Additional Ultrastructural Observations of the Gill Epithelium of the Water Flea Daphnia Magna with Reference to Ionic and Macromolecular Transport

Abstract

The ultrastructural features of the gill epithelium of adult Daphnia magna are consistent with
their dual function of ion transport from the surrounding medium to the hemolymph and transport of
macromolecules from the hemolymph towards the cuticle. The thin cuticle of the gill epithelium displays
short, thin epicuticular tubercles and pits. Silver grains penetrate the cuticle in AgNO3 treated specimens.
The single layer of flat epithelial cells is of one type only, the dark cells. The epithelial cells display extensive
infoldings of the apical and basal plasma membranes associated with mitochondria and delicate, folded
lateral membranes enclose narrow intercellular spaces. They have large irregular nuclei with prominent
nucleoli. Their cytoplasm is rich in mitochondria, rough and smooth endoplasmic reticulum, microtubules,
and vesicles, while Golgi complexes are sparse. The basal cytoplasm displays dense tubular elements, coated
vesicles, multivesicular bodies, and lysosomes. These ultrastructural features are characteristic of ion
transporting epithelia and of cells engaged in protein and lipid synthesis.