Student Theses and Dissertations

Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


This paper presents the results of a twelve-month field study of the behavior and ecology of gymnotid fish from the Rupununi District of Guyana, South America, and from Trinidad, W.I. There are 13 species of gymnotids in the Rupununi District, of which six to eight species occur frequently in the main study area in Moco-moco Creek. All species produce electrical discharges which are presumably used for object location and for electrical communication. Gymnotids are active at night and inactive during the day. Some species tend to specialize on the types of daytime hiding places selected, while others generalize. Eigenmannia virescens and Sternopygus macrurus select a wide range of hiding places, but Sternopygus individuals are randomly dispersed whereas Eigenmannia tend to clump together. The breeding season of most gymnotids starts with a migration into a quiet, flooded swamp, immediately after the first flood at the beginning of the rainy season. Inspection of gonadal condition and plots of the growth rates of gymnotid larvae show that most species delay breeding until the onset of the rains. Sternopygus is an exception in that it begins breeding in the main creek before the beginning of the rains. Both Eigenmannia and Sternopygus produce electric discharges that differ from all other species of gymnotids from Moco-moco Creek. Among the fish with tone discharges, Sternopygus produces the one with lowest frequency (50 to 150 Hz) and Eigenmannia produces the one with next lowest (250 to 600 HZ). Each species produces an appropriate speciesspecific response when presented with an electric sine wave stimulus of the appropriate frequency. Sternopygus exhibits a sexual difference in the resting frequency of its normal discharge. Reproductive males discharge at 55 to 70 Hz whereas females discharge at 100 to 140 Hz. This frequency difference has communicative significance. Males, showing site-attachment during the breeding season, respond to playback of sine waves of frequencies within the female range with transient frequency increases and discharge cessations. Males do not respond to playback of sine waves of frequencies corresponding to the other males nor to other species of tone fish from the Rupununi. Further evidence indicates that variations in the discharge of males may function in the courtship of this species. Although there is a statistical difference in the discharge frequency between male and female Eigenmannia, the overlap is extensive. Males discharge at 250 to 600 Hz whereas females discharge at 340 to 560 Hz. The agonistic behavior of Eigenmannia was observed in aquaria in the field, and was described in terms of the component actions, both electrical and motor. Aside from the normal discharge, electrical actions included Rises and Interruptions. Rises that lasted longer than 2 seconds (Long Rises) were given by subordinate fish, while they were retreating from attacks from their opponent. Some Rises were given spontaneously, and they tended to elicit attacks from the dominant fish. Long Rises act as a submissive display in Eigenmannia. Interruptions were given by dominant fish. They were given in bouts of varying numbers depending on differences in the likelihood of attack. Interruptions cause the opponent to retreat, and are thus classed as a threat display. During playback experiments with Eigenmannia, Attacks and Interruptions were given in response to playback of Eigenmannia-like sine waves, and of tape-recorded signals from Eigenmannia. Playback of non-Eigenmannia-like frequencies, or of other species from Moco-moco Creek, were ineffective.


A thesis submitted to the Faculty of The Rockefeller University in partial fulfillment of the requirements for the degree of Doctor of Philosophy

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