A highly distributed Bragg stack with unique geometry provides effective camouflage for Loliginid squid eyes

Cephalopods possess a sophisticated array of mechanisms to achieve camouflage in dynamic underwater environments. While active mechanisms such as chromatophore patterning and body posturing are well known, passive mechanisms such as manipulating light with highly evolved reflectors may also play an...

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Bibliographic Details
Published in:Journal of the Royal Society interface 2011-10, Vol.8 (63), p.1386-1399
Main Authors: Holt, Amanda L., Sweeney, Alison M., Johnsen, Sönke, Morse, Daniel E.
Format: Article
Language:English
Subjects:
Animals
Bragg Reflector
Camouflage
Eye - anatomy & histology
Loligo - physiology
Microscopy - methods
Microscopy, Atomic Force
Ocular Physiological Phenomena
Optical Phenomena
Reflectin
Refractometry
Scattering, Radiation
Self-Assembly
Spectrum Analysis - methods
Squid
Transfer-Matrix
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Summary:Cephalopods possess a sophisticated array of mechanisms to achieve camouflage in dynamic underwater environments. While active mechanisms such as chromatophore patterning and body posturing are well known, passive mechanisms such as manipulating light with highly evolved reflectors may also play an important role. To explore the contribution of passive mechanisms to cephalopod camouflage, we investigated the optical and biochemical properties of the silver layer covering the eye of the California fishery squid, Loligo opalescens. We discovered a novel nested-spindle geometry whose correlated structure effectively emulates a randomly distributed Bragg reflector (DBR), with a range of spatial frequencies resulting in broadband visible reflectance, making it a nearly ideal passive camouflage material for the depth at which these animals live. We used the transfer-matrix method of optical modelling to investigate specular reflection from the spindle structures, demonstrating that a DBR with widely distributed thickness variations of high refractive index elements is sufficient to yield broadband reflectance over visible wavelengths, and that unlike DBRs with one or a few spatial frequencies, this broadband reflectance occurs from a wide range of viewing angles. The spindle shape of the cells may facilitate self-assembly of a random DBR to achieve smooth spatial distributions in refractive indices. This design lends itself to technological imitation to achieve a DBR with wide range of smoothly varying layer thicknesses in a facile, inexpensive manner.
ISSN:1742-5689
1742-5662
DOI:10.1098/rsif.2010.0702