Visual biology of salmonids with special reference to polarised-light sensitivity




Parkyn, Daryl Charles

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The visual biology and its relevance to orientation was examined in fishes in the subfamily Salmoninae (salmon, trout, and char) by characterising the spectral and polarisation sensitivity of their visual systems. Integrated spike activity of axons from the optic nerve was used as a measure to determine the polarised-light sensitive mechanism underlying the ability of rainbow trout (Oncorhynchus mykiss) to orient in down-welling, linearly-polarised light. Relative sensitivity curves were then obtained for the five types of photoreceptor cells in this trout's retina: rods, ultraviolet sensitive cones (UV), short wavelength cones (blue-sensitive) (S), medium wavelength cones (green-sensitive) (M), and long wavelength cones (red-sensitive) (L). Under scotopic conditions (dark adapted), no sensitivity to e-vector was apparent. Under photopic conditions (light adapted), trout parr exhibited on-responses with e-vector sensitivity in two orthogonal channels. No evidence of polarisation sensitivity was observed in the on-responses of larger fish (50–78 g smolts) under UV-isolating background conditions, whereas the off-responses were unchanged.M and L cones, in contrast, retained their unimodal response. The decrease in UV-polarisation sensitivity in larger fish was found not to be attributable to size-related changes in the ability of the ocular media of the eye to transmit polarised light. Rainbow trout was used as a model species and spectral and polarisation sensitivity were compared with steelhead (an anadromous form of O. mykiss), cutthroat trout (O. clarki), kokanee (land-locked form of O.nerka), and brook char (Salvelinus fontinalis ). Visual pigment templates from rainbow trout were corrected for ocular media absorbance and overlaid on the spectral sensitivity curves for the purposes of comparison. Some differences in sensitivity were observed among species. In particular, on-responses were dominated by L-cones in most species.However, in kokanee, the M- cone was dominant.These differences may be related to the photic environments from which these fish originate. The first physiological evidence of near-UV sensitivity in the genus Salvelinus is also provided. It was therefore concluded that UV sensitivity is ubiquitous in the subfamily Salmoninae. Polarisation sensitivities in the above species were modelled using periodic regression analysis. Comparisons suggest that all of these species have a similar dual-channel polarisation detection system. The effect of the tuberculostatic drug Ethambutol on the visual physiology of rainbow trout was also examined. This drug appears to cause a decrease in sensitivity analogous to chromatic adaptation. Fish were fed daily for one month and then spectral sensitivity and polarisation sensitivity of Ethambutol-treated fish were compared to control fish. Relative to controls the visual systems of treated fish were dominated by the M-cone mechanism. Spectral and polarised light sensitivity of UV and L-cones were reduced. Finally, orientation responses of juvenile rainbow trout, steelhead, and brook char to a polarised-light stimulus were examined under controlled laboratory and semi-natural field conditions. Trained fish of all species oriented to the plane of polarised light, whereas untrained fish could not. Fish trained in the lab were tested at twilight and were found to be able to orient under natural skylight. Under semi-natural conditions, fish in floating net-pens in a lake were provided food rewards at a specific compass bearing. Their orientation responses were assessed under various natural and artificial conditions. When the sun was visible the fish typically had a unimodal distribution. However, they were typically not oriented when presented with only brightness and spectral cues at Zenith or on cloudy days. In contrast, both steelhead and sockeye were oriented correctly at civil twilight if the horizon was not obscured by heavy cloud or if blue sky was visible at Zenith.



Salmonidae, Visual pathways