Localization and distribution of antigens related to calcium-rich deposits in the gills of several freshwater bivalves

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Abstract
In the gills of freshwater unionid mussels, calcium concretions may account for up to 50% of the dry weight of the gill and are composed primarily of calcium phosphate with an associated glycoprotein. The role of these structures in relation to calcium physiology of bivalves is unknown. A study of the origin and distribution of these structures and their possible relationship to specific organ systems or tissue types was undertaken. An antiserum to decalcified concretions was prepared and utilized in indirect immunofluorescence studies on fresh-frozen sections of excised gill. Electron microscopy was employed to examine areas of interest revealed by the immunofluorescence microscopy. Calcium concretions provided the bulk of the fluorescence when the sections were examined. In addition to the concretions, other structures including the chitinous rods of the gill filaments, and nerve tracts intimately associated with the concretions also bound the antiserum, suggesting the possibility of some shared antigens. Some previously undescribed cells, also found to react with the antisera, were shown by electron microscopy to contain what appears to be concretion material in stages of assembly and likely represent the synthesis site of the concretions in the gill. Antisera were produced from isolated concretions of two different genera; each reacted similarly when tested against tissues from several different unionid species. These data suggest a similar composition for calcium phosphate concretions within the Unionidae.