Cartilaginous fish (sharks, rays — class Chondrichthyes) are unique among fish in retaining high concentrations of urea in their blood and tissues (~2–2.5% urea). This serves as an osmolyte to maintain osmotic balance with seawater. Bony fish excrete mostly ammonia; insects and birds excrete uric acid; mammals excrete urea but are not fish. Ureotelic animals include cartilaginous fish, adult amphibians, and mammals.