Pereopodal disk: a new type of extrabranchial ion-transporting organ in an estuarine amphipod, Melita setiflagella (crustacea)

Abstract

A discoid organ, ‘pereopodal disk (PD)’, was found on the medial surface of the basipodite of each pereopod, except the third and the fourth, in an estuarine amphipod, Melita setiflagella. The silver methods showed that PD is an extrabranchial ion-permeable area of the body surface. The ultrastructural study revealed that PD is covered by a thin and soft cuticle layer suggesting high permeability to gases and ions, and is composed of a thick, transporting-type epithelium. This epithelium is characterized by deep basal infolding systems (BIS) of cell membranes exceeding two-thirds of the epithelial thickness and complicated interdigitations between adjacent epithelial cells, both associated with many mitochondria. Apical infolding systems (AIS) are shallow and not accompanied by any mitochondria. These characteristics resemble those of the sternal epithelia and form a striking contrast in the polarity of the infoldings to the gill epithelia, which are characterized by a well-developed AIS and sparse BIS. The results suggest that this unique organ may be involved in the active transport of electrolytes to maintain constant osmotic pressures of the body fluids under widely fluctuating salinities of the estuarine environments.