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
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.