Vanadium (V): Currently there is no specific MCL for vanadium. Vanadium may cause respiratory problems and inhibition of Na and K in ATP production.

Venation: The arrangement of veins in a leaf.

Venter: The undersurface.

Ventral Fins: Paired fins on the lower part of the fish, located below the gill covers. Not all fish have these.

Ventral: On the bottom or underside area of a fish.

Vent: A term that refers to the genital pore or sexual organ of fishes. See also Hydrothermal vents.

Venturi: A type of valve that produces bubbles by drawing air into fast flowing water. It is sometimes used on protein skimmers. When used with protein skimmers, it has a cylindrical body to draw air in.

Vernation: The manner in which a leaf, or the parts of a leaf, are rolled up in the bud. The same is true for the fronds of ferns.

Vertebrates: Animals that possess a backbone.

Vertical Integration: In this context, an economic term describing the U.S. pork industry. It means that a small number of companies are involved in more than one, and usually several, phases of pork production. Large factory farming companies ("integrators") combine company-owned farms, contract farms, and farming-related businesses (feed mills, transportation, etc.) under a single company banner.

Vertically Homogeneous Estuary: An estuary in which, at any given location, wind or tidal mixing homogenizes salinity throughout the water column.

VHO (Very High Output) Lighting: Very High Output fluorescent bulbs have a much higher output than NO bulbs. They require a special ballast and end caps. VHO lighting is used mostly on freshwater plant tanks or marine reef tanks.

Virus: A non-cellular infectious agent that reproduces only in living cells.

Vitamin: Chemical substances (micro-nutrients) required in trace concentrations acting as a cofactor with enzymes in catalyzing biochemical reactions.

Viviparous: Refers to the development of an organism through the juvenile stage within a parent.

Volatile Organic Compounds: Natural or manmade gaseous hydrocarbons which can be active components in atmospheric chemistry.