PEREOPODAL DISK - A NEW-TYPE OF EXTRABRANCHIAL ION-TRANSPORTING ORGANIN AN ESTUARINE AMPHIPOD, MELITA-SETIFLAGELLA (CRUSTACEA)

A discoid organ, 'pereopodal disk (PD)', was found on the medial surfa ce of the basipodite of each pereopod, except the third and the fourth , in an estuarine amphipod, Melita setiflagella. The silver methods sh owed that PD is an extrabranchial ion-permeable area of the body surfa ce. The ultrastructural study revealed that PD is covered by a thin an d soft cuticle layer suggesting high permeability to gases and ions, a nd is composed of a thick, transporting-type epithelium. This epitheli um is characterized by deep basal infolding systems (BIS) of cell memb ranes exceeding two-thirds of the epithelial thickness and complicated interdigitations between adjacent epithelial cells, both associated w ith many mitochondria. Apical infolding systems (AIS) are shallow and not accompanied by any mitochondria. These characteristics resemble th ose of the sternal epithelia and form a striking contrast in the polar ity of the infoldings to the gill epithelia, which are characterized b y a well-developed AIS and sparse BIS. The results suggest that this u nique organ may be involved in the active transport of electrolytes to maintain constant osmotic pressures of the body fluids under widely f luctuating salinities of the estuarine environments.