Visual pigments and dichroism of anchovy cones: a model system for polarization detection

The retinas of anchovies have two unique photoreceptor types: "bifid" and "long" cones (Fineran & Nicol, 1976). The outer segments of these cells contain multiple layers of membranes (lamellae) oriented longitudinally (axially). This orientation is distinct from that in all other vertebrate rods and cones, where the lamellae are stacked transversely with their planes perpendicular to the incident light path. Although the common arrangement provides optimal absorption for normally incident light rays, it is also insensitive to the rays' direction of vibration (i.e. their polarization). In contrast, the two mutually perpendicular sets of axially oriented lamellae segregated into bifid and long cones could function as the principal analyzers for linearly polarized light, as previously hypothesized (Fineran & Nicol, 1976, 1978). Here, we report on a microspectrophotometric study that shows (1) the presence of two spectrally distinct visual pigments in the three photoreceptor types of the bay anchovy retina; these are typical vertebrate pigments in that they bleach, when exposed to light, and have absorption spectra like all other vitamin A1-based visual pigments; (2) that the rods and cones exhibit dichroic absorption of light in accordance with their lamellar orientation, and (3) that the two cone types of the retina contain a spectrally indistinguishable pigment with peak absorbance (lambda(max)) around 540 nm, while the rods contain a rhodopsin-like pigment with lambda(max) near 500 nm. Compared to other vertebrates, anchovies are remarkable for using a monochromatic cone system with unusual specializations supportive of a polarization detection system.