Video camera-computer tracking of nematodeCaenorhabditis elegans to record behavioral responses

A new method is used to analyze responses to changes in the concentration of two chemical stimuli. Nematodes are allowed to move around on the surface of a thin layer of agar across which a stream of air blows to carry volatile stimuli. Darkfield illumination provides high-contrast images of the worms which are acquired by a video camera and fed to a microcomputer which is programed to simultaneously track and record the movements and changes in direction of as many as 25 animals. The results are reported in real time. The worms respond to an increase in CO₂ concentration by decreasing the number moving and increasing the number of changes of direction. Both responses adapt to steady-state levels in about half a minute. This suggests that they respond by changing the probability of initiating a reversal bout. This observation adds a repellent to the class of stimuli thatC. elegans reponds to by klinokinesis. The resonses to changes in oxygen concentration are somewhat different. Movements and changes in direction both decrease when the oxygen concentration falls and increase when the concentration rises. No adaptation is seen within the one-minute time span observed. This observation provides further evidence that the response to oxygen differs from the response to other chemicals and may be sensed internally. These observations demonstrate that computer tracking is a sensitive method of analyzing animal behavior. It is further demonstrated that a significant response can be detected to a relatively weak stimulus in less than 5 min.