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O.B.: An abbreviation for Orange-Blotched, which refers to a fish with an orange-blotched or spotted morphology. See Orange-Blotched.

O.D.: An abbreviation for outside diameter, used when measuring tubing dimensions.

Oceanic Ridge: A sinuous ridge rising from the deep-sea floor.

Oceanic: Associated with sea-water environments seaward of the shelf-slope break.

Ocellus: (pl. ocelli) A simple eye.

Octocoral: Octocorals have eight tentacles on each polyp. There are many different forms, which may be soft, leathery, or even those producing hard skeletons.

Oligotrophic: Refers to water bodies or habitats with low concentrations of nutrients.

Omnivore: An animal that eats both living plant and animal matter.

Omnivory: Being able to feed in more than one distinct way (i.e., an organism capable of carnivory and herbivory).

Oodinium: See Velvet Disease.

Operculum: A hard, bony plate which covers the gills.

Optimal Foraging Theory: A theory designed to predict the foraging behavior that maximizes food intake per unit time.

Orange-Blotched: Refers to a fish whose coloring is orange-blotched or spotted. Also annotated as O.B. or marmelade cat.

Organic Nutrients: Nutrients in the form of molecules synthesized by or originating from other organisms.

Organic: Deriving from living organisms.

Osmoconformer: An organism whose body fluids change directly with a change in the concentrations of dissolved ions in the external medium.

Osmoregulator: An organism that regulates the concentration of dissolved ions in its body fluids irrespective of changes in the external medium.

Osmosis: The movement of pure water across a membrane from a compartment with relatively low dissolved ions to a compartment with higher concentrations of dissolved ions.

Outer Lip: The portion of a shell around the right side of the aperture in a gastropod.

Outwelling: The outflow of nutrients from an estuary or salt-marsh system to shelf waters.

Overdominance: Selection favoring heterozygotes.

Overfishing: The harvest of fish or marinelife at a rate and volume that destroys the ocean's ability to provide such harvest in the future; the unsustainable harvest of fish and marinelife.

Oviparous: Producing eggs which are fertilized, develop and hatch outside the body.

Ovoviviparous: Producing eggs (usually with yolk) that are fertilized internally. Hatching may occur internally or external to the mother.

Oxygen Dissociation Curve: A curve showing the percent saturation of a blood pigment, such as hemoglobin, as a function of oxygen concentration of the fluid.

Oxygen Minimum Layer: A depth zone, usually below the thermocline, in which dissolved oxygen is minimal.

Oxygen Minimum Zone: Zone in which respiration and decay reduce dissolved oxygen to a minimum, usually between 800 and 1000m.

Oxygen Technique: The estimation of primary productivity by the measurement of the rate of oxygen increase.

Ozone (O3): A gas used in aquarium systems to break down organic waste and as a sterilizing agent to kill bacteria and small organisms in the water. It is used in an ozone reactor or protein skimmer. It is essential to reef keeping, but water should be filtered through carbon before returning it to the tank, as excess ozone can harm fish and other creatures. Ozone is explosive and is harmful to fish and to humans if in large quantities.
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