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The ultrastructure of the pillar cells in the gill lamellae of the freshwater shrimp Macrobrachium olfersii was examined to evaluate the routes of salt and water movement across the gill epithelium and into the hemolymph. Alterations were morphometrically quantified in shrimp maintained in fresh water (FW, &lt;0.5{permill} salinity) and after acclimation to saline media (21{permill} or 28{permill} salinity). The tissue interface between the hemolymph and the external medium consists exclusively of the thin apical flange regions of the pillar cells, the upper membrane of which is highly amplified by dense microvilli and overlain by a thin cuticle. The lower flange membrane, bathed by the hemolymph, is smooth and not invaginated. Contiguous flanges are strongly bound by junctional structures including desmosomes and septate junctions. The basal surface of the pillar cell perikaryon is linked to the adjacent septal cells through many basolateral junctions. The septal cell plasmalemma is abundantly and deeply invaginated, each infolding enclosing numerous mitochondria; these characteristics are typical of salt-transporting machinery. After shrimps were acclimated to saline media for 10 days, the thickness of the pillar cell flanges was significantly reduced (from 1.3 to {approx}0.4 {mu}m), as was the height (from 0.8 to 0.3 {mu}m) and density (from 4.0 to {approx} 1.8 microvilli/{mu}m) of the apical microvilli. This reduction in the apical surface area of the pillar cells appears to lead to decreased ionic permeability, concomitant with a reduction in Na+/K+-ATPase activity, thus limiting Na+ uptake. In contrast to the brachyurans, in which the respiratory and ion-transporting mechanisms are differentially located in the anterior and posterior gills, in palaemonid shrimps the pillar cells apparently play a dual role: ions move preferentially through ion transporters in the microvilli above the pillar cell perikaryon, while respiratory gases are exchanged through the fine flange regions in contact with the hemolymph.

biological bulletin

The Route of Ion and Water Movements Across the Gill Epithelium of the Freshwater Shrimp <i>Macrobrachium olfersii</i> (Decapoda, Palaemonidae): Evidence From Ultrastructural Changes Induced by Acclimation to Saline Media

The Route of Ion and Water Movements Across the Gill Epithelium of the Freshwater Shrimp Macrobrachium olfersii (Decapoda, Palaemonidae): Evidence From Ultrastructural Changes Induced by Acclimation to Saline Media