P— Continued 



predation — A relationship between two 

 species or populations where one direc- 

 tly attacks (hunts, grasps, kills and eats) 

 the other. 



predator— Animals that actively hunt other 

 living animals as a food source. 



primary productivity— The rate at which 

 energy is stored by photosynthesizing 

 organisms (chiefly green plants) in the 

 form of organic substances. 



primary succession — Succession which 

 starts from a virgin area. Compare with 

 secondary succession. 



productivity— The rate at which energy is 

 stored by anabolic processes in orga- 

 nisms. See also primary productivity. 



profundal zone— In lakes, the bottom and 

 deepwater areas beneath the light com- 

 pensation level. 



protista — Acellular organisms having a 

 nucleus, chromosomes and mitotic cell 

 division (e.g., protozoans, most algae, 

 slime molds and fungi). 



psammolittoral habitat— Refers to the area 

 among the sand grains at the water's 

 edge. 



psammon— Interstitial organisms found be- 

 tween sand grains. 



pyramids— The use of pyramids to show an 

 organism's tropic level. There are three 

 basic types: numbers, biomass, and 

 energy. See also eltonian pyramids. 



--R 



red tide— Discoloration of surface waters, 

 most frequently in coastal zones, caused 

 by large concentrations of micro- 

 organisms, particularly dinoflagellates. 

 They may or may not be toxic. 



redd— A bed prepared in sand or gravel to 

 received the spawn of fishes. 



respiration— The chemical and physical 

 processes involved in providing oxygen 

 for metabolism and removing the carbon 

 dioxide resulting from metabolism . 



rheotaxis— The alignment of an organism in 

 relation to a current. 



--S-- 



salt marsh— See marsh. 



saprophytic nutrition — The process by 

 which certain plants absorb food from 

 their surroundings, usually after the food 

 has been dissolved externally. 



saprotroph— An organism that feeds on 

 dead organic matter. 



saprozoic— The process by which certain 

 animals absorb food from their sur- 



roundings, usually after the food has 

 been dissolved externally. 



scavengers— Those animals that eat dead 

 organisms. 



sea grass— Members of marine seed plants 

 that grow chiefly on sand or sand -mud 

 bottom. They are most abundant in 

 water less than 9 meters deep. The 

 common types are: Eel grass (Zostera), 

 Turtle grass (Thallasia), and Manatee 

 grass (Syringodium). 



15 



