J. N. Lythgoe

The spectral clustering of visual pigments.

Evidence is presented that the λmax of visual pigments are not distributed uniformly throughout the spectrum but, on the contrary, are clustered around certain discrete positions. Eight of these positions for the A1 pigments have been determined from a consideration of published data and a comparably large amount of new data presented here for the first time. These positions are 478.5, 486.5, 493, 501, 506, 511.5, 519 and 528 nm. The relationship between A1 and A2 pigments, and the structural implications of the clustering phenomenon, are discussed.

The ecology of cone pigments in teleost fishes

Abstract The visual pigments in the retinae of 18 species of fishes were measured microspectrophotometrically and assigned to specific cone types. The major ecological implications of these data are seen by grouping the fishes into habitat classes based on spectral quality of the water and depth. It is seen that double and twin cones in the examined species contain a visual pigment roughly matching the water background spacelight, while single cones occupying typically the “additional” position in a square mosaic unit are invariably blue-sensitive and offset from the water transmission maximum. In photopic dichromats the central single cone of a square unit was found to contain a pigment identical to that found in the twin cones. The relevance of these findings to contrast enhancement, adaptation to rapid changes in spectral quality of the water, and formation of “ghost” pigments through opponancy is also discussed.

The visual pigment basis for cone polymorphism in the guppy, Poecilia reticulata

Long-wavelength visual pigment polymorphism, similar to that found in primates, was found in the guppy using microspectrophotometry (MSP). Guppies have a rod pigment with a wavelength of maximal absorbance (lambda max) at 501 nm and cone pigments with peak absorbance at 408 and 464 nm. In addition individuals may have one, two or three cone classes in the yellow-green region of the spectrum with mean lambda max values of 533, 543 and 572 nm. Unlike primates this variation is not sex-linked and may be based on only two visual pigments which occur either on their own in outer-segments of the 533 nm and 572 nm cone classes or as a mixture in the 543 nm cone class.

Visual pigment polymorphism in the guppy Poecilia reticulata.

Visual pigment polymorphism similar to that found in primates is described in the photoreceptors of wild-caught guppies (Poecilia reticulata). Microspectrophotometric examination of retinal cells revealed rod visual pigments with a lambda max close to 503 nm. Classes of cones with lambda max around 410 and 465 nm were found, together with a population of pigments in the 529-579 nm range. It is in these long-wavelength cones that polymorphism occurs. Male guppies are highly polymorphic for body colour and it is possible that the cone polymorphism is related to the appreciation of the different yellow, orange and red carotenoid colour spots that are used in sexual display.

Visual pigments and visual range underwater

In shallow water the spectral radiance of a grey object differs from that of the water background spacelight. The spectral absorbance of the visual pigments present in the eye will therefore affect the perceived contrast between an object and its water background, and hence the range at which the object can be seen underwater. A diver-operated instrument is described that allows the spectral contrast between a grey target and its water background to be measured. The data so obtained show that in Mediterranean waters those visual pigments with a λmax corresponding to the wavelength of maximum light transmission through the water are best suited for detecting large very dark or very bright objects. But “offset” visual pigments are more suitable for detecting small grey objects in shallow water.

The Adaptation of Visual Pigments to the Photic Environment

In 1936 Clarke wrote: “These results [clear ocean water selectively transmits blue light] raise the question of the possibility of a shift in sensitivity of the eyes of a deep water fish towards the blue end of the spectrum.” This prediction must be one of the most accurate in biology, for twenty-one years later Denton and Warren (1957) and Munz (1957) published papers showing that bathypelagic fishes did indeed possess large quantities of visual pigments with λmax located in the blue region of the spectrum.

The ecology of the visual pigments of snappers (Lutjanidae) on the Great Barrier Reef

The visual pigments in the retinal photoreceptors of 12 species of snappers of the genus Lutjanus (Teleostei; Perciformes; Lutjanidae) were measured by microspectrophotometry. All the species were caught on the Great Barrier Reef (Australia) but differ in the colour of the water in which they live. Some live in the clear blue water of the outer reef, some in the greener water of the middle and inshore reefs and some in the more heavily stained mangrove and estuarine water. All the species had double cones, each member of the pair containing a different visual pigment. Using Bakers and Smiths (1982) model to predict the spectral distribution of ambient light from chlorophyll and dissolved organic matter it was found that the absorption spectra of the visual pigments in the double cones were close to those that confer the maximum sensitivity in the different water types. Single cones contained a blue or violet-sensitive visual pigment. The visual pigments in the rods showed little variation, their wavelength of maximum absorption always being in the region 489–502 nm.

Light and Vision in the Aquatic Environment

The conditions for vision underwater are more exacting than on land. As the depth increases, the daylight gets progressively less bright, and that which remains comes mostly from above and is restricted to a narrow and variable band of the spectrum. Deeper than about 1,000 m in even the clearest water, there is not enough daylight for vision, and animals must produce their own light. At all depths, light is more strongly scattered than is usual on land, and it is principally for this reason that it would be unusual indeed to be able to see farther than 100 m through the water. Both air and water scatter, refract, and absorb light; but the effects are quantitively much greater in water, and it is often possible to recognize visual adaptations to the particular conditions for vision underwater, and several of these will be considered in this chapter. However, an animal is concerned with the real problems of finding food, finding mates, and avoiding getting eaten, and several optical problems may be involved at any one time. If we are to understand how an animal is adapted to the real world around it. an attempt must be made to take a more integrated view of the problems it encounters in its visual environment.

The influence of light on the twilight migrations of grunts

SynopsisBehaviors that precede the daily migrations of mixed-species schools of juvenile grunts (Pomadasyidae), from patch reefs to grass beds at dusk and vice versa at dawn, are defined and utilized to ascertain the precision of the migrations. Although premigratory behaviors differ at dusk and dawn, the migrations are precise twilight events which occur at the same light intensities during dawn and dusk. Histological sections of the retina reveal that both cones and rods are fully exposed to ambient light during the migrations. Under the difficult photic conditions that prevail during migration, the retina is structured photomechanically to maximize the absorption of ambient light. Body colorations of the grunts, which consist mostly of intense colored stripes during the day, are replaced at night by cryptic melanic patterns. The precision of migration, the photomechanical movements in the retina, and the changes in body coloration are considered adaptive because they reduce predation on grunts when they migrate and are most vulnerable to attack. In support of this conclusion, the migrations take place just before the evening and just after the morning ‘quiet period’ - thus they avoid that period during twilight when predation is highest in tropical fish communities.

Visual pigments in the individual rods of deep-sea fishes

SummaryThe visual pigments in the rods of 15 species of deep-sea fish were examined by microspectrophotometry. In 13 species a single visual pigment was found. The λmax of these pigments, which ranged from 475 nm to 488 nm, suggest they give the fish maximum sensitivity to the ambient light in the deep, blue ocean waters where they live. In two species two visual pigments were found in separate rods.Bathylagus bericoides had rhodopsins of λmax 466 nm and 500 nm andMalacocephalus laevis had two rhodopsins of λ max 478 nm and 485 nm. It is noted that the species with two visual pigments tend to be dark in colour and live in deeper, darker, water.