Fine structure of the kidneys of osmotically stressed Mytilus, Mercenari, and Anodonta

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Abstract
Osmotically induced ultrastructural changes in the kidneys of the freshwater bivalve Anodonta and the marine bivalves Mytilus and Mercenaria were studied. Osmotic stresses were given to Anodonta by keeping them in distilled water or in 6% seawater, and to Mytilus and Mercenaria by keeping them in 50% seawater for various periods. In all of these bivalves, the convoluted, single cell layered kidney epithelia displayed wide lateral intercellular spaces as well as extracellular spaces in the basal membrane infoldings during hyposmotic stress. These spaces were greatly reduced when the animals were kept in isosmotic media (i.e., isosmotic to their respective hemolymphs). The kidney cells contained abundant cytoskeletal elements and microfilaments were often observed in bundles in the basal membrane infoldings. Actin was observed in the basal membrane infoldings using the specific fluorescent stain nitrobenzoxadiazole-phallacidin. The cell contacts of the kidney epthelia were studied in platinum replicas of freeze-fractured tissues. The lateral cell membrane and basal membrane infoldings contained many gap junctions. Many rows of dense intramembrane particles of septate junctions were observed in the kidneys of animals from isosmotic media. The septate junctions in the kidneys of aminals from hyposmotic media contained either fewer intramembrane particle rows or many sinuous intramembrane particle rows. The site of prourine formation in mollusks are discussed.