The Ecology of Teleost Fish Visual Pigments
219
are rare in the photoreceptors of marine animals. All visual pigments with A max
beyond ca. 570 nm are based on 3-dehydroretinal.
All opsins have a similar structure, consisting of a chain of around 350 amino acids
that crosses the outer segment disc membrane seven times in the form of a-helices.
Isolated retinal and 3-dehydroretinal absorb at ca. 380 and 400 nm respectively.When
bound to the opsin, always via a Schiffs base linkage at a lysine on the seventh
transmembrane helix, the amino acids tune the chromophore to absorb at longer
wavelengths. Thus, the absorption spectrum of a visual pigment depends on both the
identity of the chromophore and the amino acid composition of the opsin surrounding
that chromophore (Bowmaker 1991, 1995; Partridge and Cummings 1999 for
reviews).
3 Shallow-Water Teleost Rod Visual Pigments
Since rods are used under scotopic conditions, they are concerned with maximising
photon capture. Most simply this involves matching the absorption spectrum of the
visual pigment to the ambient illumination (Partridge 1990) (sensitivity hypothesis).
Consequently, fish inhabiting freshwater, which is an environment containing
predominantly longwave radiation, generally have more red-sensitive visual pigments,
using 3-dehydroretinal as the chromophore, than those inhabiting green-based coastal
waters (Munz 1964; Schwanzara 1967; Munz and McFarland 1973, 1977; Lythgoe
1972). However, even among rods this correlation is not always good (e.g.,
Bowmaker et al. 1994) and animals from seemingly quite different habitats, for
example, often have similar rod pigments (Lythgoe et at. 1994 ).
However, expecting a simple match between the background light and the visual
pigment is in many cases an oversimplification because it assumes the animal is
attempting to maximize sensitivity to the surrounding spacelight when in fact it is
often detecting a specific target (Partridge and Cummings 1999). For example, a
visual pigment with ' A max offset from the background will enhance the contrast of
certain targets and hence increase the range at which they can be detected (Lythgoe
1968, 1972; McFarland and Munz 1975) (contrast hypothesis). An animal's
behaviour and the requirements this imposes on the visual system must therefore also
be considered along with the surrounding illumination. The spectral composition of
the environment also changes throughout a 24-hour period and Hobson et at. (1981)
have shown that scotopic visual pigments may be better matched to photopic
conditions at twilight rather than the starlight and moonlight prevailing at night.
However, in most instances a case can be made for explaining the A max of most rod
visual pigments based on either the contrast or the sensitivity hypothesis. These
arguments assume that sunlight is the only, or major source of radiation available.
While this is certainly true for freshwater species and shallow oceanic animals, the
situation is very different in the deep ocean.
219
are rare in the photoreceptors of marine animals. All visual pigments with A max
beyond ca. 570 nm are based on 3-dehydroretinal.
All opsins have a similar structure, consisting of a chain of around 350 amino acids
that crosses the outer segment disc membrane seven times in the form of a-helices.
Isolated retinal and 3-dehydroretinal absorb at ca. 380 and 400 nm respectively.When
bound to the opsin, always via a Schiffs base linkage at a lysine on the seventh
transmembrane helix, the amino acids tune the chromophore to absorb at longer
wavelengths. Thus, the absorption spectrum of a visual pigment depends on both the
identity of the chromophore and the amino acid composition of the opsin surrounding
that chromophore (Bowmaker 1991, 1995; Partridge and Cummings 1999 for
reviews).
3 Shallow-Water Teleost Rod Visual Pigments
Since rods are used under scotopic conditions, they are concerned with maximising
photon capture. Most simply this involves matching the absorption spectrum of the
visual pigment to the ambient illumination (Partridge 1990) (sensitivity hypothesis).
Consequently, fish inhabiting freshwater, which is an environment containing
predominantly longwave radiation, generally have more red-sensitive visual pigments,
using 3-dehydroretinal as the chromophore, than those inhabiting green-based coastal
waters (Munz 1964; Schwanzara 1967; Munz and McFarland 1973, 1977; Lythgoe
1972). However, even among rods this correlation is not always good (e.g.,
Bowmaker et al. 1994) and animals from seemingly quite different habitats, for
example, often have similar rod pigments (Lythgoe et at. 1994 ).
However, expecting a simple match between the background light and the visual
pigment is in many cases an oversimplification because it assumes the animal is
attempting to maximize sensitivity to the surrounding spacelight when in fact it is
often detecting a specific target (Partridge and Cummings 1999). For example, a
visual pigment with ' A max offset from the background will enhance the contrast of
certain targets and hence increase the range at which they can be detected (Lythgoe
1968, 1972; McFarland and Munz 1975) (contrast hypothesis). An animal's
behaviour and the requirements this imposes on the visual system must therefore also
be considered along with the surrounding illumination. The spectral composition of
the environment also changes throughout a 24-hour period and Hobson et at. (1981)
have shown that scotopic visual pigments may be better matched to photopic
conditions at twilight rather than the starlight and moonlight prevailing at night.
However, in most instances a case can be made for explaining the A max of most rod
visual pigments based on either the contrast or the sensitivity hypothesis. These
arguments assume that sunlight is the only, or major source of radiation available.
While this is certainly true for freshwater species and shallow oceanic animals, the
situation is very different in the deep ocean.