RHEOTROPIC RESPONSES OF EPINEPHELUS STRIATTJS 



443 



TABLE 1 



Successive positions of orientation of a group of seven hamlets observed at about two minute intervals 



^ 



X * ^T 



C s 



the exact nature of this reaction whether the two positions are due 

 to differences between individual fishes or are phases of one reac- 

 tion. Single groupers, however, when studied continuously showed 

 that both positions are assumed in the course of one reaction. 



2. In individual yis/ies. Individual hamlets were tested in a spacious 

 oblong aquarium (fig. 2) provided with plane glass front and back to 

 allow uninterrupted observation from the sides as well as from above. 

 A glass tube 1 cm. in diameter and so directed as to make about 

 equal angles with two adjacent 

 sides of the aquarium, delivered 

 a strong current (C) diagonally 

 across the tank. The volume of 

 this current was approximately 

 0.1 liter per second. It is the 

 only one which is significant for 

 the purposes of this investigation. 

 Those peripheral to this were not 

 of sufficient strength or regularity 

 to influence the fish in any con- 

 sistent manner. These currents 

 were studied by the use of floating and suspended objects, and the 

 results plotted show their main features. 



The fish under investigation (fig. 3) seemed to prefer the region of 

 the strongest current; that is, it remained near the source of the cur- 

 rent, shifting from one position of quiet to another, settling to the bot- 

 tom, or remaining suspended at a fixed place in the current for a few 

 moments, and then again changing position slightly. All the while, 

 however, the fish assumed either posterior or lateral orientation to the 



2 ' Diagram of aquarium (20 by 30 



inches) and currents as seen from above. 

 ^ Main diagonal current 



