Effects of Habituation to Different Light Intensities on the Head Retraction Response in Earthworms (Dendrobaena veneta)

This study investigated the effects of habituation to different light intensities on the head retraction response in the earthworm Dendrobaena veneta using a t1−t2 experimental design. Twenty-four adult earthworms were randomly assigned to two groups, each habituated with either 700 lux or 6,300 lux light (80 trials). Head retraction responses to 700, 2,100, and 6,300 lux were measured in pre- and posthabituation phases, with stimuli presented in a pseudorandomized order. Statistical analyses showed that, prior to habituation, responsiveness increased with light intensity across all subjects. During habituation, both groups exhibited significant declines in responsiveness, but the group exposed to 6,300 lux showed a steeper and more sustained decrease compared to the 700 lux group. Posthabituation testing demonstrated that worms habituated with 6,300 lux exhibited a generalized reduction in responsiveness across all test intensities, whereas the 700 lux group maintained an intensity-dependent response pattern. These findings challenge the traditional view that lower stimulus intensity leads to stronger habituation and instead indicate that higher intensity stimuli can produce more generalized and persistent habituation effects. The results emphasize the importance of distinguishing between learning and performance in habituation studies and highlight the value of standardized procedures for comparative research on nonassociative learning in invertebrates.