Effects of acoustic vibration on the reorientations of <italic>C. elegans</italic>

Abstract

As a typical model organism, the nematode C. elegans detect and respond to diverse environmental stimuli, such as light, temperature, odors and chemicals. The underline neuronal circuitry has been defined by decades of work by C. elegans researchers. However, there’s no work done on its perception to acoustic vibration, which is a critical environmental cue in the natural habitat. For the first time, we quantified worm’s response to acoustic vibration with precisely controlled amplitude and frequency. In an isotropic environment, worm’s navigation can be described as a random walk. Periods of forward movements are interrupted by random reorientations. C. elegans respond to environmental cues by biasing its reorientations. In this work, we delivered acoustic wave with defined amplitude and frequency to freely moving C. elegans . A custom-built real-time computer vision system was used to record the movements of individual young adult worms navigating the agar surface. Data collected was analyzed using customized particle-tracking and shape analysis algorithms. To automatically flag reorientations, we considered both the posture of the animal and the movement of its center of mass. Rapid reorientations ( Ω turns or reversal-turns) were flagged when the heading change of the center of mass trajectory was > 60° over 1 second. It was found that worms reorientate more when sound is on and reorientate less when sound is off. To quantify animal’s response to acoustic wave, we defined a non-dimensional acoustic sensitive index according to the formula. The index approaches 0 or 1 if there is no response or big response, respectively. We tested animal’s response to acoustic wave of varying displacement amplitude and fixed frequency. It was found that the acoustic sensitive index is proportional to the displacement amplitude. However, when we further tested animal’s response to acoustic wave of fixed displacement amplitude and varying frequency, the data shows that the acoustic sensitive index first increases then decreases with the increase of frequency. To characterize the mechanical vibration of the agar surface during the acoustic stimuli, we used a laser vibrometer (Polytec). It was showed that there’s obvious response when the sound cause vibration is as low as ~200 nm. The vibration level is much lower than other mechanical stimuli, such as gentle hair touch or plate tapping, which were used for studying the mechanical sensation of C. elegans in most labs. Our results suggest that worm can sense acoustic vibration. It responds to acoustic vibration by biasing the reorientation rate during navigation. The acoustic sensitive index depends on the frequency and displacement amplitude of the acoustic wave. The vibration level C. elegans can sense is much lower than people normally thought. Our work laid the ground work for studying C. elegans acoustic sensation. It would be interesting to further understand the neural circuits and cellular mechanism of this behavior.