Spectral and ultraviolet-polarisation sensitivity in juvenile salmonids: Acomparative analysis using electrophysiology

Spectral and polarisation sensitivity were compared among juvenile (parr) r ainbow trout (Oncorhynchus mykiss), Steelhead (O. mykiss), cutthroat trout (O. clarki clarki), kokanee (O. nerka) and brook char (Salvelinus fontinali s) using multi-unit recording from the optic nerve, Although reared under t he same conditions, differences in photopic spectral sensitivity were evide nt. Specifically, ON-responses were co-dominated by L- and M-cone mechanism s in all fish except O. nerka, consistent with an M-cone mechanism sensitiv ity. The sensitivity of OFF-responses was dominated by the M-cone mechanism for all fish, but 0, mykiss appeared to show an additional contribution fr om the L-cone mechanism. Using chromatic adaptation, an independent ultravi olet-sensitive mechanism is described for the first time for the salmonid g enus Salvelinus, In addition, this ultraviolet-cone mechanism was present i n the members of the genus Oncorhynchus that were examined. Thus, ultraviol et sensitivity appears to be common to the major extant clades of the subfa mily Salmoninae. All species showed differential sensitivity to both vertic al and horizontal linearly polarised light. This sensitivity differed betwe en ON- and OFF-responses. The ON-responses were maximally sensitive to both vertically and horizontally polarised light, whereas the OFF-responses dis played maximal sensitivity to horizontally polarised light in all species, with reduced sensitivity to vertically polarised light compared with ON-res ponses. Because of the similarity in the physiological characteristics of p olarisation sensitivity among the salmonid species examined, no relationshi p between the degree of migratory tendency and the ability to detect polari sed light could be identified.