Long-term Behavioral Tracking of Freely Swimming Weakly Electric Fish

Abstract

Long-term behavioral tracking can capture and quantify natural animal behaviors, including those occurring infrequently. Behaviors such as exploration and social interactions can be best studied by observing unrestrained, freely behaving animals. Weakly electric fish (WEF) display readily observable exploratory and social behaviors by emitting electric organ discharge (EOD). Here, we describe three effective techniques to synchronously measure the EOD, body position, and posture of a free-swimming WEF for an extended period of time. First, we describe the construction of an experimental tank inside of an isolation chamber designed to block external sources of sensory stimuli such as light, sound, and vibration. The aquarium was partitioned to accommodate four test specimens, and automated gates remotely control the animals' access to the central arena. Second, we describe a precise and reliable real-time EOD timing measurement method from freely swimming WEF. Signal distortions caused by the animal's body movements are corrected by spatial averaging and temporal processing stages. Third, we describe an underwater near-infrared imaging setup to observe unperturbed nocturnal animal behaviors. Infrared light pulses were used to synchronize the timing between the video and the physiological signal over a long recording duration. Our automated tracking software measures the animal's body position and posture reliably in an aquatic scene. In combination, these techniques enable long term observation of spontaneous behavior of freely swimming weakly electric fish in a reliable and precise manner. We believe our method can be similarly applied to the study of other aquatic animals by relating their physiological signals with exploratory or social behaviors. Video LinkThe video component of this article can be found at

Enhanced sensory sampling precedes self-initiated locomotion in an electric fish

“…1A) (Jun et al, 2012;Jun et al, 2014). Underwater sound was recorded during experiments by a hydrophone (TC4013, Reson Inc., Goleta, CA, USA) and amplified (50 dB gain; VP1000, Reson Inc.) to check for animal movements triggered by external vibrations.…”

“…Underwater sound was recorded during experiments by a hydrophone (TC4013, Reson Inc., Goleta, CA, USA) and amplified (50 dB gain; VP1000, Reson Inc.) to check for animal movements triggered by external vibrations. Detailed descriptions of the experimental chamber, the aquarium tank designs and water conditions are provided elsewhere (Jun et al, 2014).…”

“…We quantified the activity level of an animal by integrating the EOD amplitude fluctuations from all recording channels. The activity level computed from the EOD recording accurately quantified the animal's movements without requiring a concurrent video recording, which demands significantly greater storage space due to high-speed and high-resolution imaging for sensitive motion detection [see Jun et al (Jun et al, 2014) for the method of computing the activity level].…”

“…The temporal precision of the EOD-based movement-onset times was improved to the level of a visual detection method ( Fig. 5A-F) by applying the following procedures (Jun et al, 2014). First, we selected a subset of up-transitions exhibiting clearly discernible movement onset and clear down-and up-states based on the following criteria.…”

“…We conclude that each EOD constitutes a discrete sampling event and that EODR is therefore a direct measure of sampling rate over some time interval. Pulse-type electro-sensation thus offers a distinct experimental advantage because each sampling event can be readily and non-invasively monitored from an unrestrained animal (Jun et al, 2012;Jun et al, 2014). In gymnotiform species, EOD pulses are directly driven by a hindbrain pacemaker that is modulated by diencephalic and medullary prepacemaker activity (Heiligenberg et al, 1981;Kawasaki et al, 1988;Dye, 1988;Metzner, 1993;Caputi et al, 1993;Zupanc and Maler, 1997;Wong, 1997;Comas and Borde, 2010).…”