The Biology of Transparency
◐ The Invisible Man: Have you ever wished to be invisible? Transparency is quite common in biology, being particularly useful as camouflage in the open ocean where there is nothing to hide behind. There is an astonishing variety of transparent jellyfish, glass squid, worms and this creepy-crawly crustacean from the “twilight zone” of the deep sea seen in the image.
◑ How does it work? To be transparent, light must pass through without being absorbed or scattered. Most organic molecules do not absorb light in the visible range, except for the visual pigments of the eyes, which must absorb light to function. Light scattering is caused by changes in refractive index which determines how light is bent as it passes through (see http://goo.gl/7l6zFC). To be perfectly transparent, the refractive index should be the same throughout. This is clearly a challenge in biological tissues, where lipid membranes and protein/DNA rich organelles (like mitochondria or nuclei) are much denser than the surrounding cytoplasm. So transparent animals resort to a number of tricks to avoid light scattering.
◐ See Right Through Me: One way is to become extremely flat! Since there is an exponential relationship between thickness and light absorption/scattering, a 1 cm thick tissue that is 60% transparent will achieve 95% transparency if it is only 1 mm thick. Some tissues, like the lens of our eyes, undergo drastic reduction of complexity, relying on neighboring cells to feed them. At the ultrastructural level, surfaces can be cloaked in submicroscopic bumps, smaller than half the wavelength of light that average out the differences in refractive indexes. Known as moth eye surfaces, these are responsible for the transparency of the beautiful glasswing butterfly Greta oto (see http://goo.gl/KS85mo).
◑ I See You!: It’s hard to keep the gut transparent, unless one only eats transparent food, like the larvae of the phantom midge that sucks out clear fluids from its prey. Also, transparency can be foiled by predators that have evolved to use UV light or even polarized light to spot their prey, since underwater light is polarized particularly in the horizontal plane. A study with squid showed that they attacked plastic beads with birefringence, preferentially over beads without this optical property. Something to think about before you invest in an invisibility cloak!
GIF: This 9 cm long amphipod is nearly completely transparent. Via http://goo.gl/bL14Oy from the video below.
Video: For a short 2:41 minute video of more stunning transparent creatures, watch Deep Sea Creatures – Nature’s Microworlds – Episode 11 Preview – BBC Four
Musical Inspiration: Queen – ‘The Invisible Man’