Odor Perception and Taste 221 



Some Uses of Odor Perception 



Odor perception is less limited by water than is vision and hearing. 

 For this reason the olfactory organs of fishes are highly developed and 

 extensively used for many purposes. 



Schooling. Wrede ^^' discovered that schools of elritze were held to- 

 gether by a substance in the mucus of that species; the introduction of 

 mucus from these minnows into a part of an aquarium would cause the 

 minnows in the aquarium to congregate there. Vision appeared to be 

 important in schooling during daylight hours, but the stimulus of odor 

 held the schools together in darkness. "Both blinded and hind-brain- 

 extirpated fish could be trained to these substances, indicating chemo- 

 reception, although animals with nose intact responded best."^'^'^ Schools 

 of young silver salmon in an aquarium remained intact as long as the 

 room was lighted, but when the light was extinguished and infrared sub- 

 stituted, dispersal of the school was complete in a matter of a few seconds, 

 indicating that in this case, schooling depended entirely upon vision.^ 



Fright Reaction. Blinded elritze that did not respond to pike mucus, 

 were placed in an aquarium with a pike that captured some of them. 

 When those remaining were tested with a combination pike and elritze 

 odor, they gave a severe fright reaction. Thus when these minnows show 

 a reaction to odors of several other fishes, it may be that the reaction is 

 due to past experience with these fishes rather than an innate response. 



In a long series of experiments von Frisch •^- tested a substance exuded 

 by an injured elritze minnow which he called "Schreckstoff" (alarm 

 substance). This material would cause an alarm reaction (Schreck- 

 reaktion) when released in a school of uninjured minnows. Quan- 

 titatively, when 100 ml of this solution was poured into aquaria of 

 25- to 150-liter capacity, the minnow gave pronounced fear reactions. 

 The alarm-causing material was mostly in the skin, because extracts from 

 minnow intestine and liver gave no reaction and extracts from gills, 

 muscle, and ovaries were from one-tenth to one-hundredth as active as 

 skin extracts. Minnows with severed olfactory nerves would not respond 

 to the substance; for this reason von Frisch concluded that the alarm 

 substance was odoriferous. Huttel ^^ pointed out that the alarm substance 

 of the elritze was a purin- or pterin-like substance. 



That the alarm reaction may be transmitted by sight alone was 

 demonstrated by Verheijen ^^ when he placed two aquaria containing 

 elritze side by side and dropped tissue juice into one aquarium to produce 

 typical fright reactions. Minnows in the adjacent aquarium eventually 

 huddled together in a tight "fright" school, but were about 10 seconds 

 slower in their reaction than w^ere the minnows stimulated with "fright" 

 juice. 



