Light sensitivity of the rhinophores and eyes ofAplysia

Summary

The rhinophores (posterior tentacles) of the head ofAplysia (Fig. 1) are well known as chemical and tactile sensors. Here, they are shown to be excellent photoreceptors too.

• 1. Illumination of the rhinophore of an isolated preparation of the rhinophores-cerebral ganglion-eye (Fig. 2) evoked neuronal activity in cerebral neurons and efferent activity in the optic nerve (Fig. 3).

• 2. Evoked activity in the ipsilateral optic nerve was a phasic burst of unitary spikes at “on”. The number of spikes was approximately proportional to the log of intensity (Fig. 4). After the phasic “on” response the normally ongoing efferent activity originating in the cerebral ganglion and recorded in the optic nerve was suppressed briefly but resumed if the light was continued for minutes (Fig. 3).

• 3. The spectral sensitivity of the rhinophore and eye are similar. Both have a peak near 500 nm and sensitivity is 1 log unit less at 405 nm and 1.7 log units less at 660 nm than at 500 nm (Fig. 6).

• 4. The greatest photosensitivity of the rhinophore is at the tip in the sensory groove (Fig. 2) directed laterally and upward from the head.

• 5. The rhinophores may serve as extraocular photoreceptors to the circadian systems or modulators of ocular photosensitivity and they may also function in simple visual discriminations since orienting movements of the rhinophores were evoked by illumination of the head of intactAplysia.