THE GYMNOTID EELS OF TROPICAL AMERICA. 159 



other Gyninotids and was unable to saj^ whether the method described for E. 

 eledrophorus was common to all. 



In his phylogenetic-ethologic study of the Gymnotidoe, Schlesinger (1910) 

 states that the use of the anal fin described by Sachs must be general throughout 

 the family. He bases his conclusion entirely on a morphologic comparison between 

 the Mormyrid Gymnarchus and the Gymnotids Eledrophorus and Gymnotus. In a 

 footnote he adds that Dr. Franz Steindachner told him that he had seen living 

 Gymnotidse in Brazil swimming in the method described. 



While in British Guiana in the summer of 1910 I had opportunity to study the 

 motion of a number of species in their normal environment. Three species in 

 particular were examined, Eigenmannia virescens and Sternopygus macrurus, which 

 are abundant in the trenches in and about Georgetown, and Gymnotus carapo in 

 Hubabu Creek. Several other species were also seen ahve. In every case two 

 methods of swimming were observed, (1) the use of the anal fin alone, and (2) the 

 use of the anal aided by the body proper or the pectorals or both. 



When at rest the Gymnotids face the current of the stream, the entire body 

 and caudal appendage being in a straight line and the pectorals laid back against 

 the body. The anal fin was kept moving just enough to counteract the motion 

 of the stream, and the pectorals gave an occasional stroke. The movements of 

 the anal fin were similar to those described by Sachs for the electric eel. From the 

 cephalad end of the anal fin a series of undulating waves passed caudad so that a 

 longitudinal section of the entire anal fin in motion resembles a fairly regular sine 

 curve. There were usually six of these waves traversing the fin at any one time, 

 rarely five or seven. The speed of the wave varies with the speed of the current 

 of the stream — always being just sufficient to maintain the position of the fish. 

 If the current varied in direction the fish responded at once with a stroke of one or 

 both of the paddle-shaped pectorals, which kept the long axis of the fish parallel 

 to the direction of the current. 6therwise the pectorals were not used. During 

 these resting periods the caudal appendage streams out behind the fish. 



If a resting fish were slightly disturbed it merely increased the speed of the 

 waves traversing the anal and moved away. If frightened (all of the Gymnotidce 

 were very easily frightened) it swam rapidly away by the same motion of the anal 

 fin, the use of the pectorals being more frequent in guiding the fish. If it became 

 necessary for the fish to make a sudden turn, the entire body was slightly curved 

 in the desired direction. This curving of the body together with the rapid use of 

 the pectorals enables these long fishes to make quite abrupt turns. 



The second method of swimming involved the use of the entire body as well 

 as the fins. When the fish was being pursued, the anal fin moved, as before, in a 

 series of rapid waves, but in addition the entire body was at the same time moved 

 in a serpentine fashion. In this way it was able to swim very rapidly. An indi- 

 vidual would move the anal fin rapidly in the pecufiar manner of swimming when 

 held in the air. 



