JUVENILE salmon adjust their colour vision as they move into deeper waters to feed. The switch from making one kind of photoreceptor pigment to another may help them see better in the different quality of light.
I帽igo Novales Flamarique and Christiana Cheng of Simon Fraser University in Burnaby, Canada, measured the sensitivity of light-sensitive cones in young pink salmon鈥檚 eyes to different wavelengths of light. Just after the salmon hatched, one type of cone, known as a 鈥渟ingle鈥� cone, was most sensitive to ultraviolet light with a wavelength of about 370 nanometres.
By the time the fish had doubled in size, however, single cones responded best to blue light with a wavelength of about 430 nanometres. Since blue light penetrates deeper into water than ultraviolet, this change may help the older fish see more acutely as they switch from plankton feeding at the surface to preying on small fish deeper down (Nature, vol 423, p 279).
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The team also showed that the fish make different photoreceptor pigments, or opsins, before and after the switch. During the transition, individual cones may contain both opsins. Cones containing more than one opsin have been observed in several other vertebrates, including some mammals, so this strategy for adjusting colour vision may be more common than scientists have realised, says Novales Flamarique.