Photosensitivity of the central nervous system of Limulus polyphemus

The photosensitivity of the central nervous system (CNS) of the horseshoe crab, Limulus polyphemus, was investigated by analyzing changes in motor nerve activity in the segmental nerves of prosomal and opisthosomal ganglia. Spontaneous efferent impulses were recorded in the dark from all the investigated segmental nerves. Impulse trains from the 7th dorsal nerve in the prosomal CNS were inhibited in response to illumination of the whole CNS. Impulse trains from each of the 9-13th dorsal nerves in the isolated opisthosomal CNS were inhibited, and the impulse train from each the 14-16th dorsal nerve was elicited or inhibited upon illuminating the whole CNS. Spontaneous rhythmic bursts at 20-80 s intervals were recorded in the dark from the ventral nerves of the isolated opisthosomal CNS. In the presence of light, the rhythmicity of spontaneous bursts disappeared and other species of impulse trains were elicited. In single ganglion preparations, isolated from the rest of the CNS by surgically severing the connectives, similar photoresponses were recorded before and after isolation. These results demonstrate that the CNS of Limulus is a photosensitive organ.