2 ULTRASTRUCTURALLY DISTINCT TYPES OF TRANSPORTING TISSUES, THE BRANCHIOSTEGAL AND THE GILL EPITHELIA, IN AN ESTUARINE TANAID, SINELOBUS-STANFORDI (CRUSTACEA, PERACARIDA)

On each lateral side of the cephalothorax segments, the adult Sinelobu s stanfordi has a branchial chamber which contains an elongated bag-sh aped gill and is covered by a thick branchiostegite. The ultrastructur al study revealed that the inner surface of the branchiostegite is com posed of a transporting-type epithelium which is morphologically disti nct from the gill epithelium. Both epithelia are covered by extremely thin (about 80 nm) cuticle layers, suggesting high permeability to gas es, ions, and water. In contrast, the outer surface of the branchioste gite consists of ordinary epithelium covered by a very thick cuticle l ayer in common with the body surface. The inner branchiostegite epithe lium (4-10 mu m thick) has a shallow (about 1 mu m deep) apical infold ing system of the cell membrane (AIS) and an extensive three-dimension al tubular network (about 120 nm in diameter) which is formed by the i nvagination of the basolateral cell membrane (TNB). The TNB is associa ted with slender mitochondria and occupies the majority of the cytopla smic area of the epithelial cell. The gill epithelium, on the other ha nd, is about 10 mu m thick and characterized by an abundance of oval m itochondria, well-developed (4-5 mu m deep) AIS, and a smooth basal ce ll membrane lacking any infoldings. These morphological features indic ate that not only the gill epithelia, but also the inner branchiostega l epithelia, are involved in the ion-transporting processes. The ultra structural differences between these two kinds of epithelia also sugge st their different roles in the osmoregulation of this animal, since i t inhabits estuaries which are subject to extreme changes in salinity.