Osmotic Problem: They are hypotonic to their environment. Water will tend to diffuse INTO their bodies. Salts will diffuse out.
Role of Kidney: Removes excess water. Produces large amounts of dilute urine. Kidneys also reabsorb salts. They also rarely drink water.
Urine: Large amount but dilute.
Osmotic Problem: Hypertonic to environment. Water diffuses out. High salt levels present in the water
Role of Kidney: Continually drinks water. Kidneys reabsorb water, while actively secreting salts. Small amounts of concentrated urine. Salt is also excreted across gills.
Urine: Small, concentrated amount.
Osmotic Problem: Water needs to be conserved.
Role of Kidney: Regulates concentration of blood, while at the same time excretes urea and conserves water.
Urine: Concentration changes with the availability of water, as well as temperature and water loss through sweat. Water levels in blood rise, urine amount rises, and concentration decreases and vice versa.