Aglomerularism in Harpagifer bispinis: a subantarctic notothenioid fish living at reduced salinity

We investigated the renal morphology, histology and ultrastructure of Harpagifer bispinis, as a first step toward understanding the morpho-functional basis of its adaptation to potentially freezing brackish seawater. Fish were separated into two groups of ten individuals each, and acclimated to 2 and 38 salinity. A study of complete serial sections of the kidney revealed that the nephrons were aglomerular. At the highly convoluted proximal segment two different regions were evident, a feature that has not been previously reported for other aglomerular species. In electron photomicrographs we distinguished light and dark cells in the proximal tubule epithelium, with highly infolded basolateral membranes and closely associated mitochondria. The dark cells also had a large number of mitochondria in the apical region. The intercellular spaces at the epithelium of the proximal tubule were larger in fish acclimated at 2 salinity, a modification that might facilitate urine secretion, thus contributing to the survival of an aglomerular fish in a hyposmotic medium.