A review of vertebrate and invertebrate ocular filters

Abstract

The spectral information available to an animal's visual system depends both on the wavelengths reaching its outer segments and the visual pigments contained within them. The former is governed not only by the chromatic stimuli present in the environment, but also by the degree to which these are modified through intraocular filters, before being absorbed by the visual pigments. Although the ocular media of the majority of animals are transparent to light above about 310 nm, their primary function being either refractive or nutritive, in some, pigments are present that filter the spectral content of the light reaching the retina. Light impinging on the visual pigments may have its spectrum further modified by both filters within the retina itself and by reflective structures behind the retina. This review sets out to describe the spectral characteristics of such ocular filters in both vertebrates and invertebrates, and outlines their location, the biochemical nature of the filtering pigments, and their function. In the following sections, filters are described by sequentially following the path of light through the eye's optical components from the outside world into the photoreceptors and, where present, reflection back by a tapetum. Most vertebrate filters, whatever their origin and placement, remove short wavelengths. Their function is therefore dealt with in a separate section. The eyes of invertebrates are more varied in their design and consequently possess a greater variety of functionally distinct filters. Their suggested functions are detailed within each section.