Acquired immunity to amyloodiniosis is associated with an antibody response

The dinoflagellate Amyloodinium ocellatum, which causes amyloodiniosis or 'marine velvet disease', is one of the most serious ectoparasitic diseases plaguing warmwater marine fish culture worldwide. We report that tomato clownfish Amphiprion frenatus develop strong immunity to Amyloodinium ocellatum infection following repeated nonlethal challenges and that specific antibodies are associated with this response. Reaction of immune fish antisera against dinospore and trophont-derived antigens in Western blots indicated both shared and stage-specific antibody-antigen reactions. A mannan-binding-protein affinity column was used to isolate IgM-like antibody from A. frenatus serum. The reduced Ig consisted of one 70 kD heavy chain and one 32 kD light chain with an estimated molecular weight of 816 kD for the native molecule. Immunoglobulin (Ig) isolated from immune but not non-immune fish serum significantly inhibited parasite infectivity in vitro. An enzyme-linked immunosorbent assay (ELISA) was developed using polyclonal rabbit antibody produced against affinity-purified A. frenatus Ig. Anti-Amyloodinium serum antibody was not always detectable in immune fish, although serum antibody titers in immune fish increased after repeated exposure to the parasite. These results suggest that there may be a localized antibody response in skin/gill epithelial tissue, although antibody was rarely detected in skin mucus.