Marine vertebrates live in a hypertonic environment. One strategy employed by bony fish is to reduce the size of the corpuscle and process only small quantities of urine. This prevents the loss of water. Salt and urine are excreted by a separate process from the gills. Sharks retain a large glomerulus but have evolved the ability to tolerate high levels of urea in the body.
Body fluids are isotonic with the sea.
The urea displaces salt and the kidneys process large quantities of blood to dump both salt and water. Tetrapods derived from fresh-water ancestors, but they face a similar problem of water loss to that of marine fish and use similar solutions. Reptiles, for example, have reduced the size of the corpuscle and process less urine. Marine turtles and sea birds excrete salt from special glands on the face to supplement the action of the kidney. Mammals and birds face an additional problem.