The present study was conducted to elucidate and compare the adaptive renal structures of freshwater, Bagrus bayad, brackish water Tilapia zilli and seawater Scomber japonicus. The trunk kidneys of the selected fish species were investigated morphometrically, histologically and ultrastructurally. The recorded results showed that the nephrons of Bagrus bayad and Tilapia zilli were distributed within the renal tissue as clusters, while those of Scomber japonicus were scattered solitary within the renal tissue. Bagrus bayad displayed abundant of well vascularized large glomeruli along with thin filtration barrier and numerous wide filtration slits as well as numerous mitochondria of both proximal and distal tubules. However, Scomber japonicus exhibited few number of small glomeruli along with abundance mesangial cells, extremely narrow filtration slits and relatively thick filtration barrier as well as unique giant mitochondria. The structural organizations of the Tilapia zilli nephrons were inter-between Bagrus bayad and Scomber japonicus. As a whole, the results of the current study showed that the kidney of the examined fish species exhibited specific adaptive renal structural features to fulfill their variable osmoregulatory demands.

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