Ultraviolet sensitivity in the torus semicircularis of juvenile rainbow trout (Oncorhynchus mykiss).

The spectral sensitivity of single units in the torus semicircularis (TS) of small (< 30 g) and large (> 60 g) juvenile rainbow trout, Oncorhynchus mykiss, was investigated. All examined units (n = 39) showed inputs from the long and medium cone mechanisms. In addition, a majority of units (28 of 39) in both size groups of fish had inputs from the UV cone mechanism, and both groups had several types of color-coded units. The TS of large trout differed from small fish by having a significantly higher proportion of luminance or non-color-coded units relative to color-coded units. Additionally, large fish had a reduced number of UV-sensitive units and an increased number of short-wavelength-sensitive units relative to small fish.