Glossary, continued. 



TROCHOPHORE— A molluscan larval stage (except 

 in Cephalopoda) following gastrulation (embryonic 

 stage characterized by the development of a simple 

 gut). It is commonly ciliated, biconically shaped, and 

 free-swimming; it establishes an evolutionary link be- 

 tween annelids and molluscs, since both groups dis- 

 play a similar life stage. 



TROPHIC LEVEL— The feeding level in an ecosystem 

 food chain characterized by organisms that occupy a 

 similar functional position. At the first level are auto- 

 trophs or producers (e.g., algae and seagrass); at the 

 second level are herbivores (e.g., copepods and mol- 

 luscs); at the third level and above are carnivores (e.g., 

 fishes). Omnivores feed at the second and third levels. 

 Decomposers and detritivores may feed at all trophic 

 levels. See FOOD WEB and PRODUCTION. 



TURBELLARIA— A class of mostly aquatic, non-para- 

 sitic flatworms that are leaf-shaped and covered with 

 cilia. 



TWO-YEAR-OLD— A fish that is a member of age- 

 group II, in its third calendar year. 



UMBO — A dorsal protuberance on each shell (valve) 

 of a bivalve mollusc, which rises above the line of 

 articulation and is the oldest part of the shell. 



UPWELLING — The process whereby prevailing sea- 

 sonal winds create surface currents that allow nutrient- 

 rich cold water from the ocean depths to move into the 

 euphotic or epipelagic zone. This process breaks 

 down the thermocline and increases primary produc- 

 tivity, and ultimately fish abundance. 



YEAR-CLASS — Refers to animals of a species popu- 

 lation hatched or born in the same year at about the 

 same time; also known as a cohort. Strong year- 

 classes result when there is high larval and juvenile 

 survival; the reverse is true for weak year-classes. The 

 effects of strong and weak year-classes on population 

 size and structure may persist for years in long-lived 

 species. Variation in year-class strength often affects 

 fisheries. See DISTRIBUTION and STOCK. 



YEARLING — A fish that is a member of age-group I, in 

 its second calendar year. 



YOLK SAC LARVA— A larval fish still bearing yolk, also 

 called a prolarva. 



YOUNG-OF-YEAR— Young fish of age-group 0, from 

 transformation into juvenile until January 1 . 



ZOEA — An early larval stage of various marine crabs 

 and shrimp; zoea have many appendages and long 

 dorsal and anterior spines. 



ZOOPLANKTON— Animal members of the plankton. 

 Most range in size from microscopic to about 2.54 cm 

 (1 inch) in length. They reside primarily in the epipe- 

 lagic zone and feed on phytoplankton and each other. 

 Although they have only a limited ability to swim 

 against currents, many undertake diel migrations. Taxa 

 include protozoa, jellyfish, comb jellies, arrowworms, 

 lower chordates, copepods, water fleas, krill, and the 

 larvae of many fish and invertebrates that are not 

 planktonic as adults. 



VELICONCHA — A bivalve larval stage. A veliconcha 

 has two larval shells and moves by using its velum. 



VELIGER — A ciliated larval stage common in mol- 

 luscs. This stage forms after the trochophore larva and 

 has some adult features, such as a shell and foot. 



VELUM — The ciliated swimming organ of a larval mol- 

 lusc. 



VIVIPAROUS— Refers to animals that produce live 

 offspring; eggs are retained and fertilized in the female 

 (as compared to OVIPAROUS). 



WATER COLUMN — The water mass between the 

 surface and the bottom. 



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