Both photons and fluoride ions excite limulus ventral photoreceptors.

Illuminating Limulus ventral photoreceptors with dim light results in a response composed of discrete waves. In the dark, adding 10 mM sodium fluoride to the artificial seawater that bathes the photoreceptor includes discrete waves. The fluoride-induced waves are similar to those evoked by light. These findings suggest that fluoride induces a process similar or identical to visual excitation of the photoreceptor.