الفهرس 
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Abstract
The main goal of the current work was to elucidate and evaluate the different adaptive structural and functional aspects of the main osmoregulatory organs of three different fish species inhabiting diverse habitats. The structural and functional characteristic features of the renal and extra-renal organs including gills, integument and the anterior intestinal portion were investigated in freshwater fish (Bagrus bayad), brackish water fish (Tilapia zilli) and seawater fish (Scomber japonicus) in order to reveal the adaptive structural and functional features that could be related to the nature of their habitats.
The kidneys of both brackish water fish species, Tilapia zilli and seawater species, Scomber japonicas were slender, elongated organs and mostly fused along their entire length. While the kidneys of freshwater species, Bagrus bayad were relatively short broad organs.
Each kidney was composed of two main grossly undistinguishable kidneys, the anterior or head and the posterior or trunk kidneys.
Bagrus bayad had the greatest number of nephrons followed by Tilapia zilli then Scomber japonicus which exhibited the fewest number comparing with the other two species.
The size of the glomeruli as well as the glomerular filtration surface area of Bagrus bayad were markedly higher if compared with the corresponding values of both Tilapia zilli and Scomber japonicus.
The capillary loops of the glomeruli of the various studied fish species were lined with attenuated and fenestrated endothelial cells. The mesangial cells were situated peripherally and among the walls of the capillary loops. The mesangial cells were relatively few in both Bagrus bayad and Tilapia zilli comparing to those of Scomber japonicus which showed abundance of mesangial cells.
Scomber japonicus exhibited the highest value of renal sodium content followed by Tilapia zilli while Bagrus bayad showed the lowest value.
Scomber japonicus had the highest values of renal potassium and magnesium contents followed by Bagrus bayad then Tilapia zilli which having the lowest values.
Scomber japonicus also displayed the highest level of renal chloride contents, while both Bagrus bayad and Tilapia zilli had very closed values.
Renal ammonia content of Tilapia zilli was obviously higher comparing to the corresponding values of both Bagrus bayad and Scomber japonicus.
The branchial sodium, magnesium and chloride contents of Scomber japonicus fish species were certainly higher when compared with the corresponding values of either Bagrus bayad or Tilapia zilli.
Tilapia zilli had the highest values of both branchial potassium and ammonia contents followed by Bagrus bayad then Scomber japonicus which showed the lowest values.
The findings of the present investigation revealed that both renal and extra-renal osmoregulatory organs of the selected fish species displayed specific adaptive structural and functional aspects to fulfill their osmoregulatory demands in according to the nature of their habitats. As a result, all the examined fish species have an ability to maintain the volume and composition of their extracellular body fluid within narrow limits regardless to the nature of their habitat in terms of salinity.