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Ron H. Douglas
Since vertebrates appear to locate relatively few visual pigments at specific points
of the spectrum, it would seem likely that these points have some specific adaptive
significance to the animal. This, however, is extremely difficult to demonstrate, since
it is usually not clear what precisely the animal is trying to achieve with its pigments.
The pigment may, for instance, be designed to simply maximize quantum catch by
matching its absorption to the surrounding spacelight, but in most instances is
probably detecting a more specific visual target (Partridge and Cummings 1999).
Correlations between a pigment's 'A max and the visual stimulus are most apparent
in fish, which of all vertebrates have the greatest visual pigment diversity. This is
because the underwater environment is optically more diverse than any other. The
spectral composition ofbodies of water is highly variable due largely to differences
in the quantity and identity of the substances dissolved or suspended within them
(Douglas 1991 ). Thus, much of the open ocean, and certain bodies of freshwater,
approach the transmission characteristics of pure water and are a clear blue,
preferentially transmitting short-wave radiation. In coastal water, on the other hand,
intermediate wavelengths tend to dominate, giving it a yellow/ green colour due to the
presence of chlorophyll and other dissolved organic matter. Freshwater habitats are
highly variable, but in contrast to oceanic and coastal waters they are often red/brown
in colour, transmitting mainly longwave radiation due to suspended organic matter
and run-off from the surrounding land (Lythgoe 1972; Loew and McFarland 1990).
Furthermore, the spectral characteristics of the underwater environment, especially
near land, may vary seasonally or over a shorter time scale, due to variations in the
production of organic matter within the water and changes in the identity or quantity
of material reaching it from land. Not only does the spectral content of the underwater
light environment vary, so does its intensity, decreasing both with depth and turbidity.
Consequently, fish live in almost every conceivable optical environment.
2 Visual Pigment Structure
All vertebrate visual pigments consist of two components: the chromophore, an
aldehyde of vitamin A, which absorbs the light, and a protein, opsin, which
determines the spectral absorption characteristics of the chromophore. They are
located within the membranes of photoreceptor outer segment discs (Fig. 2).
The chromophore in most vertebrates is retinal, a derivative of vitamin A 1•
However, some fish, reptiles and amphibians possess an additional chromophore, 3dehydroretinal, derived from vitamin A2• All visual pigments with retinal as their
chromophore are known as rhodopsins, whereas vitamin A2-based pigments are
referred to as porphyropsins. A visual pigment consisting of a given opsin and using
retinal as the chromophore, will have a narrower absorption spectrum peaking at
shorter wavelengths than a pigment composed of the same opsin bound to the AT
derived 3-dehydroretinal (Fig.l).These two pigments, based on the same opsin but a
different chromophore, are known as a pigment pair. Since porphyropsins are more
longwave-sensitive, they are common in the rods and cones of freshwater fishes but
Ron H. Douglas
Since vertebrates appear to locate relatively few visual pigments at specific points
of the spectrum, it would seem likely that these points have some specific adaptive
significance to the animal. This, however, is extremely difficult to demonstrate, since
it is usually not clear what precisely the animal is trying to achieve with its pigments.
The pigment may, for instance, be designed to simply maximize quantum catch by
matching its absorption to the surrounding spacelight, but in most instances is
probably detecting a more specific visual target (Partridge and Cummings 1999).
Correlations between a pigment's 'A max and the visual stimulus are most apparent
in fish, which of all vertebrates have the greatest visual pigment diversity. This is
because the underwater environment is optically more diverse than any other. The
spectral composition ofbodies of water is highly variable due largely to differences
in the quantity and identity of the substances dissolved or suspended within them
(Douglas 1991 ). Thus, much of the open ocean, and certain bodies of freshwater,
approach the transmission characteristics of pure water and are a clear blue,
preferentially transmitting short-wave radiation. In coastal water, on the other hand,
intermediate wavelengths tend to dominate, giving it a yellow/ green colour due to the
presence of chlorophyll and other dissolved organic matter. Freshwater habitats are
highly variable, but in contrast to oceanic and coastal waters they are often red/brown
in colour, transmitting mainly longwave radiation due to suspended organic matter
and run-off from the surrounding land (Lythgoe 1972; Loew and McFarland 1990).
Furthermore, the spectral characteristics of the underwater environment, especially
near land, may vary seasonally or over a shorter time scale, due to variations in the
production of organic matter within the water and changes in the identity or quantity
of material reaching it from land. Not only does the spectral content of the underwater
light environment vary, so does its intensity, decreasing both with depth and turbidity.
Consequently, fish live in almost every conceivable optical environment.
2 Visual Pigment Structure
All vertebrate visual pigments consist of two components: the chromophore, an
aldehyde of vitamin A, which absorbs the light, and a protein, opsin, which
determines the spectral absorption characteristics of the chromophore. They are
located within the membranes of photoreceptor outer segment discs (Fig. 2).
The chromophore in most vertebrates is retinal, a derivative of vitamin A 1•
However, some fish, reptiles and amphibians possess an additional chromophore, 3dehydroretinal, derived from vitamin A2• All visual pigments with retinal as their
chromophore are known as rhodopsins, whereas vitamin A2-based pigments are
referred to as porphyropsins. A visual pigment consisting of a given opsin and using
retinal as the chromophore, will have a narrower absorption spectrum peaking at
shorter wavelengths than a pigment composed of the same opsin bound to the AT
derived 3-dehydroretinal (Fig.l).These two pigments, based on the same opsin but a
different chromophore, are known as a pigment pair. Since porphyropsins are more
longwave-sensitive, they are common in the rods and cones of freshwater fishes but