Conditions of illumination during rearing affect specific components of phototactic behavior of laboratory-reared Hermissenda

We have investigated the effect of 3 different conditions of illumination during rearing on phototactic behavior of the marine mollusk Hermissenda. Rearing Hermissenda in the dark results in a significant increase in the variability of positive phototactic behavior of adults and decreases responsiveness to illumination. The decrease in the positive phototactic response, as measured by the significant increase in the time taken by dark-reared Hermissenda to enter an illuminated area, was due to a decreased velocity of locomotion in the light gradient. These changes in phototactic behavior were not observed for Hermissenda reared on 12 h light-12 h dark cycles or under constant illumination. The affects of dark rearing on phototactic behavior cannot be the result of abnormal photoresponses since the photoreceptors of dark-reared Hermissenda exhibited normal generator potentials in response to light. A cellular analysis of the behavior of laboratory-reared Hermissenda will be useful in studying phototaxis since learning and experiences during rearing have two quite different behavioral consequences.