Structural color has been a quite famous topic on this blog; which is justified as it is a feature that assures communication within many species and can be decisive in reproductive success. When it comes to defense, strategies involving pigmentation and bioluminescence to alert and dazzle potential predators are well documented. But did you know some organisms use structural color to simply disappear? Indeed, invisibility cloaks are not just a Harry Potter fantasy. Check out the following video:
While studies of structural colors usually emphasize specific flashy, attention-grabbing traits, recent studies about marine species have revealed structural colors can also be used for the opposite purpose – concealment.
The example above shows how a sea sapphire, a small crustacean, relies on photonics to mislead predators. According to the American Chemical Society, its structural invisibility is enabled by layered hexagonal guanine crystals. These nanoscopic mirrors deflect light into the UV spectrum. The more they’re angled, the more light is shifted, to the point where no visible light is reflected, and the sea sapphire is rendered invisible to the human eye and to the sea sapphire’s enemies.
Similarly, fish have been using the guanine crystals trick. Scale orientation varies along their body length such that reflections conceal its thickness. This structural approach might be more advantageous for species with flatter morphologies. Furthermore, a study from Bristol University showed not only do sardines and herrings scales play with reflection angle but also with polarization. A highly reflective structure that keeps polarization low can produce hyper-realistic effects since the reflected image is barely distorted. As a result, fish can’t be easily distinguished from their surroundings and seem to disappear.
According to these researchers, it’s an ingenious design that outdoes current non-polarizing reflectors. On the other hand, structural reflection of UV might be more widespread than we think, it may be present in eggshells – as studied by Daphne – or in iridescent flower petals. Since UV production requires lots of energy, a passive structure deflecting available natural light is far less energy-consuming.Therefore, who knows how these two bio-inspirations could lead to sci-fi optical systems and mind-blowing illusionism?
Articles can be checked here:
http://pubs.acs.org/doi/abs/10.1021/jacs.5b05289 (sea sapphire)
http://www.nature.com/nphoton/journal/v6/n11/full/nphoton.2012.260.html (sardines and herrings)