Photoreceptors, visual pigments, and ellipsosomes in the killifish, Fundulus heteroclitus: A microspectrophotometric and histological study

The photoreceptor layer of F. heteroclitus was examined by light and electr on microscopy. We identified four cone visual pigments with maximum absorba nce (lambda(max)) 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 t hat peaked in the middle wavelengths (503 nm). Electron-dense bodies, ellip sosomes and pseudoellipsosomes, were present in the distal ellipsoids of lo ng/middle (L/M) and long/long (L/L) wavelength double cones and in single s hort wavelength (S) cones, respectively. The light absorption of ellipsosom es 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 upp er half of the retina while the highest cone density was round in the centr o-ventral retina. An analysis of potential quantum catches for each cone ty pe us a function of retinal sector and underwater irradiance characteristic s revealed higher overall quantum catches for cones in the upper retina whe n the light field was assumed homogeneous, and higher quantum catches for c ones in the lower retina when downwelling. horizontal, and upwelling irradi ances 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 ir radiance sensed by double cones.