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 (λmax) around 540 nm, while
the rods contain a rhodopsin-like pigment with λmax
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.