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

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 that non-ocular photosensory systems play remain elusive. We found that the terrestrial slug Limax valentianus avoids light and escapes into dark areas even if it is blinded by the removal of the bilateral superior tentacle. The escape behaviour was more evident for short-wavelength light. Illumination to the head with blue but not red light 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, to a lesser extent, rhodopsin was expressed in the brain according to RT-PCR. Light-evoked neural responses were recorded from the left cerebro-pleural 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 L. valentianus detects and avoids light by using its brain as a light-sensing organ in the absence of eyes.