The red pigment in the eyes of the squid, blue crab, and horseshoe crab becomes photosensitive when treated with formalin, and bleaches in the light. The resulting change in density is approximately symmetrical around a maximum at 480 mµ in the blue green. This difference absorption spectrum is in rough agreement with the spectral sensitivity of the cephalopod eye and differs only slightly from the difference absorption spectrum of vertebrate visual purple.
The formalin-sensitized pigment is not melanoid. Its bleaching in squid retinas releases large quantities of retinene.
It is suggested that the light sensitivity of the normal squid photopigment may be independent of its light stability.