A type of climate characterized by insufficient moisture to support appreciable plant life. It is a climate of extreme aridity, usually of extreme heat, and of negligible rainfall. (From McGraw-Hill Dictionary of Scientific and Technical Terms, 4th ed)
Marine ridges composed of living CORALS, coral skeletons, calcareous algae, and other organisms, mixed with minerals and organic matter. They are found most commonly in tropical waters and support other animal and plant life.
The relationship between two different species of organisms that are interdependent; each gains benefits from the other or a relationship between different species where both of the organisms in question benefit from the presence of the other.