Functional mapping of ultraviolet photosensitivity during metamorphic transitions in a salmonid fish, Oncorhynchus mykiss

Ultraviolet visual sensitivity appears to be reduced and, possibly, lost du ring smoltification in anadromous populations of salmonid fishes. Similar c hanges occur in non-anadromous salmonids over a mass range that is associat ed with smoltification in their anadromous conspecifics. However, in sexual ly mature adult salmonids, ultraviolet-sensitive cones are present in the d orsotemporal retina, suggesting that ultraviolet sensitivity (i) may be reg ained with sexual maturity or (ii) might never be completely lost. Both smo ltification and the transition to sexual maturity are regulated, in part, b y the hormone thyroxine. Thyroxine treatment of juvenile Oncorhynchus mykis s results in precocial developmental changes that mimic smoltification, inc luding a reduction of ultraviolet sensitivity. However, whether loss of ult raviolet sensitivity in O. mykiss or in other species of salmonids is compl ete during normal development (or in response to thyroxine treatment) is un clear. In the present study, we have 'mapped' topographically ultraviolet p hotosensitivity during natural and hormone-induced smoltification. Thyroxin e-treated O. mykiss juveniles and anadromous steelhead O. mykiss smolts wer e examined for ultraviolet visual sensitivity by recording compound action potentials from the optic nerve. By selectively illuminating either the dor sal or the ventral retina, we have shown that the reduction of ultraviolet sensitivity occurs primarily in the ventral retina in both groups of fish. Ultraviolet sensitivity remains intact in the dorsal retina.