Nanoscale reflective structures have evolved in many animals. Through
their intricate organizational properties and structural efficiency these can
offer design inspiration for optical technologies. The need for an effective
mirror is especially acute in the open ocean where silvery pelagic fish
use reflective camouflage to radiance-match their photic environment
and render themselves invisible. This requires the sides of the fish to
act as a vertical plane mirror that has a broad spectral bandwidth and is
non-polarizing; an effect achieved by an elegant multilayer arrangement
of birefringent guanine crystals in their skin. In this paper we characterize
this structural mechanism and draw design inspiration for an efficient
omnidirectional broadband mirror. Through the application of anisotropic
multilayer theory we establish how the angle of maximum polarization is
controlled by the orientation and birefringence magnitude of the guanine
crystals, and for the first time demonstrate how an evolved structure
is able to produce near omnidirectional reflectivity. We then perform
structural optimization and establish that high broadband reflectivity is
characterized by variation in nanoscale periodicity (the thickness and
spacing of the guanine crystals). For a fixed amount of crystal layers,
linear systematic variation in periodicity produces higher spectrally
and angularly averaged reflectivity than random variation in periodicity
and is near optimal in performance as a broadband mirror in the visible
wavelength regime.