The kidneys of fish develop from the pronephros and mesonephros. The function of the kidney is osmoregulation. In freshwater fish, the kidney saves ions and excretes water. In saltwater fish, the kidney excretes ions and conserves water. The majority of nitrogenous waste is excreted through the gills. The other function of the kidney is hematopoiesis with hematopoietic tissue located in the interstitium of the kidney. This function is primarily in the anterior kidney but can be found throughout the entire kidney.