Student Stuff
The Kidneys
The kidneys are paired organs located in the body cavity ventral to (below) the vertebral column. They are one of the organs involved in excretion and regulation of the water balance within the fish.
Freshwater and marine fishes are faced with different problems with regard to regulating the concentration of salts within the body. Their kidneys differ considerably in structure. Freshwater fishes have larger kidneys than marine fishes. They have a higher concentration of salts in the body tissues than the surrounding water. Conversely marine fishes have a lower concentration of salts in the body tissues than the surrounding water.
The kidneys of freshwater fishes remove water and re-absorb salts and sugars. They produce large amount of very dilute urine. This helps the fish avoid becoming "waterlogged" from the large amounts of water diffusing into the fish.
The kidneys of marine fishes however conserve water. Marine fishes drink water and excrete only a small volume of very concetrated urine.
In most fishes, the gills and gut are largely responsible for the excretion of surplus salts.
Further reading
- Helfman G.S., Collette, B.B. & D.E. Facey. 1997. The Diversity of Fishes. Blackwell Science. Pp. 528.
- Lagler, K.F, J.E Bardach & Miller R.R. 1962. Ichthyology. John Wiley & sons. Pp. 545.
