ZOOLOGY: H. JORDAN 
157 
RHEOTROPISM OF EPINEPHELUS STRIATUS BLOCH 
By Hovey Jordan 
BERMUDA BIOLOGICAL STATION FOR RESEARCH. AGAR'S ISLAND. BERMUDA^ 
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 ^\oq]i) , confined in a 
cage through which flowed a current of fresh seawater, called my atten- 
tion to their pecuhar 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 
