussels.

We tested whether host fish that acquired resistance to glochidia of one mussel species were cross-resistant to glochidia of other species. Largemouth bass (Micropterus salmoides) were primed with 4-5 successive infections of glochidia of Lampsilis reeveiana. The percentage of attached glochidia that survived and transformed to the juvenile stage (transformation success) was compared between primed fish and naïve controls. Transformation success of L. reeveiana, Lampsilis abrupta, Villosa iris, and Utterbackia imbecillis was significantly lower on primed fish (37.8%, 43.5%, 67.0%, and 13.2%, respectively) than on control fish (89.0%, 89.7%, 90.0%, and 22.2% respectively). Immunoblotting was used to analyze the binding of serum antibodies from primed fish with glochidia proteins. Antibodies bound to glochidia proteins of similar molecular weight from L. reeveiana and L. abrupta. Bound proteins of V. iris differed in molecular weight from those of the Lampsilis species. There was no binding to specific glochidia proteins of U. imbecillis or Strophitus undulatus. Our results indicate that host-acquired resistance can extend across mussel genera and subfamilies and might involve both specific and nonspecific mechanisms. Understanding the specificity of acquired resistance of hosts to glochidia could enhance understanding of the evolutionary and ecological relationships between mussels and their host fishes.