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U.V.: Ultraviolet light, generally not wanted in the aquarium. Special filters or shields are required for its use.

UGF: See under gravel filter.

Ultraplankton: Plankton that are less than 2 micrometers in size. They are difficult to separate from water.

Ultraviolet (UV) Light: Ultraviolet is a high energy, short wavelength of light. It is shorter than violet in the visible spectrum and on the border of the x-ray region.

Ultraviolet Sterilizer: A device that uses UV light to kill bacteria and other tiny organisms in aquarium water.

Umbilicus: Hole or indentation in the base of the columella.

Umbo: (pl. umbones) The apparent "apex" or "beak" of each valve around which "radial" growth has proceeded.

Unconfined Aquifer: The saturated formation in which the upper surface fluctuates with addition or subtraction of water. The upper surface of an unconfined aquifer is called the water table. Water, contained in an unconfined aquifer, is free to move laterally in response to differences in the water table elevations.

Undergravel Filter: This filter provides both mechanical and biological filtration. It uses a few long, plastic tubes attached to a perforated plate on the bottom that is covered with gravel to pull water through the gravel. By pulling water through the gravel, the bacteria that help in the nitrogen cycle are oxygenated. Water is pulled through the gravel and then through the tubes with the aid of an air stone attached to an air pump. As the air bubbles rise, a current is generated which continuously brings water from the aquarium, through the substrate and to the top of the tubes. UG Filters are not recommended for planted tanks or African Cichlid setups.

Uniform Spatial Distribution: Situation in which individuals are more evenly spread in space than would be expected on the basis of chance alone.

Unsaturated Formation: The soil or other geologic material usually located between the land surface and a saturated formation where the voids, spaces or cracks are filled with a combination of air and water.

Unsustainable: The present use, consumption or harvesting of natural resources that can not be continued into the future.

Upwelling: The movement of nutrient-rich water from a specified depth to the surface.

UV Sterilizer: See Ultraviolet Sterilizer.
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