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r: The intrinsic rate of increase of a population.

R/O: See reverse osmosis.

Rachis: The continuation of the stripes in a fern that is divided.

Radial: Sculpture that extends from the center of the shell outward toward the margin in cap shaped shells.

Radiocarbon Technique: The estimation of primary productivity by the measurement of radiocarbon uptake.

Radiolaria: Protistan phylum, whose members are planktonic and secrete an often elaborate siliceous test.

Random Spatial Distribution: Situation in which individuals are randomly distributed in a space; probability of an individual's being located at any given point is the same irrespective of location in the space.

Ray: The bones which make up the skeleton in the fins.

Reactor: A device used to add a substance to the aquarium water in a controlled manner. Ozone, kalkwasser, and carbon dioxide are the most common reactors. With an ozone reactor, for example, water is forced through a pressurized column of an air-ozone mixture.

Recharge Area: The area where water predominantly flows downward through the unsaturated formation (zone) to become groundwater.

Recruitment: The residue of those larvae that have: (1) dispersed; (2) settled at the adult site; (3) made some final movements toward the adult habitat; (4) metamorphosed successfully, and (5) survived to be detected by the observer.

Redd: A salmon nest, dug out of the gravel in the stream bed by the adult female.

Red Slime: See cyanobacteria.

Red Tide: A red coloration of coastal waters that is caused by dense outburst of phytoplankton (usually dinoflagellates). Some red tides result in mass fish deaths, others contaminate shellfish, and still others produce no toxic effects.

Redox: This is a scientific term referring to the reduction-oxidation potential of the water; in other words, it is a measure of how easy it is for organic reactions to take place in the aquarium. Its measurement gives an indication of how an aquarium will be able to sustain life. A high value is better than a low one. The redox potential refers to an electrical charge on a molecule that has transformed in a chemical reaction. The measurement is expressed in millivolts by an ORP monitor.

Redox-Potential Discontinuity: That depth below the sediment-water interface marking the transition from chemically oxidative to reducing processes.

Reef Flat: The space between a fringing reef and the beach shoreline covered by water at high tide but has almost no water on it at low tide and where coral growth is minimal.

Reef Front: The outside edge of all types of coral reefs; the side of a coral reef facing the open ocean, where coral growth is most extensive; also known as the reef face.

Reference Dose (RfD): The maximum daily exposure to a chemical that is judged to be without risk of adverse systemic health effects over a person's lifetime. It formerly was called the Acceptable Daily Intake.

Refuge: A device by which an individual can avoid predation. Refuges can be either isolated containers that pull their water from the main tank, or they can be compartmentalized sections within the main tank.

Regulator: In reference to SCUBA diving equipment, a device which allows compressed air to flow from the tanks to the mouth of a diver. Also refers to an organism that can maintain constant some aspect of its physiology (e.g., body temperature) constant despite different and changing properties of the external environment.

Relationship: A connection or association between two or more things; the result of the interaction between two things, or one thing an another.

Renewable Resource: A resource that can be regenerated (e.g., a growing diatom population that is being exploited by a copepod).

Reproduction: The process of duplicating, or the production of offspring.

Reproductive Effort: The fraction of assimilated nutrients that are devoted to reproductive behavior and gamete production.

Resins: Synthetic media used for chemical filtration through a process known as ion-exchange.

Resource: A commodity that is required by an organism and is potentially in short supply.

Respiration: Metabolic processes by which food or food-storage molecules yield the energy on which all living cells depend. Also refers to the consumption of oxygen during aerobic metabolism.

Respiratory Pigment: A molecule, polymer, or other complex adapted to bind and transport oxygen efficiently, usually in a circulatory system (e.g., hemoglobin).

Respiratory Quotient: The ratio of moles of carbon dioxide produced to oxygen consumed in respiration.

Rete Mirabile: A countercurrent exchange structure of capillaries that allows gas uptake in a fish swim bladder.

Reverse Bohr Effect: An effect that occurs when lactate builds up in the blood of certain invertebrates and pH decreases, increasing the affinity of hemocyanin for oxygen.

Reverse Osmosis (RO): A water treatment method used to remove dissolved inorganic chemicals and suspended particulate matter from tap water before it is used in an aquarium. Water, under pressure, is forced through a semi-permeable membrane that removes molecules larger than the pores of the membrane. Large molecules are flushed out as wastewater. Smaller molecules are removed by an activated carbon filter. Reverse Osmosis works slowly, sometimes as slow as 10 or 15 gpd (gallons per day, and wastes a couple of gallons for every gallon of filtered water produced. Notwithstanding, it is perhaps the best method for tap water purification.

Reverse Under Gravel Filter: This variant of an under gravel filter runs in the opposite direction, pushing water up through the gravel. It requires water pumps of some sort to run, but needs cleaning less often.

Reynolds Number (Re): A number that represents the relative importance of viscous forces and inertial forces in a fluid. As Re increases, inertial forces become more important. In sea water, Re increases with increasing water velocity and with the size of the object in the water

Rhizome: A stem growing more or less horizontally near the surface of the soil or gravel and sometimes showing above it.

Ribs: Raised folds on the surface of a shell.

Ridge: Similar to ribs, but milder.

Rip Current: A stretch of strong, flowing water near the shore, directed usually away from the shore.

Riparian: Living on the bank of a lake, pond or stream.

Rip Tide: See Rip Current.

Rise: Bottom of low relief at the base of the continental slope.

Rootstock: A very short, but often thick stem, growing vertically at or just above the surface of the gravel. Often referred to as the Crown.

Rotifers: A phylum that includes organisms that have ciliated mouths and a retractable "foot" for anchoring.

ROV: Abbreviation for remotely operated vehicle, usually a submersible tethered to a ship, with facilities for video, remote sampling by grabbing arms, and precise navigation.

Runner: A horizontal stem growing just above the surface of the gravel and rooting at its nodes.

Runoff: is precipitation or irrigation water that does not infiltrate but flows over the land surface toward a surface drain, eventually making its way to a river, lake or an ocean.
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