The major nitrogenous waste of shark is urea. Urea is less toxic and less soluble in water than ammonia. Hence, it can stay for sometime in the body. Many land vertebrates (adult amphibians, mammals) and such aquatic animals, which cannot afford to lose much water (eg, elasmobranch fishes) marine bony fishes, adult froges, earthworms, nematodes turn their ammonia into urea for excretion.