Using transparency and reflection, animals residing in the ocean's featureless midwater environment can make themselves nearly invisible to potential predators. Different optical strategies are found in different types of tissues; while muscles and connective tissue can be made transparent for highly absorbing body parts such as eyes and guts, reflection is a ubiquitous strategy for camouflage. Since the pelagic light field in regions of the ocean with asymptotic light regimes is roughly cylindrical, radiance matching can be an effective strategy for reflective camouflage; if an animal can perfectly reflect, with the same intensity and spectral composition, light radiating from behind a viewer, this reflectance will also match the light radiating from behind the animal, and the animal will remain inconspicuous on its background. Since the reflectors involved in this camouflage strategy are also required to be thin (they must be thinner than the organism's skin), dielectric mirrors provide a highly effective, energy-efficient strategy for camouflage in open water.
In general, mirrors made of either a smooth metal surface or with alternating layers of contrasting refractive index [distributed Bragg reflectors (DBRs)] can be used for specular reflection of broad or narrow ranges of frequency. In the visible electromagnetic spectrum, metallic mirrors typically exhibit broadband reflectance, while Bragg mirrors typically reflect more restricted bands (called the ‘bandgap’) that span a narrower region of the spectrum. Bandgaps of periodic DBRs with a narrow range of spatial frequencies can be broadened by increasing the refractive index contrast, but the contrast required to span the visible region is greater than that found in biological materials. In the case of biological materials, broadband reflectance from a DBR can be achieved by increasing the range of layer thicknesses within the stack along the direction of incoming light.
There are several structural strategies for accomplishing an increased spatial distribution of layers in a DBR: (i) a random distribution of layer thicknesses [1
]; (ii) an ordered distribution of layer thicknesses (a.k.a. ‘chirped’); and (iii) a stack of several single spatial frequency DBRs with narrow bandgaps on top of each other resulting in broadband reflection. Nature has mastered several of these optical structures, for example, chaotically spaced silver reflectors in fish [2
], chirped bronze-coloured beetle reflectors [3
] and silver butterfly wings that use a colour-additive technique [4
]. Here, we describe for a novel optical and structural design for a broadband DBR, found in the silvery covering of squid eyes from the family Loliginidae. This silvery covering consists of packed spindle-shaped cells that achieve broadband visible reflectance by creating a large range of layer thicknesses. The silvery covering of the squid eye apparently matches the background radiance of the water column in which the animal is immersed, thereby hiding the retina by creating the illusion of transparency ().
Figure 1. (a) Loliginind squid schooling under ambient lighting in shallow water showing effective camouflage of the large eye structure. (b) Photograph of squid eye showing relationship of silver tissue to other eye structures. (c) Magnification of 10× (more ...)
The broadband reflectors found in the squid eye tissue are densely packed protein-rich spindle-shaped cells with a refractive index of 1.56 [5
] surrounded by cytoplasm with a refractive index of approximately 1.33 [6
]. The optical structure of this eye covering is intriguingly different from the reported broadband reflecting structures in fish scales, because the average size and variation of both the high- and low-refractive index regions is up to ten times that described in fish scales, while the difference between high- and low-refractive index is 0.23 rather than 0.5 (guanine, found in fish scales, has a refractive index of 1.83) [2
]. Therefore, in addition to guanine-based reflectors, evolution has also fostered the formation of proteinaceous (therefore polymer-based) broadband dielectric reflectors with layers made from entire cells as a form of midwater camouflage.
Because periodic or randomized DBRs are specular reflectors, the incident angle of incoming light is equal to the angle at which light is reflected and both the wavelength and the intensity of reflection vary with angle. However, the shape of the reflectance spectrum of the squid eye is independent of incident angle, while the reflected radiance drops slightly at oblique angles. Random DBRs are frequently examined and used as optical components such as filters, microcavities and waveguides, where broadband optical reflection can be advantageous [7
], and understanding this kind of biological system could lead to inspiration for spindle-based three-dimensional angle-independent broadband reflectors (e.g. ellipsoidal three-dimensional photonic crystals [8
For this optical design to behave like a DBR, a large contrast in refractive index must exist between the cells and the cytoplasm, which in the case of the non-crystalline reflectors in squids, probably requires proteins specifically evolved for optical function. In the tissue covering, the Loliginid squid eye, this structure achieves refractive index contrast using cells that are densely and homogeneously filled with protein for high refractive index, and an expanded extracellular space containing mostly water for low refractive index. We also investigated the biochemical composition of the high-refractive index component of this novel reflector, and found a stereotyped protein composition, reminiscent of that found in lens cells that also serve an optical function [9
]. In this case, the handful of highly expressed proteins in the tissue is comprised of reflectin homologues in addition to a novel, highly hydrophobic protein with implications for the self-assembly mechanisms responsible for forming these DBR structures.
In the context of their environment, squid eyes seem particularly inconspicuous given the high contrast nature of the large, dark pupil in the centre of a silvery eyeball structure (a). In this report, we describe the biological, cellular and optical properties of the tissue covering the eyes of Loliginid squid (b), which serves as a static reflector for optical camouflage. We focus on the manner in which the long, spindle-shaped cells in the eye tissue (c–e) are arranged to achieve broadband specular reflectance and investigate the details of the reflectance of this tissue in the context of the radiance fields in which it evolved.