The Ecology of Teleost Fish Visual Pigments
229
regain their UV sensitivity on reentering shallow freshwater (Beaudet and Hawryshyn
1999 for review). Such changes in an animal's UV sensitivity may reflect a change
in the animal's lifestyle and habitat away from a planktivorous surface feeding
existence to life in deeper water (Browman and Hawryshyn 1994; Bowmaker 1995).
The best examples of changes in the chromophore to alter a visual pigment to suit
a particular environment are provided by migratory species moving between redbased freshwater and bluer oceanic habitats. Anadromous salmon, for instance, living
in the sea, have relatively shortwave-sensitive rhodopsin visual pigments, but when
they migrate to freshwater to spawn, the retinal is replaced by 3-dehydroretinal to
yield longer wave-sensitive porphyropsins (Beatty 1984 ).
Some non-migratory species, such as the freshwater rudd, also display changes in
the rhodopsin/porphyropsin ratio of both their rod and cone pigments (Dartnall et al.
1961; Loew and Dartnall 1967; Beatty 1984; Whitmore and Bowmaker 1989).
Although this ratio is influenced not only by age, but also by changes in season, day
length, light intensity, temperature, hormonal status and diet, its significance remains
unclear and there is often no obvious relationship between photic environment and
the rhodopsin! porphyropsin ratio (Muntz and Mouat 1984).
The developmental expression of new opsins to produce novel visual pigments has
been less often described (although it is probably not less common) than chromophore
changes. Thus, new opsins are probably expressed in both the pollack (Shand et al.
1988) and the goatfish (Shand 1993) as they mature to adapt to their new, more
benthic, existence, as well as in the flounder (Evans et al. 1993) and Chromis
punctipinnis (McFarland and Loew 1994). Why such changes occur is not always
obvious (McFarland and Loew 1994). Pollack, for instance, studied in the English
channel, change the spectral sensitivity of their single cones from a A max of around
420 nm, when they are surface feeding adults, to cones with A max 450-460 nm (Fig.
6), which adapts them to their deeper existence where there is less shortwave
illumination. Conversely, however, a 580 nm cone pigment in planktivorous tropical
goatfish is replaced by one absorbing at 530 nm for deeper-dwelling adults (Shand
1993).
Catadromous eels are particularly interesting because they change both their
chromophore and express a new opsin as they migrate from relatively shallow
freshwater into the deep ocean during their spawning migration to the Sargasso Sea.
Individual rods contain a mixture of rhodopsin (A max 501 nm) and porphyropsin (A.
max 523 nm) visual pigments. In freshwater, as in other species, the A2-based pigment
predominates, but on migration towards the sea, the retina becomes primarily A 1based (Wood et al. 1992). However, in addition to changing its chromophore during
migration towards the sea, a new opsin is expressed within existing rods (Wood et al.
1992; Wood and Partridge 1993; Archer et al. 1995), resulting in a novel rhodopsin
with A max 482 nm, typical of a deepsea fish.
229
regain their UV sensitivity on reentering shallow freshwater (Beaudet and Hawryshyn
1999 for review). Such changes in an animal's UV sensitivity may reflect a change
in the animal's lifestyle and habitat away from a planktivorous surface feeding
existence to life in deeper water (Browman and Hawryshyn 1994; Bowmaker 1995).
The best examples of changes in the chromophore to alter a visual pigment to suit
a particular environment are provided by migratory species moving between redbased freshwater and bluer oceanic habitats. Anadromous salmon, for instance, living
in the sea, have relatively shortwave-sensitive rhodopsin visual pigments, but when
they migrate to freshwater to spawn, the retinal is replaced by 3-dehydroretinal to
yield longer wave-sensitive porphyropsins (Beatty 1984 ).
Some non-migratory species, such as the freshwater rudd, also display changes in
the rhodopsin/porphyropsin ratio of both their rod and cone pigments (Dartnall et al.
1961; Loew and Dartnall 1967; Beatty 1984; Whitmore and Bowmaker 1989).
Although this ratio is influenced not only by age, but also by changes in season, day
length, light intensity, temperature, hormonal status and diet, its significance remains
unclear and there is often no obvious relationship between photic environment and
the rhodopsin! porphyropsin ratio (Muntz and Mouat 1984).
The developmental expression of new opsins to produce novel visual pigments has
been less often described (although it is probably not less common) than chromophore
changes. Thus, new opsins are probably expressed in both the pollack (Shand et al.
1988) and the goatfish (Shand 1993) as they mature to adapt to their new, more
benthic, existence, as well as in the flounder (Evans et al. 1993) and Chromis
punctipinnis (McFarland and Loew 1994). Why such changes occur is not always
obvious (McFarland and Loew 1994). Pollack, for instance, studied in the English
channel, change the spectral sensitivity of their single cones from a A max of around
420 nm, when they are surface feeding adults, to cones with A max 450-460 nm (Fig.
6), which adapts them to their deeper existence where there is less shortwave
illumination. Conversely, however, a 580 nm cone pigment in planktivorous tropical
goatfish is replaced by one absorbing at 530 nm for deeper-dwelling adults (Shand
1993).
Catadromous eels are particularly interesting because they change both their
chromophore and express a new opsin as they migrate from relatively shallow
freshwater into the deep ocean during their spawning migration to the Sargasso Sea.
Individual rods contain a mixture of rhodopsin (A max 501 nm) and porphyropsin (A.
max 523 nm) visual pigments. In freshwater, as in other species, the A2-based pigment
predominates, but on migration towards the sea, the retina becomes primarily A 1based (Wood et al. 1992). However, in addition to changing its chromophore during
migration towards the sea, a new opsin is expressed within existing rods (Wood et al.
1992; Wood and Partridge 1993; Archer et al. 1995), resulting in a novel rhodopsin
with A max 482 nm, typical of a deepsea fish.