2134 INTRODUCTION The spectral quality of light in aquatic environments is spatially and temporally more variable than that in terrestrial environments. Aquatic organisms that move between different habitats, or those that inhabit seasonally variable habitats, are faced with the challenge of tuning their spectral sensitivity to maximize detection and identification of targets (predators, prey and conspecifics). In fishes, spectral sensitivity can be adjusted by the addition or loss of a photoreceptor class or by changes to the visual pigments within the photoreceptors themselves (Beaudet and Hawryshyn, 1999; Bowmaker, 1995). Visual pigments (VPs) comprise two components: an opsin protein and a chromophore. Specific amino acid sites throughout the opsin protein play key roles in spectral tuning of the resultant VP (Yokoyama, 2000; Yokoyama et al., 2007). There are five classes of vertebrate opsins that are categorized based on amino acid sequence and on spectral absorbance (reviewed in Bowmaker, 1995; Yokoyama, 2000). In addition to expressing a representative of one or more of each of these opsin classes, some fishes have recently been found to express different subtypes of the various opsin classes (Chinen et al., 2003; Matsumoto et al., 2006; Wood and Partridge, 1993). Changes in expression levels of opsin subtypes have been associated with ontogenetic changes and metamorphic transitions, some of which have been induced artificially with hormones or the light environment (Beatty, 1975; Carlisle and Denton, 1959; Fuller et al., 2005; Hope et al., 1998; Mader and Cameron, 2004; Shand et al., 2008; Shand et al., 2002; Takechi and Kawamura, 2005). The other component of the VP, the chromophore, can also be varied in some species. Many freshwater and euryhaline fishes have the ability to change which chromophore is incorporated into their VPs, shifting between retinal (aldehyde of vitamin A 1 ) and 3,4- dehydroretinal (aldehyde of vitamin A 2 ), or using mixtures of both (Beatty, 1984). The wavelength of maximum absorbance (λ max ) of a VP based on vitamin A 2 is long-wavelength shifted relative to the same opsin combined with vitamin A 1 . The long-wavelength shifted vitamin A 2 -based VPs are also less thermally stable than vitamin A 1 -based VPs, which could have implications for species that inhabit temperate waters where ambient temperatures vary seasonally. Pacific salmonids are anadromous fishes restricted to temperate climates that are equipped with a remarkably dynamic visual system that varies temporally at different time scales throughout life history (Allison et al., 2006a; Allison et al., 2003; Beatty, 1966; Browman and Hawryshyn, 1994; Hawryshyn et al., 1989; Temple et al., 2006), making this group ideal for investigating adaptive changes in visual pigment composition. Pacific salmon start life in freshwater as alevin. They become parr once their yolk sac is absorbed. They may spend anywhere from a few days to a few years in fresh water (depending on species) The Journal of Experimental Biology 211, 2134-2143 Published by The Company of Biologists 2008 doi:10.1242/jeb.009365 Effects of exogenous thyroid hormones on visual pigment composition in coho salmon (Oncorhynchus kisutch) Shelby E. Temple 1 , Samuel D. Ramsden 1 , Theodore J. Haimberger 1 , Kathy M. Veldhoen 1 , Nik J. Veldhoen 2 , Nicolette L. Carter 1 , Wolff-Michael Roth 3 and Craig W. Hawryshyn 1,4, * 1 Department of Biology, University of Victoria, Victoria, British Columbia, Canada, 2 Department of Microbiology and Biochemistry, University of Victoria, Victoria, British Columbia, Canada, 3 Faculty of Education Research, University of Victoria, Victoria, British Columbia, Canada and 4 Department of Biology and Center for Neuroscience Studies, Queen’s University, Kingston, Ontario, Canada *Author for correspondence (e-mail: craig.hawryshyn@queensu.ca) Accepted 23 April 2008 SUMMARY The role of exogenous thyroid hormone on visual pigment content of rod and cone photoreceptors was investigated in coho salmon (Oncorhynchus kisutch). Coho vary the ratio of vitamin A 1 - and A 2 -based visual pigments in their eyes. This variability potentially alters spectral sensitivity and thermal stability of the visual pigments. We tested whether the direction of shift in the vitamin A 1 /A 2 ratio, resulting from application of exogenous thyroid hormone, varied in fish of different ages and held under different environmental conditions. Changes in the vitamin A 1 /A 2 visual pigment ratio were estimated by measuring the change in maximum absorbance (λ max ) of rods using microspectrophotometry (MSP). Exogenous thyroid hormone resulted in a long- wavelength shift in rod, middle-wavelength-sensitive (MWS) and long-wavelength-sensitive (LWS) cone photoreceptors. Rod and LWS cone λ max values increased, consistent with an increase in vitamin A 2 . MWS cone λ max values increased more than predicted for a change in the vitamin A 1 /A 2 ratio. To account for this shift, we tested for the expression of multiple RH2 opsin subtypes. We isolated and sequenced a novel RH2 opsin subtype, which had 48 amino acid differences from the previously sequenced coho RH2 opsin. A substitution of glutamate for glutamine at position 122 could partially account for the greater than predicted shift in MWS cone λ max values. Our findings fit the hypothesis that a variable vitamin A 1 /A 2 ratio provides seasonality in spectral tuning and/or improved thermal stability of visual pigments in the face of seasonal environmental changes, and that multiple RH2 opsin subtypes can provide flexibility in spectral tuning associated with migration–metamorphic events. Key words: rhodopsin, porphyropsin, thyroxine, fish, vision, opsin gene sequence, expression, PCR, MSP. THE฀JOURNAL฀OF฀EXPERIMENTAL฀BIOLOGY