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

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-transp orting and non-ion-transporting types. The sterna of Sternomoera japon ica, Grandidierella japonica, Melita setiflagella, and Corophium uenoi , belonging to the former category, are covered by ion-permeable cutic ular layers lined with a thick epithelium, which is characterized by b oth a shallow apical infolding system (AIS) and a well-developed basol ateral infolding system (BIS). The AIS is composed of frequent lamella r infoldings without mitochondrial association originating from the ap ical sides of the epithelia and involves virtually every part of the a pical cell membrane, forming elaborate intercellular channels in the s ubcuticular 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 mi tochondria. This BIS involves the greater part of the cell membrane an d constitutes a complicated, giant labyrinth of the intercellular spac es above the basal lamina. The AIS and BIS never communicate in the st ernal epithelia, though they sometimes interpenetrate. On the other ha nd, 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 charac terized 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 amphipo ds of a variety of extrabranchial ion-permeable areas of the body surf ace. They also suggest that the sterna lined with thick, specialized e pithelia, together with the gills, are involved in the active transpor t of electrolytes in osmoregulation as well as respiration.