Salmonid fishes undergoing the process of smoltification

have elevated levels of thyroid hormones (Barron, 1986; Hoar, 1988; Datta et al., 1999; Eales, 1990). Thyroid hormones play a central role in controlling many of the alterations in physiology, morphology and behavior noted during this metamorphic transition in juvenile salmon (Hoar, 1988). Thyroid hormones have also been identified in altering the visual system during smoltification. During migrations from freshwater to marine environments, the visual pigment composition of the retina changes from vitamin A2 to A1, and changes in this ratio can also be induced by exposure to thyroxin (T4; Alexander et al., 1998; Allen, 1977; Cristy, 1974). The use of a deiodinase inhibitor, methimazole, blocks activation of T4 and inhibits these pigment alterations (Alexander et al., 1998). The loss of corner single cones from the square cone mosaic occurs during smoltification (Lyall, 1957b; Bowmaker and Kunz, 1987; Hawryshyn et al., 1989; Kunz, 1987; NovalesFlamarique and Hawryshyn, 1996). Corner single cones have been identified as the ultraviolet-sensitive (UVS) cone through a combination of spectral sensitivity experiments (loss of UV sensitivity), histology (decrease in number of corner single cones; Beaudet et al., 1993; Bowmaker and Kunz, 1987; Loew and Wahl, 1991) and in situ hybridization against opsin mRNAs (Allison et al., 2003). Ultrastructural evidence has shown that the UVS cones disappear as a result of apoptosis (Kunz et al., 1994). Browman and Hawryshyn (1992, 1994) mimicked smoltification by treating parr rainbow trout with T4 and described a similar loss of both UV sensitivity and UVS cones, which was thought to be a uniform loss across the retina. However, a recent study (Deutschlander et al., 2001), using functional mapping techniques, demonstrated that a T4 2665 The Journal of Experimental Biology 206, 2665-2673 © 2003 The Company of Biologists Ltd doi:10.1242/jeb.00470