U
Under-gravel filter system: Filter system at base of aquarium that contains micro-organisms that clean the water.
Understory: The vegetation that grows below the canopy of a forest.
Uplift tube: Tube through which air is pumped to draw water from the under-gravel filter.
Upthrust: A thrust in an upward direction. Used to indicate buoyancy. Weight of water displaced.
Upwelling: Currents that bring nutrient-rich bottom water up to the surface layers of water.
Urchin: A type of echinoderm invertebrate, related to sea stars, shaped like a ball and protected by long, sharp spines. Often called a "sea urchin."
Uric acid: White paste-like waste produced by seabirds.