Visual pigment 467, a photosensitive pigment present in tench retinae

Photoreceptors and eyes of pikeperch Sander lucioperca, pike Esox lucius, perch Perca fluviatilis and roach Rutilus rutilus from a clear and a brown lake

The photoreceptors and eyes of four fish species commonly cohabiting Fennoscandian lakes with different light transmission properties were compared: pikeperch Sander lucioperca, pike Esox lucius, perch Perca fluviatilis and roach Rutilus rutilus. Each species was represented by individuals from a clear (greenish) and a humic (dark brown) lake in southern Finland: Lake Vesijärvi (LV; peak transmission around 570 nm) and Lake Tuusulanjärvi (LT; peak transmission around 630 nm). In the autumn, all species had almost purely A2-based visual pigments. Rod absorption spectra peaked at c.526 nm (S. lucioperca), c. 533 nm (E. lucius) and c. 540 nm (P. fluviatilis and R. rutilus), with no differences between the lakes. Esox lucius rods had remarkably long outer segments, 1.5-2.8-fold longer than those of the other species. All species possessed middle-wavelength-sensitive (MWS) and long-wavelength-sensitive (LWS) cone pigments in single, twin or double cones. Rutilus rutilus also had two types of short-wavelength sensitive (SWS) cones: UV-sensitive [SWS1] and blue-sensitive (SWS2) cones, although in the samples from LT no UV cones were found. No other within-species differences in photoreceptor cell complements, absorption spectra or morphologies were found between the lakes. However, E. lucius eyes had a significantly lower focal ratio in LT compared with LV, enhancing sensitivity at the expense of acuity in the dark-brown lake. Comparing species, S. lucioperca was estimated to have the highest visual sensitivity, at least two times higher than similar-sized E. lucius, thanks to the large relative size of the eye (pupil) and the presence of a reflecting tapetum behind the retina. High absolute sensitivity will give a competitive edge also in terms of short reaction times and long visual range.

The photosensitive retinal pigments of fishes from relatively turbid coastal waters

Digitonin extracts have been prepared from the retinae of a dozen species of marine and euryhaline teleost fishes from turbid water habitats. Spectrophotometric analysis of the extracts shows that the photosensitive retinal pigments of these species have maximum absorption above 500 mµ. In nine species there are retinene₁ pigments with λ_max between 504 and 512 mµ. In the marine but euryhaline mullet, Mugil cephalus, there is a porphyropsin with λ_max 520 mµ. A mixture of rhodopsin and porphyropsin in an extract of a marine puffer, Sphoeroides annulatus, was disclosed by partial bleaching with colored light. In addition, one other species has a 508 mµ pigment, of which the nature of the chromophore was not determined. The habitats in which these fishes live are relatively turbid, with the water greenish or yellowish in color. The spectral transmission of such waters is probably maximal between 520 and 570 mµ. It is suggested that the fishes have become adapted to these conditions by small but significant shifts in spectral absorption of their retinal pigments. These pigments are decidedly more effective than rhodopsin in absorption of wavelengths above 500 mµ. This offers a possible interpretation of the confusing array of retinal pigments described from marine and euryhaline fishes.

Morphology and spectral sensitivities of retinal and extraretinal photoreceptors in freshwater teleosts

Fish eyes possess a complicated morphological and neural organisation of retinal and extra-retinal receptors. Features such as photoreceptor mosaic array and photoreceptor grouping are unique among vertebrates. Spectral sensitivities of these photoreceptors range from UV to the red portion of the visible spectrum. Moreover, these sensitivities can change with the age of the animals. In this review we will examine thoroughly the morphology, and the spectral sensitivities of retinal and extra-retinal receptors and the influence upon them of factors such as hormones, ageing, season, habitat light conditions, and migration.

The contributions of the orientated photosensitive and other molecules to the absorption of whole retina

Observations have been made on the intact retinae of various animals chosen so as to give examples of golden, red and purple retinae. It is confirmed, by a method giving the absorption curve of unbleached retina, that the retinae of deep-sea fish have golden photosensitive pigments in optical densities so high that they absorb over 90% of blue-green light striking them. Despite the fact that light has passed through all the retinal layers the spectral total density curves correspond closely to those obtained on highly purified retinal extracts. After making a small correction for losses of light in the layers of retina other than the receptor layer the D min. / D max. ratios were about 0·25 for chrysopsin retinae and 0·25 for porphyropsin retinae. The losses of light in retinae from which the rod and cone layer has been brushed off, although varying little with wavelength, rise slowly in the near ultra-violet and red from a minimum in the yellow. The edge-fold preparation, useful for studying the dichroism of the retinal rods is described. Using such preparations it is shown that the spectral curves of dichroic difference in density are very close to those of total retinal density and therefore there can be little material absorbing light between 400 and 720 m μ orientated along the axes of unbleached rods. There is, however, such material absorbing near ultra-violet light present in unbleached retinae whose principal photosensitive pigment is derived from vitamin A 2 . For both vitamin A 1 and vitamin A 2 retinae the sense of the dichroism in the near ultra-violet is reversed in the first hour following the bleaching of the photosensitive pigments. This shows that the molecules of visual white (vitamin A ) in the bleached isolated retina are orientated with their axes of resonance parallel to the axes of the rods, a conclusion confirmed by studies of the polarization of the fluorescence of visual white in the bleached retina.

Axial chromatic aberration of the human eye: frequency or wavelength?

The axial chromatic aberration of the human eye is nearly perfectly described by a linear function when expressed in terms of frequency rather than wavelength. Since linear functions are simple to work with and more readily understood, there are advantages for the expression of these data in terms of frequency.

Visual pigments and the labile scotopic visual system of fish

Among mammals, birds, most reptiles and chondrichthians, only rhodopsins are present. Among agnathans, osteichthians, amphibians and certain freshwater turtles there are species having only porphyropsins or only rhodopsins or, more interestingly, both pigments, either sequentially or together. This latter grouping represents the paired-pigment species. Associated with the presence of paired-pigments is the possibility that the proportions of rhodopsin and porphyropsin may change. Depending on the characteristics of each paired-pigment species, naturally occurring changes in visual pigment ratios are related to migrations in anadromous and catadromous teleosts and anadromous cyclostomes and to seasonal variation in several teleosts. In addition, the visual pigment composition of certain species of teleosts has been altered by the specific effects of light, temperature, diet and hormones. Of two possible mechanisms for altering spectral sensitivity, varying the proportion of rhodopsin and porphyropsin is far more common than utilizing a single chromophore and changing the opsin. In addition to the long established evidence that extractable rod pigment ratios may change during the life cycle or in response to specific exogenous factors, there is the more recent recognition from microspectrophotometry that cone pigment ratios may also change in concert. The effect of lighting conditions and temperature on the visual pigment composition of certain paired-pigment species is presented.

Spectral distribution of the negative potential in the cichlid electroretinogram

A negative potential occurring in the ERG of certain cichlid fishes shows a unique set of properties when evoked by monochromatic light. Spectral distributions determined for Cichlasoma octofasciatum and Hemichromis bimaculatus show a blue (460-478 nm) maximum. no Purkinje shift upon light adaptation, and secondary peaks correlated with the species' behavioral preferences in reproduction and parental care.

Rhodopsin kinetics in the cat retina

The bleaching and regeneration of rhodopsin in the living cat retina was studied by means of fundus reflectometry. Bleaching was effected by continuous light exposures of 1 min or 20 min, and the changes in retinal absorbance were measured at 29 wavelengths. For all of the conditions studied (fractional bleaches of from 65 to 100%), the regeneration of rhodopsin to its prebleach levels required greater than 60 min in darkness. After the 1-min exposures, the difference spectra recorded during the first 10 min of dark adaptation were dominated by photoproduct absorption, and rhodopsin regeneration kinetics were obscured by these intermediate processes. Extending the bleaching duration to 20 min gave the products of photolysis an opportunity to dissipate, and it was possible to follow the regenerative process over its full time-course. It was not possible, however, to fit these data with the simple exponential function predicted by first-order reaction kinetics. Other possible mechanisms were considered and are presented in the text. Nevertheless, the kinetics of regeneration compared favorably with the temporal changes in log sensitivity determined electrophysiologically by other investigators. Based on the bleaching curve for cat rhodopsin, the photosensitivity was determined and found to approximate closely the value obtained for human rhodopsin; i.e., the energy Ec required to bleach 1-e-1 of the available rhodopsin was 7.09 log scotopic troland-seconds (corrected for the optics of the cat eye), as compared with approximately 7.0 in man.