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

Abstract

ABSTRACT The microanatomy and ultrastructure of the sixth gill of the fresh-water shrimp Macrobrachium olfersii was examined in shrimps maintained either in fresh water or acclimated to a high salinity medium (HSM, sea water of 21‰) for 10 days. At its base in the gill shaft, 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 cells disposed perpendicularly above and below the septum. The pillar cells possess apical microvilli apposed to the cuticle, while their basal regions are strongly attached to the septal cells by areas of desmosomal contact. Wide apical flanges spread radially from the columnar pillar cells as sheets of thin cytoplasm of about 850 nm in thickness. The flanges also exhibit microvilli, and adjacent flanges are connected by areas of septate junctions; there is no elaboration of the basolateral membrane. The pillar cells and their flanges are overlaid by a thin cuticle (175 nm thick), resulting in a hemolymph-water diffusion barrier of about 1 μm in the region of the flanges. The septal cells are rich in mitochondria and exhibit numerous infoldings of their plasma membrane which individually envelop each mitochondrion, probably providing an energy source for active salt transport through the pillar cells. The gill can thus be considered a mixed function gill with a dual role in gas exchange and salt transport. The 10-day acclimation period to HSM induced changes in this basic arrangement, quantified by morphometric analysis. The surface density of the plasma membrane infoldings increased significantly from 2.01 ± 0.15 μm2 membrane/μm3 cytoplasm in shrimps maintained in fresh water to 2.91 ± 0.19 μm2 membrane/μm3 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 invagination). The mitochondrial volume fraction (12.56 ± 0.69 cf. 14.26 ± 0.71%) was unchanged as was the length to width ratio of the mitochondrial profiles (2.74 ± 0.17 cf. 3.23 ± 0.18). The data are discussed in terms of the significance of the physical coupling between membrane invaginations and mitochondria in the septal cells, and in terms of the possible evolution of the lamellar structure in the phyllobranchiate gills of the Palaemonidae.