FINE-STRUCTURE OF THE GILLS OF THE FRESH-WATER SHRIMP MACROBRACHIUM-OLFERSII (DECAPODA) - EFFECT OF ACCLIMATION TO HIGH SALINITY MEDIUM ANDEVIDENCE FOR INVOLVEMENT OF THE LAMELLAR SEPTUM IN ION UPTAKE

The microanatomy and ultrastructure of the sixth gill of the fresh-wat er shrimp Macrobrachium olfersii was examined in shrimps maintained ei ther in fresh water or acclimated to a high salinity medium (HSM, sea water of 21parts-per-thousand) for 10 days. At its base in the gill sh aft, each phyllobranchiate lamella possesses a central afferent vessel and 2 lateral efferent vessels. The lamella exhibits a nonfenestrated , medial septum over its full extension which divides the lamella into 2 compartments through which hemolymph flow is directed by pillar cel ls disposed perpendicularly above and below the septum. The pillar cel ls possess apical microvilli apposed to the cuticle, while their basal regions are strongly attached to the septal cells by areas of demosom al contact. Wide apical flanges spread radially from the columnar pill ar cells as sheets of thin cytoplasm of about 850 nm in thickness. The flanges also exhibit microvilli, and adjacent flanges are connected b y areas of septate junctions; there is no elaboration of the basolater al membrane. The pillar cells and their flanges are overlaid by a thin cuticle (175 nm thick), resulting in a hemolymph-water diffusion barr ier of about 1 mum in the region of the flanges. The septal cells are rich in mitochondria and exhibit numerous infoldings of their plasma m embrane which individually envelop each mitochondrion, probably provid ing an energy source for active salt transport through the pillar cell s. The gill can thus be considered a mixed function gill with a dual r ole in gas exchange and salt transport. The 10-day acclimation period to HSM induced changes in this basic arrangement, quantified by morpho metric analysis. The surface density of the plasma membrane infoldings increased significantly from 2.01 +/- 0.15 mum2 membrane/mum3 cytopla sm in shrimps maintained in fresh water to 2.91 +/- 0.19 mum2 membrane /mum3 cytoplasm in those acclimated to HSM. The infoldings themselves became arranged in stacked layers separating the mitochondria (7.07 +/ - 1.51 cf. 13.73 +/- 2.48 test segments intersecting more than one inv agination). The mitochondrial volume fraction (12.56 +/- 0.69 cf. 14.2 6 +/- 0.71%) was unchanged as was the length to width ratio of the mit ochondrial profiles (2.74 +/- 0.17 cf. 3.23 +/- 0.18). The data are di scussed in terms of the significance of the physical coupling between membrane invaginations and mitochondria in the septal cells, and in te rms of the possible evolution of the lamellar structure in the phyllob ranchiate gills of the Palaemonidae.