The kidneys are paired organs located in the body cavity either side of the backbone.
They are one of the organs involved in excretion and regulation of the water balance within the fish.
Seawater contains more (and freshwater contains less) dissolved salts than the body a fish. This means that freshwater and marine fishes have different problems with regard to regulating the concentration of salts within their bodies. Their kidneys differ considerably in structure.
Copyright © Australian Museum, 2006