THE PHOTOMOVEMENT OF Caenorhabditis elegans, A NEMATODE WHICH LACKS OCELLI. PROOF THAT THE RESPONSE IS TO LIGHT NOT RADIANT HEATING

Abstract

Abstract— Caenorhabditis elegans adults were tested at constant temperature with 10 s periods of monochromatic light alternated with 20 s dark periods. Stimuli at effective intensities and wavelengths caused an increase in the frequency of ecclitic (phobic, avoidance) responses, which was measured as an increase in the probability of a temporary reversal in direction of movement. For monochromatic stimuli ranging from 420 to 680 nm at a constant 56 picoeinsteins s ‐1 cm ‐2 , only those at520–600 nm elicited significant responses. At 540 nm the threshold fluence rate was approximately 30 pE s ‐1 cm ‐2 . At saturating intensities the mean reversal probability was increased to 0.20 in 10 s from a background level of 0.12. approximately. Because C. elegans lacks ocelli and is very sensitive to temperature, possible sources of radiant heating were considered in detail, including (a) infrared present in the stimuli, (b) absorption of light by the arena, and (c) absorption of light by a nematode pigment. All possible sources were found to cause a negligible temperature rise, on the order of or less than the natural temperature fluctuations inside the worm, 1.5 times 10 ‐6 °C. A 2 times 10 ‐4 °C temperature rise produced by a 1230 nm infrared stimulus had no significant effect on reversal frequency. It was concluded that the response to illumination must have been to light, and not to temperature changes. Large, + or ‐ 2 °C changes from the acclimation temperature caused significant increases in the background frequency of ecclitic responses (a thermoecclisis or thermoklinokinesis). However, neither the threshold nor the saturation level of light‐induced responses was affected by the ± 2°C changes.