ULTRASTRUCTURAL EVIDENCE FOR OSMOREGULATORY FUNCTION OF THE STERNAL EPITHELIA IN SOME GAMMARIDEAN AMPHIPODS

Abstract

ABSTRACT The ion-permeabilities and the ultrastructure of the sternal epithelia were studied in 7 species of gammaridean amphipods. Evidence has been obtained that the sterna can be divided into 2 categories: ion-transporting and non-ion-transporting types. The sterna of Sternomoera japonica, Grandidierella japonica, Melita setiflagella, and Corophium uenoi, belonging to the former category, are covered by ion-permeable cuticular layers lined with a thick epithelium, which is characterized by both a shallow apical infolding system (AIS) and a well-developed basolateral infolding system (BIS). The AIS is composed of frequent lamellar infoldings without mitochondrial association originating from the apical sides of the epithelia and involves virtually every part of the apical cell membrane, forming elaborate intercellular channels in the subcuticular space. In contrast, the BIS consists of deep infoldings of the basolateral cell membranes and extensive interdigitations between epithelial cells, which are both associated with a large number of mitochondria. This BIS involves the greater part of the cell membrane and constitutes a complicated, giant labyrinth of the intercellular spaces above the basal lamina. The AIS and BIS never communicate in the sternal epithelia, though they sometimes interpenetrate. On the other hand, the sterna of Jesogammarus jesoensis, Jesogammarus hinumensis, and Haustorioides sp., belonging to the latter category, are composed of an ion-impermeable, thick cuticular layer and a thin epithelium characterized by a paucity of mitochondria and complete lack of the BIS and AIS, which are common to those of the ordinary epithelium of the body surface. The results suggest a rather widespread occurrence in amphipods of a variety of extrabranchial ion-permeable areas of the body surface. They also suggest that the sterna lined with thick, specialized epithelia, together with the gills, are involved in the active transport of electrolytes in osmoregulation as well as respiration.