RHEOTROPIC RESPONSES OF EPINEPHELUS STRIATUS 



. TABLE 2 

 Time elapsed in assuming the positions, 1 to 12, shown in figure 3 



445 



5. Experiments on regional sensitivity 



A study of the relative sensitivity of various parts of the body was 

 undertaken with the hope of finding some evidence as to the nature of 

 the causes which produce this peculiar rheotropic reaction. The 

 method employed to determine the sensitivity of different areas of the 

 surface of the fish to a current was to direct a jet of water at close 

 range against limited areas (approximately 25 sq. cm.) and to note the 

 response, if any. For this purpose a long glass tube with a bore 1 cm. 

 in diameter was used, and the average flow of water through it was 

 roughly 1/28 liter per second. In this way all parts of the dorsal and 

 lateral surfaces were explored. To prevent as far as possible any com- 

 plication that might result from sight stimuli, the observer was screened 

 from the fish by a curtain covering the side of the tank. The curtain 

 was provided with a hole for observation; the tube was nearly invis- 

 ible. In all cases the local current stimulation produced a negative 

 reaction, a swimming or backing away from the current. It is pos- 

 sible, however, to divide the body into five regions based on their rela- 

 tive susceptibility to stimulation by such a current. In the order of 

 promptness of reaction these are as follows : lip region (seven seconds) ; 

 caudal fin (sixteen seconds) ; dorsal fin, posterior part (twenty-two sec- 

 onds); 3 cheek and operculum (twenty-five seconds); sides of body (about 

 thirty seconds). The belly was not tested because of its inaccessi- 

 bility. Thus it appears that the lip region is by far the most sensi- 

 tive part of the integument tested. If stimulation of the lips is pro- 

 longed, the hamlet becomes very vigorous in its attempts to escape. 



3 That the fins are not essential in rheotropism is indicated by the fact that 

 when either dorsal or caudal fins are removed, the normal reaction is unaltered. 

 It was also noticed that fishes whose fins had become badly frayed by long cap- 

 tivity were normal in their responses to currents shown in figure 3. 



