THE ULTRASTRUCTURE OF THE STERNAL GILLS FORMING A STRIKING CONTRAST WITH THE COXAL GILLS IN A FRESH-WATER AMPHIPOD (CRUSTACEA)

Sternomoera yezoensis has specialized sterna with 21 sternal gills in addition to six pairs of coral gills. Despite a common high permeabili ty to chloride ions, the epithelia of these two kinds of gills are dia metrically opposed in the polarity of the cell membrane-mitochondria c omplex. The coral gill epithelium (4-6 mu m thick) is characterized by a well developed AIS (apical infolding system) associated with a huge number of large mitochondria. The AIS exceeds two-thirds of the epith elial thickness and forms a highly sophisticated, subcuticular labyrin th. On the contrary, the sternal gill epithelium, an extension of the sternal epithelium proper, is extremely thick (10-15 mu m) and is char acterized by a very deep BIS (baso-lateral infolding system) associate d with numerous slender mitochondria. The BIS reaches nine-tenths of t he epithelial thickness and forms a giant, baso-lateral labyrinth. Sha llower, less elaborate AIS and BIS without mitochondrial association o riginate from the opposite sides of these epithelia. Although AIS and BIS interpenetrate in the sternal gill epithelium, they never communic ate. The results indicate that in addition to the coral gills, the ste rna with the sternal gills function as transporting as well as respira tory organs, though the functional difference between these two kinds of gills remains to be elucidated.