The photoreceptor layer of F. heteroclitus was
examined by light and electron microscopy. We identified
four cone visual pigments with maximum absorbance (λmax)
in the UV (363 nm), short (400 nm), middle (463 nm), and
long (563 nm) wavelength regions of the spectrum and a
rod visual pigment that peaked in the middle wavelengths
(503 nm). Electron-dense bodies, ellipsosomes and pseudoellipsosomes,
were present in the distal ellipsoids of long/middle (L/M)
and long/long (L/L) wavelength double cones and in single
short wavelength (S) cones, respectively. The light absorption
of ellipsosomes indicated the presence of reduced cytochrome-c
with the highest optical densities found in the M members
of L/M double cones. By contrast, S cones contained pseudo-ellipsosomes
which had very low optical density. UV cones were present
everywhere as part of square or row mosaics in the retina
of F. heteroclitus. Cone packing was on average
higher for locations in the upper half of the retina while
the highest cone density was found in the centro-ventral
retina. An analysis of potential quantum catches for each
cone type as a function of retinal sector and underwater
irradiance characteristics revealed higher overall quantum
catches for cones in the upper retina when the light field
was assumed homogeneous, and higher quantum catches for
cones in the lower retina when downwelling, horizontal,
and upwelling irradiances were considered separately. At
dusk, quantum catch was highest for M cones and the contribution
to the overall retinal quantum catch by UV and S cones
was much greater than during daylight hours. We propose
that UV and S cones may be used to detect targets of interest
against the background irradiance sensed by double cones.
Light in the Dark: Retrieving Underwater Irradiance in Shallow Eutrophic Waters From AC-S Measurements
