Glossary 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.