Light avoidance by non-ocular photosensing system in the terrestrial slug Limax valentianus

Abstract

Although the eye is the best studied photoreceptive organ in animals, the presence of non-ocular photosensing systems has been reported in numerous animal species. However, most of the roles non-ocular photosensory systems play remain elusive. We found that the terrestrial slug Limax valentianus avoids light and escape into dark areas even if it is blinded by the removal of the bilateral superior tentacle. The escape behaviour was more evident for the short-wavelength light. Illumination with blue but not red light, to the head elicited avoidance behaviour in the blinded slugs. Illumination to the tail was ineffective. The light avoidance behaviour of the blinded slugs was not affected by the removal of the penis, which lies on the brain in the head, suggesting that the penis is dispensable for sensing light in the blinded slug. mRNA of Opn5A, xenopsin, retinochrome, and that of rhodopsin to a lesser extent, were expressed in the brain according to RT-PCR. Light-evoked neural responses were recorded from the left cerebro-pleuro connective of the isolated suboesophageal ganglia of the brain, revealing that the brain is sensitive to short wavelengths of light (400 – 480 nm). This result is largely consistent with the wavelength dependency of the light-avoidance behaviour of the blinded slugs that we observed in the present study. Our results strongly support that the terrestrial slug Limax valentianus detect and avoid light by using their brain as a light sensing organ in the absence of eyes.