RHEOTROPIC RESPONSES OF EPINEPHELUS STRIATUS 447 



ing to prefer muscular repose to exertion, the fin movements being 

 few and slow; pectoral fins are vibrated about twenty-three times per 

 minute. The application of a localized current of little force was suf- 

 ficient to start the fishes from a position of complete rest, but when they 

 were beyond the range of the current, they "again settled to the bot- 

 tom. This behavior indicates that the agreeableness of muscular ef- 

 fort is not sufficient to cause any prolonged swimming. On the other 

 hand, the exertion and possible fatigue involved in maintaining a rela- 

 tively constant position in a current which is broad enough to cover 

 the whole fish might be expected eventually to produce a negative re- 

 action. Instead of this, however, the fishes remained for an hour or 

 two in the strongest part of the current (fig. 3), seeming to prefer it 

 to the quiet water. This leads one to the conclusion that the muscular 

 effort necessitated in these reactions is not in itself a deterrent factor. 

 Many times, too, when a localized current was directed against the side 

 of the fish, lying at the bottom near the wall of the aquarium, it was 

 observed that one of the pectoral fins extended horizontally to the 

 wall served to keep the body of the fish from contact with the side 

 of the tank. In these cases the effort involved in maintaining this 

 position did not cause the reaction time to vary. 



With a view to ascertaining what importance, if any, attaches to 

 the pressure sense, I made use of a current of water directed through 

 the glass tube (1 cm. in diameter) already referred to in other experi- 

 ments. If the pressure sense is a factor in rheotropic response, it is to 

 be expected that the response to a very strong current would be more 

 prompt than to a weaker one. Accordingly I repeatedly subjected the 

 same fish successively to a weak current (about 1/28 liter per second) 

 and to a stronger one (about 1/8 liter per second). Though the latter 

 was of sufficient force to produce an appreciable indentation of the skin 

 and musculature of the body, the reaction time was not shorter than 

 in the case of the weaker current. I may also add that fishes can be 

 pressed by the hand against the side of the aquarium with considerable 

 force without causing any definite response. In his study of the 

 pressure sense as a possible cause of rheotropism, Lyon (2) (p. 154) 

 enclosed fishes (silver sided minnows) in long stoppered bottles which 

 were floating down the stream. He found that under these conditions, 

 with all pressure stimulation thus eliminated, the fishes responded 

 normally, by swimming in a direction opposite to the drift of the bot- 

 tle, in an attempt to keep the visual environment constant. My ex- 

 periments, though not of such fundamental importance as Lyon's, 



