GLOSSARY 491 



Pantothenic Acid One of the essential B-complex vitamins. 

 Para-aminobenzoic Acid (PABA) A vitamin- like substance thought to 



be essential in the diet for maintenance of health of certain fishes. No 



requirement determined for fish. 

 Parasite An organism that lives in or on another organism (the host) 



and that depends on the host for its food, has a higher reproductive 



potential than the host, and may harm the host when present in large 



numbers. 

 Parasite, Obligate An organism that cannot lead an independent, non- 

 parasitic existence. 

 Parasiticide Antiparasite chemical (added to water) or drug (fed or 



injected). 

 Parasitology The study of parasites. 

 Parr A life stage of salmonid fishes that extends from the time feeding 



begins until the fish become sufficiently pigmented to obliterate the 



parr marks, usually ending during the first year. 

 Parr Mark One of the vertical color bars found on young salmonids and 



certain other fishes. 

 Part Per Billion (ppb) A concentration at which one unit is contained 



in a total of a billion units. Equivalent to one microgram per kilogram 



(l /ug/kg), or nanoliter per liter (l nl/liter). 

 Part Per Million (ppm) A concentration at which one unit is contained 



in a total of a million units. Equivalent to one milligram per kilogram 



(l ml/kg) or one microliter per liter (l yul/liter). 

 Part Per Thousand (ppt or /oo) A concentration at which one unit is 



contained in a total of a thousand units. Equivalent to one gram per 



kilogram (l g/kg) or one milliliter per liter (l ml/liter). Normally, this 



term is used to specify the salinity of estuarine or sea waters. 

 Pathogen, Opportunistic An organism capable of causing disease only 



when the host's resistance is lowered. Compare with Secondary Invader. 

 Pathology The study of diseases and the structural and functional 



changes produced by them. 

 Pectoral Fins The anterior and ventrally located fins whose principle 



function is locomotor maneuvering. 

 Peduncle Disease A chronic, necrotic disease of the fins, primarily the 



caudal fin, caused by invasion of a myxobacterium (commonly Cyto- 



phaga psychrophilia) into fin and caudal peduncle tissue of an unhealthy 



fish. Synonyms: fin rot disease; cold water disease. 

 Pelvic Fins Paired fins corresponding to the posterior limbs of the 



higher vertebrates (sometimes called ventral fins), located below or 



behind the pectoral fins. 

 Peritoneum The membrane lining the abdominal cavity. 

 Perivitelline Fluid Fluid lying between the yolk and outer shell 



(chorion) of an egg. 



