Refractive index gradients subserve optical isolation in a light‐adapted reflecting superposition eye

Abstract

AbstractThe freshwater shrimp Macrobrachium rosenbergii (de Man) has reflecting superposition compound eyes. Upon adaptation from dark to light conditions, the eye converts from superposition to apposition optics by screening pigment migration. The clear zone, typical of dark‐adapted superposition eyes, persisted after adaptation to 500 lux, when the eye optically behaved like an apposition eye. In this state, optical isolation is maintained by a proximal refractive‐index gradient in the crystalline cone, which only requires the proximal pigment to be in the light‐adapted position around the junction between the cone and rhabdom. Thus, the distal pigment need not invade the clear zone for the eye to convert to apposition optics. This mechanism probably decreases the time required for light adaptation. Furthermore, as a result of total internal reflections in the cone tip, the proximal gradient increases the acceptance angle.