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 coxal gills. Despite a common high permeability to chloride ions, the epithelia of these two kinds of gills are diametrically opposed in the polarity of the cell membrane-mitochondria complex. The coxal gill epithelium (4-6 microm 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 epithelial thickness and forms a highly sophisticated, subcuticular labyrinth. On the contrary, the sternal gill epithelium, an extension of the sternal epithelium proper, is extremely thick (10-15 microm) and is characterized by a very deep BIS (basolateral infolding system) associated with numerous slender mitochondria. The BIS reaches nine-tenths of the epithelial thickness and forms a giant, baso-lateral labyrinth. Shallower, less elaborate AIS and BIS without mitochondrial association originate from the opposite sides of these epithelia. Although AIS and BIS interpenetrate in the sternal gill epithelium, they never communicate. The results indicate that in addition to the coxal gills, the sterna with the sternal gills function as transporting as well as respiratory organs, though the functional difference between these two kinds of gills remains to be elucidated.