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the spectral absorbance of MWS and LWS cones differ among freshwater
(alevin and parr) and ocean (smolt) phases (Temple et al., 2008a).
Ultraviolet wavelength-sensitive (UVS) cones are present throughout
the retina of parrs in the land-locked rainbow trout (Allison et al., 2003),
in migratory sockeye salmon (Novales Flamarique, 2000), as well as
salmonids from the genus Salmo (Kunz, 1987; Kunz et al., 1994). In larger
postmetamorphic rainbow trout, a decrease in UVS cone distribution is
observed and is correlated with the reduced sensitivity to UV light (Allison
et al., 2003) and during smoltifi cation, salmonids lose most of their UVS
through programmed cell death (Allison et al., 2006a). In sockeye salmon,
UVS cones may disappear from the entire retina at the smolt stage (Novales
Flamarique, 2000). On their return migration back to fresh water, some
of these UVS cones are regenerated (Beaudet et al., 1997; Allison et al.,
2006a).
Sea trout smolts, compared to parr, had downregulated ultraviolet-
sensitive opsin (SWS1) expression in all quadrants of retina, lower long
wavelength-sensitive opsin (LWS) expression dorsally, higher rod opsin
(RH1) expression nasally and higher middle wavelength-sensitive opsin
(RH2) expression dorsally (Veldhoen et al., 2006).
The change in visual pigment composition was speculated (Jacquest
and Beatty, 1972; Evans and Fernald, 1990) to be a feature of smoltifi cation
process. Alexander and collaborators (1994) confi rmed experimentally that
the change from porphyropsin- to a rhodopsin-dominated visual system
in coho salmon ( O. kisutch ) was one of the developmental processes of
smoltifi cation and not simply responses to the light environment being
coincidental to the parr-smolt transformation (« migration-metamorphosis »
hypothesis). The authors concluded that this shift of visual pigments
could be used as an index of smoltifi cation. However, later on, Temple
and collaborators (2006) reported a shift in A1/A2 ratio correlated with
season in both 0+ (<12 months old) coho parr that remained in fresh water
for another year and in oceanic juvenile coho. They concluded that their
fi ndings support the hypothesis that the A1/A2 pigment pair system in coho
salmon is an adaptation to seasonal variations in environmental variables
(« seasonal » hypothesis) rather than to a change associated with migration
or metamorphosis (Temple et al., 2006).
The shift in opsin expression for MWS and LWS cones occurs between
parr and smolt stages, prior to seaward migration, as a means to prepare
the visual system for a different photic environment and visual tasks. The
fl exibility of the spectral tuning mechanisms in salmonids might permit
precise spectral tuning to the variable spectral environments, which they
inhabit, while maintaining some optimum signal-to-noise ratio in the face
of seasonal variation in temperature and light conditions (Temple et al.,
2008a, 2008b).
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