Subterranean Fishes of the World

4268

2019

AIM 2019

18

Troglobitic animals often exhibit dramatic differences in behavior and in the size and structure of sensory organs when compared to their epigean relatives. These differences can include the loss of eyes and the emergence or elaboration of other sensory organs. Recently, a species of troglobitic Eigenmannia (a genus of Gymnotiform weakly electric fishes) was discovered in a cave in Brazil (San Vincente II) within the Terra Ronca State Park (13°30' - 13°50' S, 46°0' - 46°30’W). We compared electromotor behaviors and the morphology of electrosensory systems of these cavefish with their epigean relatives from a nearby river (Rio da Lapa). Electrosocial and swimming behaviors were recorded at these field sites using a 16-electrode grid with 0.5 meter spacing that was placed into the streams. Fish swam freely through the grid and could be visually observed in the crystal clear water of this watershed. The epigean Eigenmannia showed a diurnal pattern of locomotor and signalling behavior, hiding in rocks and root systems along the sides of the streams and rivers during daylight hours, but emerging overnight with active swimming over distances of at least several meters. During the day, epigean Eigenmannia maintained nearly constant electric field frequencies, whereas at night these animals modulated their electric field frequencies. These modulations are similar to previously described social signals in this genus. In contrast, we saw no evidence of diurnal modulation of behavior in the hypogean Eigenmannia. These fish, which are either eyeless or have vestigial eyes, made short-duration excursions into slower-moving open water from their rocky hiding places on the edges of the stream. Underwater videos showed that these fish can make precise movements relative to conspecifics in the absence of visual cues. The hypogean fish produced similar categories of modulations of the electric fields as seen in the epigean fish. However, the electric field strength of the hypogean fish were dramatically increased relative to their epigean relatives. We made CT scans of 3 epigean and 3 hypogean Eigenmannia: the electric organs of the hypogean fish were hypertrophied relative to the epigean fish.

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