Biology Reference
In-Depth Information
In wild masu salmon, black pigmentation/darkening of dorsal,
pectoral and caudal fi n margins due to diffusion of melanin granules in
melanophores, follows with development of seawater adaptability (Ura et
al., 1997; Mizuno et al., 2001b). The level of dorsal fi n pigmentation and
ATPAse activity increased signifi cantly from January or March to May
during smoltifi cation (Mizuno et al., 2004).
Epidermis is thinner in May (at the smoltifi cation period) than in
November (at the breeding season) (Rydevik, 1988). The increases in
epidermal and dermal thickness in mature salmonids are correlated with
androgen levels (Pottinger and Pickering, 1985; Rydevik, 1988).
The biological interface between fi sh and their aqueous environment
is a mucus coat composed of biochemically diverse secretions, such as
lysozyme (Schrock et al., 2001), which activity and levels peak 1 month
before T4 surge during smoltifi cation (Fagan et al., 2003). A sharp decrease
in the numbers of mucus cells was observed in the skin of Atlantic salmon
at the beginning of smoltifi cation (O'Byrne-Ring et al., 2003).
6.2.2
Eye
Visual pigments (VP) comprise two components: an opsin protein and a
chromophore. Some fi shes have recently been found to express different
subtypes of the fi ve classes of vertebrate opsins and changes in expression
levels of these subtypes have been associated with ontogenetic changes and
metamorphic transitions. The other component of the VP, the chromophore
can also be varied in some species. In order to compensate for the changes
in spectral transmittance of water, salmonids are able to use mechanisms
that alter spectral sensitivity, including gain or loss of a photoreceptor class
(Allison et al., 2003, 2006a), changes in chromophore class [retinal (A1) or
3,4-dehydroretinal (A2)] (reviewed by Beatty, 1984; Temple et al., 2006) and
expression of different opsin classes or subtypes within a photoreceptor
class (reviewed by Bowmaker and Loew, 2008).
Salmonids have a paired-pigment visual system with photoreceptors
possibly containing rhodopsin or porphyropsin. During smoltifi cation,
retina exhibits a shift from parr-stage porphyropsin (3-dehydroretinal;
vitamin-A2-based visual pigment) dominance to a dominance in rhodopsin
(retinal; vitamin-A1-based visual pigment) at smolt-stage. This is related
to migration from FW, a medium that favours longer wavelengths of
light, to marine (seawater) environment (Bridges and Delisle, 1974). Coho
salmon possess rod photoreceptors and four classes of cone photoreceptors:
ultraviolet wavelength-sensitive (UVS), short wavelength-sensitive (SWS),
medium wavelength-sensitive (MWS) and long wavelength-sensitive
(LWS). These photoreceptors express all fi ve vertebrate opsin classes: RH1,
UVS, SWS, RH2 and LWS, respectively (Dann et al., 2004). In coho salmon,
