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The Bivalvia — Proceedings of a Memorial Symposium in Honour of Sir Charles Maurice Yonge, Edinburgh, 1986. (Edited by Brian Morton). Hong Kong University Press, Hong Kong, 1990. OSMOTIC EFFECT S O N TH E FIN E STRUCTUR E O F THE KIDNEYS AN D HEART S O F SOM E BIVALVES : TH E SIT E OF URIN E FORMATIO N H.R. Kha n an d A.S.M. Saleuddi n Department of Biology, York University, North York, Ontario M3J 1P3, Canada ABSTRACT The fin e structur e o f th e heart s an d kidney s o f osmoticall y stresse d marin e Mytilus, Mercenaria and freshwater Anodonta were studied. Osmotic stresses were given to Mytilus and Mercenaria by keeping them in 50% sea water, and to Anodonta by keeping them in distilled water and 6% sea water for various periods. Epicardial cells of the auricle and the ventricle in these species are not podocyte-like cells. Epicardial cells of the auricle and the ventricle of Anodonta and Mercenariaform a thin epithelium, in which the cells are apically attached wit h zonul a adherente s an d wel l forme d zonula-typ e (bel t junction ) septat e junctions. The auricle of Mytilus varies from the other two species by the presence of surface pericardial glands that cover most of its surface. The auricle wall of this animal also contains cuboidal epicardium except where the pericardial glands are attached. The tubular pericardial glands consis t o f podocyte-like cells . These cells ar e also dispersed i n th e auricl e wal l underneath th e epicardial cells . The podocyte-like cell s stai n for iro n i n Perl's reaction . Electron dens e particles from 5-1 0 n m in diameter ar e observed i n granules an d in the extracellular space s of the foot-processes o f these cells. The podocyte-like cell s display endocytotic profiles. These cells are similar to ubiquitous molluscan pore cells. The kidney is composed of a convoluted single cell layered epithelium. The lateral intercellular spaces and the spaces between the basal membrane infoldings of kidney epithelium are influenced by osmotic stress. The role of the podocyte-like cells, hydrostatic pressure and the kidney epithelium in urine formation is discussed. INTRODUCTION Sir Maurice Yong e describe d th e bivalves a s ' . . . most sluggis h animal s an d have withdrawn from contact with their surroundings to the extreme extent of losing the head and substituting sense-organ s i n th e mantl e margi n fo r thos e originall y possesse d o n th e head.. .oyster has taken matters still further by its permanent attachment...' (Yonge, 1960). 238 H.R. KHA N AN D A.S.M. SALEUDDI N Osmoregulation in these filter-feeding, sluggish and attached species like oyster and Mytilus could be intricate, since they cannot readily avoi d adverse water conditions. Freshwater molluscs are osmoreguiators and the brackish water and marine species are osmoconformers. However, when brackish water and marine molluscs are exposed to media of different ioni c composition they display some capacity of ion regulation (Burton, 1983). Physiological and ultrastructural studies suggest that in many molluscs primary urine (prourine) i s formed b y ultrafiltration (Potts, 1967 ; Martin, 1983) . The exact sit e of thi s process remains controversial. Picken (1937) proposed that the heart is the primary site of urine formation. It is generally believed that in bivalves the hemolymph is ultrafiltered across the cardiac tissue into the pericardial space by the hydrostatic pressure of the hemolymph (Florey and Cahill, 1977 ; Jones and Peggs, 1983; Hevert, 1984) . The presence of reverse hydrostatic gradient, which may prevent the ultrafiltration of hemolymph across the wall of heart into the pericardial cavity, has also been reported (Pierce, 1970; Tiffany, 1972; Willmer, 1978). Several ultrastructural studies have revealed the presence of podocyte-like cells (pore cells) in the auricle and pericardial glands of many molluscs, and thus suggested that these cells are involved in ultrafiltration (Andrews , 1976, 1979; Pirie and George, 1979; Martin, 1983; Meyhofer etal., 1985) . Hawkins etal. (1980) and Watts etal. (1981) also reported the presence o f podocyte-lik e cell s i n tw o bivalve s bu t wer e cautiou s abou t thei r rol e i n ultrafiltration. In the present paper we report the fine structure of heart and kidney of three osmotically stressed bivalves. The podocyte...

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