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abyssal zone — deepest part of the seafloor, below 2,000 meters.

anadromous — fish that breed in freshwater but spend most of their adult life in the ocean, such as salmon.

aphotic — depths of the ocean below which no light penetrates.

autotroph — an organism that produces its own nutrients, usually through photosynthesis.

baleen — fringed filter in the mouth of some whales that is used to strain food from seawater.

bathyal zone — region of the seafloor from the shelf edge (200m) to the start of the abyssal zone (2,000m).

bends — dangerous condition caused by gas bubbles in the blood stream, suffered by divers who ascend from pressurized depths too quickly.

benthic — relating to the ocean bottom.

Biological Oxygen Demand (BOD) — the amount of dissolved oxygen that will disappear from an enclosed water sample as aerobic bacteria decompose the organic material in the water.

bioluminescent — organisms capable of generating light.

blue-green algae — bacteria capable of performing aerobic photosynthesis.

continental shelf — the shallow bottom just offshore of most continents between water's edge and a sharp dropoff where the bottom plunges steeply.

Coriolis effect — drifting of a moving object to the right in the northern hemisphere and the left in the southern; or the direction in which it initially begins motion, due to the rotation of the Earth.

diadromous — organism that spends part of its life cycle in freshwater and part in saltwater.

diatom — a photosynthetic single celled organism enclosed by a shell of silica.

doldrums — area along the meteorological equator where surface sea winds are weak and variable and rainfall is heavy.

food chain — simplest ecological feeding relationship, in which one species eats only one species of prey and is eaten by only one species of predator.

intertidal — shore between highest and lowest tides; also called the littoral zone.

kelp — very large brown algae or seaweed, often growing in oceanic "forests."

littoral — see intertidal.

neap tide — interval when high tides are moderately high and low tides moderately low; neap tides, the opposite of spring tides, occur every other week when the sun and moon are at right angles.

neritic — water and organisms over the continental shelves.

pelagic — organism that lives in open water off the ocean's bottom.

pinniped — fin-footed marine animal such as a walrus, seal, or sea lion.

plankton — drifting organisms that lack the ability to swim against currents.

spreading center — region of the seafloor where newly formed crustal material moves in opposite directions

swim bladder — gas-filled pouch found in most bony fishes, used for buoyancy, sound production, and detection.

thermocline — narrow range of depths at which temperature changes abruptly between surface warmth and deep cold.