Glossary, continued. 



REPRODUCTIVE POTENTIAL— The total number of 

 offspring possible for a female of a given species to 

 produce if she lives to the maximum reproductive age. 

 This is found by multiplying the number of possible 

 reproductive periods by the average numberof eggs or 

 offspring produced by females of each age class. This 

 potential is seldom realized, but this and the age of first 

 reproduction, or generation time, determine the maxi- 

 mum rate of population increase under ideal condi- 

 tions. 



RHEOTAXIS — A response movement by an animal 

 toward or away from stimulation by a water current. 



RIVERINE— Pertaining to a riverorformed by a riveror 

 stream. 



ROE— The egg-laden ovary of a fish, or the egg mass 

 of certain crustaceans. 



RUN — A group of migrating fish (e.g., a shad run). 



SALT WEDGE — A wedge-shaped layer of salt water 

 that intrudes upstream beneath a low-density fresh- 

 water lens that has "thinned" while flowing seaward. 



SCAVENGER — Any animal that feeds on dead ani- 

 mals and remains of animals killed by predators. See 

 DECOMPOSER and DETRITIVORE. 



SCHOOL — A group of aquatic organisms, usually of 

 the same size, mutually attracted to each other, that 

 swim together in an organized fashion. 



SEAWATER ZONE— The portion of an estuary with 

 annual depth-averaged salinities of greater than 25 

 parts per thousand. 



SEDENTARY— Refers to animals that are attached to 

 a substrate or confined to a very restricted area (or 

 those that do not move or move very little). See 

 SESSILE. 



SETTLEMENT — The act of or state of making a per- 

 manent residency. Often refers to the period when fish 

 and invertebrate larvae change from a planktonic to a 

 benthic existence. 



SHOAL— (1) A sand bar in a body of water that is 

 exposed at low tide. (2) An area of shallow water. (3) 

 A group of fish (school). (4) As a verb, to collect in a 

 crowd or school. 



SILT — Soil with particles intermediate in size between 

 sand and clay. 



SIPHONS — The "necks" or tubes of clams and other 

 bivalves that carry water containing food and oxygen 

 into the gills (inhalant siphon), and then expel water 

 containing waste products (exhalent siphon). 



SLOUGH-A shallow inlet or backwater area whose 

 bottom may be exposed at low tide. Sloughs are often 

 adjacent to open estuarine waters, and may have a 

 channel passing through them. 



SPAT — Juvenile bivalve molluscs which have settled 

 from the water column to the substrate to begin a 

 benthic existence. 



SPAWN— The release of eggs and sperm during mat- 

 ing. Also, the bearing of offspring by species with 

 internal fertilization. See PARTURITION. 



SPECIES— (1) A fundamental taxonomic group rank- 

 ing after a genus. (2) A group of organisms recognized 

 as distinct from other groups, whose members can 

 interbreed and produce fertile offspring. See POPU- 

 LATION, SUBPOPULATION, and SUBSPECIES. 



SPERMATOPHORE— A capsule or gelatinous packet 

 (extruded by a male) containing sperm and used to 

 transfer sperm to females. Spermatophores are pro- 

 duced by certain invertebrates and some primitive 

 vertebrates. 



SEMELPAROUS— Animals that have a single repro- 

 ductive period during their lifespan; compare with 

 ITEROPAROUS. 



SESSILE — Refers to an organism that is permanently 

 attached to the substrate. See SEDENTARY. 



SESTON— Microplankton; all bodies, living and non- 

 living, floating or swimming in water. 



SPICULE — A sharp, pointed, siliceous or calcareous 

 body, as in those forming the endoskeleton of sponges, 

 corals, and certain protozoans. 



SPIT— A long, narrow sand bar or peninsula extending 

 into a body of water which is at least partly connected 

 to the shore. See SHOAL. 



SPOROCYST— A simple larval stage of parasitic trema- 

 tode worms. Contact with the host causes a metamor- 

 phosis from an earlier stage to this stage. 



351 



