Reprinted from the Proceedings of the NATIONAL ACADEMY OF SCIENCES, 

 Vol. 3, pp. 157-159, March, 1917. 



RHEOTROPISM OF EPINEPHELUS STRIATUS BLOCH 



By Hovey Jordan 



BERMUDA BIOLOGICAL STATION FOR RESEARCH. AGAR'S ISLAND. BERMUDAi 

 Communicated by E. L. Mark and read before the Academy. November 14. 1916 



An unusual, but orderly, arrangement displayed by several groupers 

 or hamlets (a marine fish, Epinephelus striatus Bloch), confined in a 

 cage through which flowed a current of fresh seawater, called my atten- 

 tion to their peculiar rheotropism. The tails of all were directed into 

 the current. When this was shut off their arrangement became pro- 

 miscuous, indicating that their novel posterior orientation was a true 

 rheotropic response. 



This phenomenon led me to investigate in detail the behavior of these 

 fishes both in groups and individually, in order to determine whether 

 this posterior orientation to a current which, so far as I have been 

 able to learn, is undescribed is a normal response of the grouper. 

 For this purpose a number of fishes were placed in the cage and a record 

 was made of the positions which they assumed at two-minute intervals. 

 These observations were made both at night and during different parts 

 of the day. In one record, which is fairly typical, the positions of each 

 of 7 fishes at 30 successive intervals in all 210 observations were 

 noted. Of the 210 observations 141 showed the fishes to be tail into 

 the current (posterior orientation), 67 side to the current (lateral orien- 

 tation) , and only 2 head into the current (anterior orientation) . In 

 order to determine whether posterior and lateral positions indicate dif- 

 ferent responses by individual fishes, or are simply phases of one reac- 

 tion, fishes were studied singly. For this I used a small aquarium 

 (30 X 20 inches) across which a moderate current of water flowed diag- 

 onally. Each of the fishes tested remained most of the time near the 

 inlet in the region of the strongest current. It assumed in succession 

 slightly different positions, chiefly by rotating the long axis of the 

 body through an arc of 90 to 180 around its own center, which pre- 

 served a comparatively fixed position in the axis of the current, so that 

 either one side of the body or the tail was at any given instant directed 

 toward the current, into which, however, the head was never pointed. 

 After assuming approximately a dozen such temporary positions, 

 which required about three or four minutes, the grouper tailed directly 

 into the current and remained in this position for about three minutes. 

 It then began a second series of changes similar to the first. In this 

 sequence of positions it is perhaps most natural to regard as a single 



