Proc.NatlAcad.Sci.USAhttp://doi.org/bmkp(2016)

Credit: © JEFF ROTMAN/ALAMY STOCK PHOTO

That the octopus succeeds in camouflaging itself is something of a puzzle. And here's why: like most cephalopods, they have a single unfiltered photoreceptor type — they're colour blind in the traditional sense. But Alexander and Christopher Stubbs think they might have hit on the mechanism by which these organisms discriminate between colours. And it comes down to having a funny-shaped pupil.

Equipped with a lens that has a wavelength-dependent index of refraction, the octopus experiences chromatic blur, in which different wavelengths come into focus at different distances from the lens. The mechanism that Stubbs and Stubbs put forward, and study numerically, suggests that the spectral content of the organism's environment could be inferred from monitoring the variation in blurriness as the lens-to-retina distance is altered. The off-axis geometry of the pupil maximizes the chromatic blurring, but does so at the expense of acuity — implying it may have evolved to facilitate this special type of spectral discrimination.