The nucleus occupies the basal region of the pillar cell perikaryon [ILLUSTRATION FOR FIGURE 2 OMITTED], which is coupled to the adjacent cells of the intralamellar septum by regions of thick, basolateral junctions [ILLUSTRATION FOR FIGURE 12 OMITTED].
The regions of contact between adjacent pillar cell flanges constitute highly structured junctional complexes [ILLUSTRATION FOR FIGURES 7 AND 11 OMITTED].
With the reduction in thickness of the pillar cell flanges, and to a lesser extent of the intralamellar septal cells, there is a corresponding, marked increase in the volume of the hemolymph lacunae ([ILLUSTRATION FOR FIGURE 3 OMITTED], cf [ILLUSTRATION FOR FIGURE 2 OMITTED]).
vannamei (Taylor and Taylor, 1992, [ILLUSTRATION FOR FIGURE 25A OMITTED]), and in the palaemonid Palaemonetes pugio (Doughtie and Rao, 1978), the pillar cell perikarya appear to constitute part of the intralamellar septum.
In FW-acclimated shrimps, the gill epithelium appears to be a tight epithelium, well protected from paracellular water and ion movements by extensive and characteristic junctions - constituted by a small desmosome and a lengthy septate junction - between adjacent pillar cell flanges.
The alterations occurring at the apical pillar cell interface, and other changes in the characteristics of the intralamellar septal cells, including the reduction in [Na.sup.+]/[K.sup.+]-ATPase activity, thus appear to reflect the structural transformations underlying the molecular mechanisms of long-term adaptation to hyperionic media in freshwater palaemonids, particularly those that restrict the uptake of [Na.sup.+].
Such salt-transporting gills exhibit a typical microanatomy: the highly flattened, cuticle-bounded gill lamellae essentially consist of a continuous layer of epithelial cells enclosing a narrow hemolymph space; linked pairs of sustaining pillar cells extend across this space between the opposing epithelial sheets, their apical flanges often forming part of the epithelium (Mantel and Farmer, 1983; Taylor and Taylor, 1986; Maina, 1990).