Marine fish regulate hypo-osmotically as they tend to gain salts and lose water. They have an aglomerular kidney which produces very little urine while secreting magnesium and sulphate ions, seawater is drunk copiously (up to 12% of body weight per day), sodium ions are actively pumped from the gut to body fluids and chlorine ions and water follow passively, the gut is relatively impermeable to magnesium and sulphate ions, the chloride cells actively secrete sodium and chlorine ions follow, and the gill membranes are relatively impermeable to water.