TEMPLE, S.E.*; HAIMBERGER, T.J.; HAWRYSHYN, C.W.; Univ. of Victoria: Investigations into the functional significance of the visual pigment shift in coho salmon (Oncorhynchus kisutch)
Chromophores are the light absorbing molecules found in the visual pigments of the photoreceptor cells that begin visual transduction when isomerized by incoming light. The primary chromophore in most vertebrates is based on a derivative of vitamin A1 called retinal. However, some vertebrates can synthesize a second chromophore based on vitamin A2 called 3,4-dehydroretinal. An added double bond in the A2 chromophore causes a red shift in the absorbance curve of these visual pigments. For fishes migrating between marine and freshwater it is believed that the red shift is associated with the redder nature of light in the freshwater environment relative to the blue/green oceans. Recently it was suggested that this shift in visual pigments could be used as an indicator to determine when coho salmon are ready to be released from hatcheries and migrate to the marine environment. Using a CCD based microspectrophotometer we have measured the absorption spectra from individual photoreceptor cells from coho in the wild and in hatcheries throughout the year, to establish if the change in chromophores is correlated to the physiological changes that prepare a young salmon to tolerate life in salt water. The shift from A2 to A1 follows an annual cycle that is closely correlated to water temperature. This annual cycle is still present however, in the absence of water temperature variation. Although the shift from A2 to A1 occurs simultaneously with the physiological changes that prepare the salmon for life in saltwater, we have also found that younger fish that would not be entering the sea at that stage were also shifting pigments, suggesting that the shift may be seasonal rather than habitat related.
