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Today I wanted to write more about giant clams and their astonishing ability to “farm” algae within their body (and then live off of the sweet sugars which the algae produce).  I still want to write about that, but it proving to be a complicated subject: giant clams mastered living on solar energy a long time ago, and we are still trying to figure out the full nature of their symbiotic systems.

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Today, instead we are going to look at the phenomenon which gives the mantles of giant clams their amazingly beautiful iridescent color. It is the same effect which provides the shimmering color of hummingbird feathers and blue morpho wings, or the glistening iridescence of cuttlefish.  All of these effects are quite different from pigmentation as generally conceived:  if you grind up a lapis lazuli in a pestle, the dust will be brilliant blue (you have made ultramarine!) but if you similarly grind up a peacock feather, the dust will be gray, alas! This is because the glistening reflective aqua-blue of the feather is caused by how microscopic lattices within the various surfaces react with light (or I suppose, I should really go ahead and call these lattices “nanostructure” since they exist at a scale much smaller than micrometers). These lattices are known as “photonic crystals” and they appear in various natural iridescent materials—opals, feathers, and scales.  Scientists have long studied these materials because of their amazing optic properties, however it is only since the 1990s that we have begun to truly understand and engineer similar structures on our own.

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Physicists from the 19th century onward have understood that these iridescent color-effects are caused by diffraction within the materials themselves, however actually engineering the materials (beyond merely reproducing similar effects with chemistry) was elusive because of the scales involved.  To shamelessly quote Wikipedia “The periodicity of the photonic crystal structure must be around half the wavelength of the electromagnetic waves to be diffracted. This is ~350 nm (blue) to ~650 nm (red) for photonic crystals that operate in the visible part of the spectrum.”  For comparison, a human hair is about 100,000 nanometers thick.

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The actual physics of photonic crystals are beyond my ability to elucidate (here is a link to a somewhat comprehensible lay explanation for you physicists out there), however, this article is more to let me explain at a sub-rudimentary level and to show a bunch of pictures of the lovely instances of photonic crystals in the natural world. Enjoy these pictures which I stole!

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But, in the mean time don’t forget about the photonic crystals! When we get back to talking about the symbiosis of the giant clams, we will also return to photonic crystals!  I have talked about how ecology is complicated.  Even a symbiotic organism made up of two constituent organisms makes use of nanostructures we are only beginning to comprehend (“we” meaning molecular engineers and materials physicists not necessarily we meaning all of us). imagine how complex it becomes when there are more than one sort of organism interacting in complex ways in the real world!

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As mentioned previously, I am a toymaker who crafts the Zoomorphs mix-and-match animal toys (you can check them out here).  I always try to make the characters whimsical but with a strong basis in reality–which means color is one of the hardest things to figure out.  If the animals’ colors are too realistic and drab, children (and other toy aficionados) will not gravitate towards the toys, but if the colors are too bright the animals become surreal.

Zoomorphs Jurassicmorphs! (*cough* available at finer toy stores)

Fortunately I have always been helped out by our magnificent extinct friends, the dinosaurs.  Paleontologists have ideas about what color dinosaurs were (based on the coloration of living reptiles and birds), but the scientists still haven’t found any conclusive answers—which means I can paint my dinosaur characters with all of the gaudy 80s colors I can find in the Pantone book!

Microraptor Fossil with plumage--found in rural China (AP Photo/Mick Ellison, American Museum of Natural History, Science /AAAS) Photo: Mick Ellison, American Museum Of Natural History, Science/AAAS / AP)

Or at least that was true until now.  On Thursday, the scientific journal, um, Science reported that an international team of Chinese and American scientists have discovered the color of microraptor, a small feathered dinosaur about the size of a crow which lived 130 million years ago.  By studying the patterns of pigment-containing cell fragments known as melanosomes which were present in an exceptional microraptor fossil, the scientists determined that the ancient animal was black with an iridescent shimmer—a common pattern for many of today’s birds.  Microraptor apparently also had two long streamer feathers on its tail.  The little dinosaur likely used its iridescence and ornate tail feathers for social signaling with other microraptors over important concerns like territory and mating.

Artist's conception of Microraptor (Photo: Mick Ellison, American Museum Of Natural History, Science /AAAS / AP)

Many of the articles I have read about the microraptor’s coloration have playfully noted that the small predator hews to the fashion adage of wearing basic black with accents.  The articles also compare the tiny dinosaur (which had two pairs of wings—on both its legs and arms) to crows, grackles, starlings and the like.  However it is important to remember that microraptor was not yet a bird—it had sharp claws on nimble little hands and a mouthful of cruel teeth (so the fashion metaphor may be even more appropriate than the bird analogy).

A grackle shows off the same colors worn by microraptor 130 million years ago.

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